THE ECOLOGICAL AND GEOMORPHOLOGICAL

CONSEQUENCES OF ARTIFICIAL CATCHMENT RESERVOIRS__________________________

A Step Towards Sustainable Sediment Management

An Independent Study By

Gray Vickery

Emory UniversityDepartment of Environmental Sciences

ENVS 494R – Dr. William SizeFall 2014

TABLE OF CONTENTS

I. INTRODUCTION______________________________________________________________________________3II. THE BENEFITS AND DRAWBACKS OF DAMS___________________________________________4

A. Historical Significance………………………………………………………..4B. Socio-economic Benefits……………………………………………………..4C. Environmental Concerns…………………………………………………….6

III. DYNAMICS OF A RIVER SYSTEM_________________________________________________________6A. Balancing Water Flow and Sediment Transport………………….6B. Sources of Water and Sediment…………………………………………..8C. Sediment Load and Deposition……………………………………………9D. Optimal Rivers for Damming…………………………………………….10E. Climatic Influences…………………………………………………………...11

IV. BIOTIC AND ABIOTIC FACTORS IN RIVER ECOSYSTEMS___________________________12A. Abiotic Factors…………………………………………………………………13B. Biotic Factors…………………………………………………………………...14

V. PROPERTIES OF NATURAL LAKES______________________________________________________15A. Ecological Zones in Natural Lakes…………………………………….15B. Thermal Stratification……………………………………………………...16C. Chemical Stratification………………………….………………………….17

VI. FRAGMENTATION OF RIVER SYSTEMS BY DAMS___________________________________20A. Creation of Abnormal Environments……………………….…….…..20B. Flooding the land………………………………………………….…………..21C. Ecological Issues within the Reservoir…………………….…………23D. Heavy Metal Bioaccumulation…………………………………………..25E. Downstream Effects…………………………………….…………………....26F. Coastal Effects……………………………………………….……………….…28G. Migratory Fish…………………………………………………..……………...30

VII. SEDIMENTATION PROBLEMS WITHIN A RESERVOIR____________________________32A. Scope of the Problem………………………………………….……………..32B. Factors Affecting Influx of Sediment………………………….………33C. Factors Affecting Sedimentation Within the Reservoir………34D. Influence of Turbidity Currents…………………………………………36E. Sediment Flushing……………………………………….……………………38

VIII. DAM DECOMISSIONING AND SEDIMENT MANAGEMENT________________________39A. Scope of the Problem………………………………………………………...39B. Restoration of Flow Regime………………………………………………40C. Effect of Sediment Release……………………………………….………...41D. Water Quality and Toxic Compounds………….……………………..42E. Additional Concerns………………………………………………………….43F. Sediment Management Strategies……………………………………..44

IX. ADDITIONAL IMPACTS__________________________________________________________________45A. Reservoir Induced Seismicity……………………….………………...….45B. Vector-borne Diseases………………………...……….……………………46C. Greenhouse Gasses………………………………………....…………………47

X. CONCLUSIONS AND RECOMMENDATIONS____________________________________________48XI. WORKS CITED____________________________________________________________________________52

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I. INTRODUCTION

There is growing concern within the scientific community as to the viability

of current reservoir management strategies. While dams play a key role in urban

infrastructure, the environmental drawbacks may be too profound to justify. By

blocking the natural flow of sediment-laden water, reservoirs disrupt the balance of

physical and ecological systems and lead to irreversible environmental damage. Our

current technology has not enabled us to overcome the problem of sedimentation

within reservoirs, which limits dam longevity and leads to negligent abandonment.

In addition to being an engineering dilemma, sediment capture also induces severe

downstream ecosystem effects extending all the way to the coastline.

An estimated 48,000 dams are operating today without the employment of

efficient or viable sediment management plans. As the demand for reliable water

increases, dam construction is forecast to accelerate dramatically. The

unsustainability of this current paradigm has brought reservoirs to the forefront of

environmental awareness. Developing a more conscious, future-oriented reservoir

management policy will require a broader understanding of the environmental

consequences associated with construction, maintenance, and removal. As a means

of elevating conventional knowledge on this serious issue, this paper explores the

interrelatedness of various natural systems to reveal the ways in which reservoirs

can alter the delicate balance between them.

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II. THE BENEFITS AND DRAWBACKS OF DAMS

A) Historical Significance

Throughout the history of modern man, human populations have nearly

always favored river valleys as the optimal locations for establishing settlements.

Around the year 2950 B.C. ancient Egyptians realized that by blocking the flow of a

river with masonry stone they could control the influx of water into their

downstream settlements.1 Suddenly, these societies found themselves with

dependable irrigation, reliable drinking water and security from floods. This pivotal

discovery enabled mankind to control life’s most precious resource, leading to rapid

advances in technology and society as a whole. Nearly three millennia later, dams

have become the symbol of our progression as a species, and our absolute

domination over nature.

B) Socio-economic Benefits

Dams have become so fundamental to the infrastructure of developed cities,

that we are currently unable to calculate how many are operational in the world

(current figures estimate 48,000).2 Their importance, though often poorly

understood by the general public, is difficult to fathom. Cities with highly developed

technical infrastructures simply cannot rely only on groundwater reserves or

imported water to satisfy the needs of immense populations. As a result, the

damming of river valleys has become routine protocol. Most dams are single-use,

1 Baxter, R.M. Environmental Effects of Dams and Impoundments. Palo Alto, CA: Annual Reviews, 1977. Print. Pg.12 http://wwf.panda.org/what_we_do/footprint/water/dams_initiative/quick_facts/

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meaning they were designed and built to perform only one task. The function of any

given dam will generally fall into one of four categories:

Irrigation - By the year 2025, it is estimated that 80% of global food

production will employ reservoir-sourced irrigation systems.

Hydroelectric power – Hydroelectric power generated by dams accounts

for 90% of global energy production.

Industrial and domestic water supply – Groundwater reserves account for

less than 2% of earth’s surface water, and once depleted, it takes hundreds of

years to replenish. Thus, the use of dams for consistent water is intuitive.

Flood control – By regulating the discharge of rivers, dams effectively

eliminate the risk for flooding downstream.3

Aside from providing these fundamental services, dams offer a number of

latent benefits. Hydroelectric dams offer cheap reliable energy and can reduce the

economic stresses associated with importation of energy resources. Also, dam

construction and maintenance creates jobs and alleviates regional unemployment.

The hydropower industries often promote their dams by encouraging recreational

activities like boating and fishing. This supports community growth around the lake.

For these reasons, the majority public perceives dam construction as a noble act and

commends organizations like Tennessee Valley Authority for their monumental

feats of engineering, as well as their positive contributions to community growth.

3 http://www.icold-cigb.org/GB/Dams/role_of_dams.asp

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C) Environmental Concerns

Although dam construction can undeniably foster social, economic and

technological progress, the environmental repercussions associated with

construction, maintenance, and removal are deeply negative. By fragmenting the

natural flow of rivers, dams prevent the transportation of sediment-laden water,

which severely alters aquatic ecosystems. As more research is done, the

environmental impacts are proving more severe and complex than we ever could

have imagined. The magnitude of these issues calls into the question our ability to

warrant dam construction as a responsible practice. Growing global population and

accelerated urbanization, however, will necessitate reliable water and thus a

continued reliance on dams. In order to develop more sustainable dam operations, it

is necessary for engineers to gain a more comprehensive understanding of the

ecological effects. With improved operations protocol, dams can be built to support

growing cities without impacting the ecosystems on which those cities depend.

III. DYNAMICS OF A RIVER SYSTEM

A) Balancing Water Flow and Sediment Transport

In order to interpret the effect of dams on river systems, it is important to

first have an understanding of natural river dynamics. Streams and rivers serve one

primary purpose: to transport inland water and sediment to the sea. Water will

always take the path of least resistance, producing a somewhat predictable fluvial

network. During the course of its journey, a river will shape the landscape through

which it flows. At any given point along a river system, terrain is altered with a

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mutable combination of three mechanisms: erosion, transportation and deposition

(figure 1).

Figure 1: The velocity required to mobilize sediment increases in correlation with grain size. As stream velocity increases, finer sediments (clay, silt) are eroded first, followed by coarser ones (sand, pebbles). The converse is true for deposition; as velocity decreases, coarser sediments are the first to deposit, followed by finer particles.

Whether the river is primarily eroding, transporting or depositing sediment

depends on the stream’s expression of potential energy, which is determined by

discharge (velocity x depth x width) and total suspended solids (TSS). Sediment

concentration must balance water volume to maintain equilibrium. Rivers respond

to equilibrium imbalance by adjusting their dimension, profile, and drainage

pattern. If sediment concentration is low and water volume is high (as it is when

dams withhold material) the river will erode and scour its channel to compensate,

resulting in a single thread geomorphological drainage pattern. This causes

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riverbank collapse and loss of riparian (streambank) habitat. Conversely, if

sediment concentration exceeds transport capacity (as it does during reservoir

flushing events) the choked stream will aggrade, and a braided profile will develop.

Apart from water volume and sediment load, the erosive characteristics of a river

system are most drastically affected by the shape of the channel and the steepness

of the watershed - eroding more laterally in gently sloping terrain and down cutting

vertically in regions with higher gradients.

B) Sources of Water and Sediment

Reservoir longevity is a function of storage volume and inflowing sediment

yield. The upstream river sources sediment from both within the stream channel,

and from erosion of surrounding hill slopes within the drainage basin. The sediment

yield of a given reservoir depends on a variety of upstream factors:

River gradient Erodability of watershed Strength, duration, and frequency of flood events Watershed vegetation Soil characteristics Bedrock composition

A streams transport capacity (competency) depends on a steady input of

water. Water typically enters rivers as runoff through base flow, and through the

formation of tributaries during times of heavy rainfall. During springtime

snowmelts in higher latitudes, a substantial percentage of discharge is owed to melt

water. It is also important to emphasize that groundwater flows are directly

connected to surface water flows. Infiltration of water on land accumulates

underground in the phreatic zone. Streams are fed by these phreatic groundwater

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flows via seepage and subsurface springs. For this reason, streams continue to flow

throughout periods of prolonged drought. Even when reservoirs limit downstream

water supply, the rivers can partially compensate for this discrepancy if sufficient

exchange occurs between groundwater and surface water.

C) Sediment Load and Deposition

Streams transport sediment in three different states (figure 2). 1. Bed load

describes the sediment that is transported along the riverbed by rolling, sliding, or

saltation (jumping). The bed load (coarse grain sands, cobbles and boulders)

experiences the least net movement of all three sediment states, as greater energy is

needed to mobilize larger particles. 2. Suspended load particles are too small to

escape suspension, but either too large or inert to dissolve. 3. Finally, the dissolved

load comprises the smallest and most soluble grains, which are dissolved into the

water and carried invisibly as chemical ions. Together, these three represent the

total sediment load of a stream, which is representative of the entire sediment yield

of the drainage basin. Because competency is velocity dependent, a river emptying

into reservoirs will first deposit its bed load sediments. As velocity slows with

increasing distance from inflow, suspended fine grain sediments will eventually

precipitate.

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Figure 2: Bed load, moves along the river bed by sliding, rolling or jumping. These grains are the largest sand gravel and cobbles. Suspended load is typically comprised of silt and clay and flows at the same speed as the stream. Dissolved load is transported as chemical ions.

E) Optimal Rivers For Damming

When prospecting a future reservoir, locations are typically chosen based on

the maturity of the river within a given water shed. The ideal location for a dam is

on a fairly high gradient response stream within a narrow river valley. Ideally, the

river valley should widen upstream away from the dam to accommodate for the

desired storage capacity. Topographically flat and wide upstream valleys serve as

the optimal location for emplacement. Furthermore, the flow rate of the dammed

river must be reliable and exhibit minimal seasonal discharge fluctuations.

Reservoirs lose water to evaporation, so the inflowing river must be competent

enough to make up for this loss. Of equal importance is the strength and

permeability of the bedrock. The underlying geology must be sturdy enough to

support the massive weight of the reservoir without folding or compressing.

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Additionally, the bedrock and its overlying horizons must be impermeable; thus,

presence of clay minerals is favored. 4

D) Climatic Influences

In river systems, climate determines temperature, precipitation and the

amount of supply water lost through evapotranspiration in the surrounding

vegetation. Many rivers only exist intermittently through out the year in response to

heavy rainfall events. During storm events, there is a significant lag time between

peak precipitation and inflow to streams and rivers (figure 3).

Figure 3: A typical hydrograph displays an individual storm event. The lag time between peak precipitation and rising discharge represents the time taken for runoff to drain into the river channel. During peak discharge, the river channel experiences the most erosion and often overflows and floods its floodplains.

In wetter climates, runoff is stiffled by watershed vegetation and intercepted by soil

infiltration, providing a cushion to protect rivers from excessive inputs. In dryer

4Degoutte, Gerard. Small Dams, Guidelines for Design, Construction and Monitoring. Cemegref Editions and Engref (France), with French Committee on Large Dams. 2002.

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climates, rivers may only carry small amounts throughout the year, and then are

completely geomorphologically altered by a single event. The frequency of storm

events is extremely important in reservoir environments, as these events represent

a concentrated influx of water and sediment.

When prospecting dam construction, the influence of a changing climate

cannot be overlooked. As the behavior of global climate patterns becomes

increasingly unpredictable, unprecedented fluctuations in precipitation are making

river flows highly variable. Large dams construction relies on the assumption that

future flow patterns will more or less resemble historic trends. Debate has been

raised concerning the safety of damming high-flow regions such as the Mekong and

the Amazon as stream flow is becoming more unpredictable. Higher frequencies of

extreme rainfall events increase the likelihood for catastrophic dam failure.

Conversely, prolonged lapses in rainfall will render hydroelectric dams functionally

useless and therefore economically counter-productive. 5

IV. BIOTIC AND ABIOTIC FACTORS IN RIVER ECOSYSTEMS

The biggest environmental concern of dam construction is the destruction of

riverine ecosystems. River ecosystems are complex, delicate, and extremely

interdependent. The drastic physical alterations associated with dam construction

have immediate and irreversible effects on these fragile ecosystems. Here we will

briefly discuss the biotic and abiotic factor influencing natural river ecology, and in

5 Wrong Climate For Big Dams. International Rivers, California, 2011.

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section VI we will see how these factors are altered by the presence of upstream

reservoirs.

A) Abiotic Factors

River environments are termed “lotic”, meaning that all ecological systems

are governed by flowing water. The flood pulse concept states that the biology of

most rivers is adapted to seasonal fluctuation in flow regime, and relies on annual

flooding to stabilize the ecosystem. Periodic flooding of the aquatic/terrestrial

transition zone (ATTZ) brings nutrients to this unique and essential ecotone. Water

temperature is of equal importance to river ecosystems. Rivers are heated at the

surface by the sun, and through conduction from the underlying substrate. Rivers

typically experience little annual temperature fluctuations and exhibit very minimal

thermal stratification, especially if fed by groundwater flows. River organisms are

typically pokilotherms, meaning that their internal temperature is flexible and

responds to their environment, allowing them to withstand seasonal temperature

changes. These species rely on annual temperature fluctuations just as they rely on

annual flood events. For example, cold winter temperatures serve as a cue for insect

eggs to break diapause.

Of all abiotic factors, lotic biota are perhaps most vulnerable to variations in

chemistry. In large streams, there is a correlation between distance from water

source and elevated nutrient content, dissolved salts, and water acidity. The

solubility of oxygen is lower in areas with higher pH. Therefore, the closer to the

river source, the more dissolved oxygen will be present. The chemical and

nutritional characteristics of a river gradually change as the river progresses

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downstream, and the resident species change accordingly. Each organism is

specifically adapted to the chemical conditions in their river-segment, so dramatic

alterations in water quality will shock populations.

B) Biotic Factors

Biotic energy inputs are equally vital to the homeostatic balance in river

ecosystems. Just as is the case in any ecosystem, the trophic chain within a river is

structured around the primary producers. These plants and algae constitute the

organic energy store that is consumed by higher orders species. The health of the

ecosystem is entirely dependent on the health of the producers. Fluctuations in

water temperature, chemistry and oxygen content impede photosynthetic activity,

causing reverberating effects throughout the food web. Organic material derived

from the surround terrestrial environment enters the stream in the form of leaf

litter. This energy input is important to the health and diversity of microbial fauna.

The reliability of this energy source relies on healthy shoreline vegetation, which in

turn relies on flood regularity.

The health of the lotic ecosystem is entirely dependent on the balance of

these biotic and abiotic factors. Unfortunately, these factors, along with others are

extremely vulnerable to perturbation from natural forces and energy fluxes. As we

will explore in section VI, the emplacement of an upstream dam will alter regularity

of flow regimes, temperature, and sediment load and dissolved oxygen, which in

turn upsets homeostatic balance across the ecosystem.

V. PROPERTIES OF NATURAL LAKES

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One objective of dam planning is to design a healthy artificial lake that

undergoes the same physical, chemical and biological processes as natural lakes.

Reservoirs environments, however, do not possess the natural conditions necessary

for the development of healthy ecology. In order to understand these differences, we

will first discuss the properties of natural lakes. Later, in section six, we will show

how reservoirs deviate from these properties and how these deviations make the

development of competent biology unmanageable.

A) Ecological Zones in Natural Lakes

A lake ecosystem is comprised of three distinct regions. The benthic region

encompasses the entire floor of the lake from the shoreline to the point of maximum

depth. A thriving invertebrate fauna occupy the benthic region, including

crustaceans (crabs and shrimp), mollusks (clams and snails) and insects. They play a

vital role in sediment bioturbation and nutrient circulation. The open water pelagic

(limnetic) zone harbors phytoplankton and aquatic plants, which are the primary

producers of lentic food webs. The presence of these organisms makes the pelagic

zone the photosynthetic epicenter of the lake. The shoreline habitat is known as the

littoral zone. In very mature lakes, a highly vegetated fringing wetland will develop

adjacent the littoral zone. If a lake is deep enough to block the transmission of light

to certain bottom stretches, another zone, the profundal zone, will exist. The flora

and fauna present in these three regions are adapted to the particular conditions of

these respective environments.

.

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Figure 4: This diagram illustrates ecological zonation in lakes. Most of a lakes photosynthetic productivity occurs in the limnetic zone. The littoral zone is a transitional habitat between open water and land. The benthic zone extends from the shoreline to the lake bottom or profundal zone, which receives no solar energy.

B) Thermal Stratification

The most important abiotic factors in lentic systems are light, temperature,

wind and chemistry. Unlike rivers, lakes experience very little turbulence. This

mildness of water flow permits the development of distinct thermal stratification.

Warm, sun-bathed surface waters make up the epilimnion. The colder, denser water

at the bottom of the lake comprises the hypolimnion. During the summer and winter,

a distinct boundary exists between the two known as the thermocline. In temperate

regions, there is a seasonal exchange between these two regions as the epilimnic

waters cool and sink, allowing colder bottom water to surface. This process, known

as lake turnover (refer to figure 3), facilitates nutrient cycling and stabilizes the

lentic ecosystem. Luckily, lentic organisms are also poikilothermic. Their internal

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body temperatures adapt to this shift in temperature within a certain range, so the

annual temperature variations are endurable, even vital to life.

Figure 5: Lake Turnover: During the summer, the epilimnion absorbs sunlight and becomes significantly warmer than the lower hypolimnion. Because sunlight does not penetrate to the lower reaches, the hypolimnion becomes extremely cold and distinct boundary develops between the two known as the thermocline. As the surface water cools during the winter, it becomes more dense and sinks. This forces the hypolimnion water to rise to the surface resulting in a complete turnover of the layers.

C) Chemical Stratification

As is the case in rivers, oxygen availability dictates all life within lakes.

Thermal stratification is closely tied to the dissolved oxygen level within any body of

water. Because it is directly exposed to solar energy, the epilimnion circulates

frequently. This maximizes contact with surface air and enriches oxygen levels. The

hypolimnion, however, does not interact with the air-water-interface and

consequentially has less dissolved oxygen. The relationship between life and oxygen

in lakes is reciprocal. Because more plant life exists in the upper pelagic zone,

surface oxygen levels are further enriched through photosynthetic processes. The

result is a sharp oxygen gradient between the upper and lower layers of typical

lakes.

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Nutrient availability in lentic systems can often be a limiting factor for a

prosperous biota. Phosphorous is perhaps the most vital nutrient in lake ecosystems

(and all ecosystems) because it is fundamental to numerous biochemical processes.

Phosphorous enters reservoirs from eroded upstream watersheds, and is

immediately absorbed by algae. These algae and macrophytes release a nonreactive

variant compound of phosphorous as a by-product of photosynthesis. The

nonreactive phosphorous is then consumed by benthic organisms, or otherwise

remineralized by microbial life back into the reactive form to continue the cycle.

Eventually phosphorous will precipitate and chemically incorporate with the

sediment, rendering it unusable to organisms.6 This cycle, along with the nitrogen

and CO2 cycle, relies on consistent turnover of the upper and lower layers, as well as

a reliable influx of sediment with the proper nutrient percentages.

The addition of excessive amounts of phosphates (usually from fertilizer

runoff, detergents, or sewage) can severely effect the chemical composition of a lake

through a process known as eutrophication. This happens when runoff nutrients

over stimulate the growth of algae, causing an explosion in phytoplankton and

machrophytes. Eventually, these algae will die and settle to the bottom. Here, the

organic debris is converted to an inorganic form through natural decomposition by

bacteria. The process of bacterial decomposition consumes vast amounts oxygen

without releasing any back into the water, so overall dissolved oxygen levels

plummet. Unable to survive in hypoxic waters, fish including trout and cisco

6 Istvanoviks, Vera. The Role of Biota in Shaping the Phosphorous Cycle in Lakes. Water Resources Management Team of the Hungarian Society of Sciences, 2008.

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asphyxiate. This provides even more organic material to the oxygen consuming

bottom decomposers, and thus a positive feedback system takes over. The zone of

depleted oxygen eventually permeates the entire lake creating a “dead zone”. In the

absence of oxygen, anaerobic decomposition of the remaining algae continues. This

releases poisonous levels hydrogen sulfide as a by-product to effectively seal the

fate of the ecosystem.7 (See figure 6)

Figure 6: The formation of algal blooms: The introduction of nutrients from industrial runoff increases the abundance of photosynthetic organisms. The eventual death of these organisms triggers a spike in bacterial decomposition, which consumes large volumes of dissolved oxygen.

VI. FRAGMENTATION OF RIVER SYSTEMS BY DAMS

A) Creation of Abnormal Environments

7 http://scholar.lib.vt.edu/theses/available/etd-01102003-162857/unrestricted/%2807%29Lit_Rev_1.pdf

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When an artificial dam is emplaced into an otherwise natural stream system,

the flow of sediment from erosional upper headwaters of the watershed to the

depositional lower reaches is interrupted. In so doing, three new environments are

essentially created: the new upstream environment, the reservoir environment

itself, and the severed, downstream environment. The morphological, chemical and

thermal conditions of each of these new zones contrast sharply with conditions in

equivalent natural systems.

The environment immediately following a reservoir is termed dam-proximal.

This segment of the river most reflects the morphology of the parent river: narrow

and windy. Newly released water is typically high clarity, making significant channel

scouring common in the dam-proximal zone as the river compensates for the

sediment shortage. As the river approaches the next reservoir, the channel begins to

widen and the velocity of the water slows. Sediment equilibrium is more or less

restored, and ecological balance is reached. Nearing the next reservoir, slackwater

begins backing up into the tributary. The reduction in velocity causes large point

bars and islands develop as the environment becomes more depositional (Figure 4).

Finally the river empties out into the reservoir and deposits all of its sediment in

three different zones: riverine (nearest the parent river), transitional, and lacustrine

(deepest part of the reservoir). These environments are purely a byproduct of

damming; they lack the natural balance and regularity of abiotic inputs necessary

for healthy stream ecology.

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Figure 7: River fragmentation results in the formation new, unnatural environments with altered morphological and biological properties. River maturity increase with distance downstream from the impoundment. Dam proximal environment exhibit reduced sinuosity from accelerated erosion of stream deposits. As the stream progresses, more sediment becomes available from the increased upstream channel scouring. Eventually, normal stream conditions stabilize and channel deposits begin to grow. Finally, as the stream approaches the next reservoir, the channel widens significantly and drowns riparian vegetation. Upon entering the reservoir, coarse sediments deposit to form deltas. Finally, the water reaches the impoundment, water velocity reduces to zero, and fine sediments precipitate.

B) Flooding the Land

With the exception of glacial lakes, natural lakes fill gradually, allowing

transitional ecotones to develop at the shoreline-water interface. Slowly, an entirely

new bionetwork takes shape with its own trophic chain. Unlike natural lakes, lakes

formed by the damming of rivers undergo a much more abrupt succession of

ecology. This hastened environmental transformation is often too fierce for

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preexisting river organisms to adapt to. Virtually every riparian and floodplain

organism is drowned in the deluge. As the upstream environment, which was once a

free flowing stream environment, is turned into a slack water reservoir, the life it

carries experiences a sudden and dramatic shift in living conditions. Conflicting

temperature, nutrient availability, and oxygen profile within the reservoir lead to

die-offs of lentic fauna. Benthic organisms in rivers, for example, are

morphologically adapted to life in flowing water, where water turbulence and low

levels of plankton keep oxygen levels high. The comparatively low levels of

dissolved oxygen within the reservoir, coupled with the pressure of an

unaccustomed physical environment cause a complete extirpation of benthic lotic

species.

Soon to be flooded river valleys are almost always densely vegetated, often

with fully mature forests. Upon flooding, the rapid submergence of organic material

(vegetation, soil, peat) stimulates a burst of heterotrophic activity that leads to

critically low oxygen levels in the newborn lake. Chironomids, being one of the few

organisms able survive in such hypoxic environments, quickly exploit the new niche.

Eventually, a succession of newer species will emerge and the ecosystem will re-

equilibrate. Of course, the fact that these environments eventually stabilize is by no

means justification for the massive species annihilation caused from the initial

valley inundation. Many dam building projects have opted to strip the valley of all

vegetation and topsoil prior to flooding it as a means of avoiding eutrophic

conditions and facilitating ecologic succession. Unfortunately, both tactics entail a

dramatic reduction of biodiversity.

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C) Ecological Issues within the Reservoir

Because reservoir ecosystems are governed by artificial energy inputs, many

ecological processes reflect the logical opposite of what nature intended. The

ecotone separating land and water is an extremely productive keystone

environment. This intertidal region typically experiences a single flood event during

the annual rain season, followed by prolonged exposure to the atmosphere for the

remainder of the year. Intertidal organisms have adapted recovery mechanisms

allowing them to endure the punctuations of flooding. In reservoirs however, water

levels are kept at flood levels for the entire year, and punctuated by drawdown

events wherein water is released from bottom outlets. This reversal of natural

trends suppresses the maturation of intertidal ecosystems. Drawdown events

severely impact benthic biodiversity, often reducing shorelines to barren mudflats.

Drawdown events also negatively impact non-migratory fish within the

reservoir. Shallow waters are ideal spawning grounds for fish, but in many

instances, poorly timed drawdowns leave eggs exposed on the beach where

hatching is improbable, and survival nearly impossible. Aquatic birds also suffer the

consequences of erratic water level fluctuations. Rising levels can flood rookeries

along the reservoir coast. During nesting seasons, the offspring are drowned before

they can even hatch.

The chemistry of natural lakes is determined by the composition of inflowing

streams as well as precipitation. This creates a moderately uniform chemical

stratification throughout the lake. Newly flooded reservoirs, however, often owe

their chemical characteristics to the leaching of soluble minerals from the newly

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flooded terrain. If these compounds are abundant and resilient, the chemical

disparity between inflow and reservoir will persist for years. This bizarre chemical

gradient facilitates the formation of large anoxic bodies of water that lie inert in the

deepest reaches of the reservoir. To aggravate the reservoir’s already eutrophic

temperament, decomposition of submerged vegetation consumes large amounts of

dissolved oxygen.

Thermal pollution describes detrimental temperature fluctuations in bodies

of water.8 Healthy large lakes experience annual lake turnover. Many reservoirs are

polymictic, meaning periodic mixing of thermal layers does not occur. The

deoxygenated hypolimnic waters, therefore, stagnate. This thwarts the development

of healthy deepwater benthos and consequently limits foraging activities for bottom

dwelling fish. 9 Dam operators employ drawdowns to empty the cold, unfertile

bottom water to facilitate thermal exchange. However, this is largely ineffective and

extremely harmful to downstream lotic ecology.

Reservoirs situated in arid regions often face the problem of over

salinization. Productive agriculture in these regions often calls for heavy irrigation.

This causes leaching and runoff of dissolved salts, which can flow downstream and

enter the reservoir. Low precipitation, and high evaporation in these regions can

further salinize reservoir water. Over the course of many years, the characteristics

of a reservoir will approach that of a natural lake, but in the meantime, local biota

suffers the hardships of an ecologically diametric habitat. A study of New England

8 Butz, Stephen. Energy and Agriculture: Science, Environment, and Solutions. Cengage Learning, 2014. 9 Cooper, C M. Effects of Abnormal Thermal Stratification on a Reservoir Benthic Macroinvertebrate

Community. The University of Notre Dame, 1980.

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reservoirs situated on un-stripped soils found that large bodies of water took an

average of 6 years to re-equilibrate.10

D) Heavy-Metal Bioaccumulation

To exacerbate the already poor water quality in newly flooded valleys caused

by eutrophication, sulfate-reducing bacteria release methyl mercury as they

consume submerged organic material. To make matters worse, as previously dry

ground is flooded, toxic substances that are present in the soil are leached into the

water. The presence of these compounds is likely owed to surface runoff from urban

stormwater, atmospheric fallout from coal-fired power plants emissions, and runoff

from mines. Mines are considered the biggest threat because they produce highly

acidic runoff replete in heavy metals like copper, zinc, nickel, iron, and manganese.

Many of these metals have a strong propensity to bioaccumulate in the food

chain. Organisms absorb these metals through respiration, ingestion, or from direct

diffusion from water to bloodstream. Phytoplankton rapidly assimilate heavy metals

and act as a vector within the ecosystem. As higher order organisms ingest the

tainted algae, biomagnification transfers the metals through the food chain. Non-

essential metals like mercury are fat soluble, so their retention time in aquatic

vertebrates can be very long. Many organisms, including crustaceans, can excrete

heavy metals without suffering any physical harm. But other organisms suffer

tremendously. Bivalves are among the species unfit to successfully regulate heavy

10 Baxter, R.M. Environmental Effects of Dams and Impoundments. Palo Alto, CA: Annual Reviews, 1977. Pg.262, 271

25

metals. 11 High concentrations of metals have been linked directly to loss of bivalve

fauna.

E) Downstream Effects

The most acute consequences of impoundments are reserved for down

stream environments. As mentioned earlier, rivers rely on flow regularity to

maintain nutrient balance, temperature normality, and proper chemistry. “What is

retained in the lake (heat, silt, inorganic or organic nutrients) is lost to the stream”

(Baxter, 271). Withholding water at dams, and then sporadically releasing it in

loosely regulated volumes puts enormous stress on downstream environments. In

many instances, downstream ecosystems have been entirely de-watered. This

inevitably leads to a die-off of the majority of lotic flora/fauna. In the reverse

scenario, large releases of water will overwhelm downstream environments. Water

volume is not the only aspect to consider. Of equal importance is temperature. When

flows are released from reservoirs, the water often stems from the lower reaches

where the water pressure helps facilitate flow. This water formed in the

hypolimnion, which can reach a frigid 4 degrees Celsius. Organisms that use

temperature as an indicator of seasonal change may mistakenly initiate breeding

processes or migrations upon influx of cold waters.

The environmental effect of blocking sediment transmission is equally

unfavorable. Downstream ecosystems rely on this sediment to nourish the food

chain and counteract erosion. Rivers respond to sediment deficiencies by eroding

their riverbanks and streambeds. This increased erosion will damage riparian

11 Osmond, D. L, et al. Watersheds. North Carolina State University, NC, 1976.

26

environments for hundreds of kilometers following a dam. Increased incising

(downward cutting through sediment or bedrock) of riverbeds establishes a new,

lower water table along rivers. This has negative impacts on riparian plants whose

roots are not long enough to tap into a deeper groundwater reservoir. Alteration of

riverbanks also impacts benthic organisms that rely on specific substrate

conditions. Depriving the river of organic detritus hinders the production of

heterotrophs that extract phosphorous and nitrogen from the sediment. Damages to

the planktonic food chain base will reverberate through the successive trophic

levels. Additionally, because the water being released is depleted of oxygen, fish

cannot achieve proper blood-oxygen concentrations.

Dams often serve to pacify the occurrence of flooding downstream and make

floodplain regions perennially habitable for development and agriculture. But

floodplain health depends on the inundation of sediment during flood events. By

eliminating the occurrence of these events, the floodplain ecosystems must readapt

to the lack of periodic siltation. The trapping of these sediments also has the

unintended effect of lessening agricultural productivity downstream. Following the

construction of the Aswan dam, the use of fertilizer in the Nile Valley became

mandatory. Before the dam was built, deposition of sediment during floods served

to naturally fertilize the land. Floodplain species are completely dependent on peak

flow events to create temporary spawning grounds and migratory routes. The

reproductive period of red-bellied piranha corresponds exactly with flooding. Flow

regulation also hinders the stability of the ATTZ, which relies heavily on nutrient

loading during periodic flooding. The oxygen provided by floodwaters also

27

facilitates decomposition in the ATTZ. Without it, leaf litter and other organic

material take longer to cycle through the ecosystem.

F) Coastal Effects

The effects of sediment trapping extend all the way to coastal outlets, where

deltas and barrier systems are starved of the life giving sediment on which they rely.

The natural maintenance of deltaic formations involves a delicate balance between

sediment deposition from the contributing rivers, and erosion from ocean currents.

If the sediment load is too plentiful, the river mouth will become choked, estuary

environments will fill with sediment, and barrier islands will regress. This often

occurs in recently logged coastal regions where loss of vegetation has accelerated

mass wasting. Conversely, if the river does not carry enough sediment, coastal

formations will undergo erosion from the persistent onslaught of longshore

currents and occasional storm surges. The ensuing erosion of deltaic structures

causes habitat loss for coastal marine flora/fauna. The latter scenario is often

witnessed when inland dams disrupt the flow of sediment. Dam construction along

the Nile River has limited the sediment budget available to organisms at its mouth,

resulting in reductions of sardine populations along the coast.

Fluctuations in river flow can also agitate coastal salinity gradients. An

estuary is at its best when river flow and tidal forces are balanced. If river flow

exceeds tidal forces, the less dense freshwater will flow above the seawater, forming

a wedge-shaped profile that narrows towards landward and prevents saltwater

intrusion. For most dammed rivers however, tidal forces exceed river output and

saltwater bodies invade freshwater estuaries as well as underground aquifers.

28

Estuarine species have specific salinity requirements, so these changes entail the

loss of biodiversity. Reservoir-induced saltwater intrusion on the North Caspian Sea

raised estuarine salinity from 8 to 11 ppt. This perturbation lead to reductions in

zooplankton, phytoplankton, and benthic populations by 2.5 times their original

numbers.12 Figure 5 summarizes the coastal effects of upstream catchment

reservoirs.

Figure 8: This diagram depicts the coastal effects of dams. Sediment reduction leads to coastal erosion; flow irregularity leads to saltwater intrusion; nutrient reduction leads to ecosystem collapse.

12 Schwartz, Maurice. Encyclopedia of Coastal Science. Hutchinson Ross, 2006.

29

G) Migratory Fish

Rivers serve as transportation pathways for a variety of migratory fish

species. Anadromous fish live the majority of their lives in the ocean, but periodically

swim inland via rivers to reach spawning grounds. The new hatch of juvenile fish

will then swim to sea and live out their lives until it is their turn to migrate to

freshwater spawning grounds. Radio-tagging experiments have shown certain

species to travel up to 300 km in search of lower-order tributaries for breeding.

Other fish are cantadromous and spend the majority of their lives in freshwater

rivers, then migrate to sea to reproduce. For both of these species, reproductive

success depends on the unobstructed continuity of the river. When dams exist on

rivers, the fish cannot reach their spawning grounds, thus reproduction is

impossible. 13

Fish rely on olfactory and tactile signals to navigate upstream. When they

approach a dam, they confront unnatural changes in chemical composition,

temperature, and presence of suspended solids. This confuses the fish and often

leads them into mistaken tributaries. Pacific salmon are cantadromous and do not

eat during their journey to coastal spawning grounds. If they swim into reservoirs,

they often deplete their energy reserves before figuring out how to traverse the

outlets. In some cases, cantadromous fish are killed by lentic predators that they

otherwise would never encounter. Fish that successfully find outlets may be

pulverized in the hydraulic turbines. This has been the fate of many American Eels

13 Gerritsen, Alida; Young, Benjamin. The Effects of Dams on Migratory Fish: A North Country Case Study. St. Lawrence University, New York, 2008.

30

on the St. Lawrence Seaway.14 Another problem for migratory fish is genetic

isolation. By obstruction migration, populations become divided and cannot

contribute to genetic growth of the species. If fish accept the fact they cannot go any

further upstream, and settle for downstream breeding grounds, they then introduce

their genes into the local population, which disrupts the ecological population

dynamics.

Fish ladders reduce the severity of the problem considerably. Fishways are

built around the reservoir with a series of flooded steps that fish can jump up to

reach the upstream reservoir tributary. Water flowing over the steps simulates river

turbulence, which signal to the fish that they are going the right direction (Figure 5).

Unfortunately, observations have shown fish ladders to be only partially effective.

Even with ladder implementation, large reductions of fish populations occur. If

reproductive success depends on successful upstream navigation, then natural

selection will favor fish that can traverse fish ladders over fish that can swim long

distances. 15

14 Baxter, R.M. Environmental Effects of Dams and Impoundments. Palo Alto, CA: Annual Reviews, 1977.15 Gerritsen, Alida; Young, Benjamin. The Effects of Dams on Migratory Fish: A North Country Case Study. St. Lawrence University, New York, 2008.

31

Figure 9: This highly advanced fish ladder at the John Day dam allows anadromous fish on the Columbia River to reach upstream breeding grounds.

VIII. SEDIMENTATION PROBLEMS WITHIN A RESERVOIR

A) Scope of the Problem

  When drafting plans for dam construction, engineers are principally

concerned with maximizing longevity without sacrificing utility. Longevity is always

described as the length of time a reservoir can remain operational before it

inevitably fills with sediment and must be decommissioned. Without the prior

implementation of sediment removal strategies, it is only a matter of time until the

water-storage capacity of any given dam is reduced to nothing as sediment displaces

volume. Despite more than six decades of research, sedimentation is still probably

the most serious technical problem faced by the dam industry. Aside from being a

32

frustration for engineers, trapped sediment is also the source of immeasurable

environmental destruction. Well-built dams will typically perform for 50 to 100

years before infill renders them useless. The unremarkable lifespan of conventional

dams proves that a continuation of current dam building trends is unsustainable.

Arguing that the short-term infrastructural benefits justify the long-term

environmental impacts would be a contentious claim. To increase the long-term

benefits of dams, more advanced, systematic management protocol must be

implemented.

B) Factors Affecting the Influx of Sediment

The types of sediment that feed reservoirs vary tremendously in volume,

composition and grain sorting due to a complex combination of factors. These

factors include the slope of the river, topography/erodibility of the watershed, flow

regime, vegetative cover, and composition of bedrock. Instances of severe rainfall

are the principle drivers of abundant sedimentation. The most potent examples

have documented complete infill of reservoirs from a single storm event. Stream

gradient is also closely related to the distribution of sediment. Steeper rivers

undergo intense erosion and mass wasting during storm events. Reservoirs built on

these streams will likely be inundated with turbid water, and poorly sorted

sediment. Soil that is highly impermeable (either from compaction, clay presence, or

over-saturation) effects detrital concentrations by facilitating runoff and increasing

stream discharge. For this reason, semi-arid regions are considered high risk for

runoff due to the highly compacted, scarcely vegetated soil. Conversely, densely

canopied watersheds are ideal homes for reservoirs because the vegetative cover

33

limits erosion, and the highly organic upper soil horizons favor groundwater

infiltration over sheet wash. Regions that have undergone a loss of vegetative cover

will experience rapid erosion of topsoil. This trend is currently underway in

Madagascar. Following irresponsible deforestation practices in highland regions, the

country’s topsoil has been gradually washed into surface flows by the tremendous

erosive power of water. This topsoil will take thousands of years to replenish.

Madagascar has served as a reminder to the importance of preserving forests.

C) Factors Affecting Sedimentation within the Reservoir

The deposition of sediment in man-made reservoirs follows a set of rules

completely dissimilar to that of natural lakes. In natural lakes, the deepest region is

typically the center, giving them a bowl-shaped longitudinal profile. Reservoirs,

however, are deepest in the region immediately adjacent the dam. This unique

longitudinal profile has earned reservoir the nickname “half-lakes”.16 The

partitioned profile causes sediment to layer problematically (this will be discussed

in later sections). Reservoirs are also unique in their geometric diversity. If multiple

streams feed the reservoir, it will take on a dendritic morphology. If placed in flat

valley with low banks, a lobed reservoir will develop. Alternatively, when dams are

built in tightly confined, high-banked rivers, the reservoir will take on an elongated,

narrow profile.17 Reservoirs come in all shapes and sizes, so standardizing the

conventional wisdom regarding sedimentation patterns is difficult. One common

16 Baxter, R.M. Environmental Effects of Dams and Impoundments. Palo Alto, CA: Annual Reviews, 1977. Pg.25817 Toniolo, Horacio, and Gary Parker. 1d Numerical Modeling of Reservoir Sedientation. IAHR Symposium on River, Coastal and Estuarine Morphodynamics. 2003

34

thread among all artificial reservoirs is that trap efficiency nearly always

approaches 100%, making sedimentation a universal problem.

As rivers empty out into reservoirs, they bring with them a bed-load of sand

range particles, and a suspended load of mud (silt-clay) range material. 18The

increased cross sectional area of the channel stifles hydraulic turbulence and

reduces the velocity of the water. This loss of momentum causes coarser grained

particles to settle out into the primary deltaic structures or topset beds. Over time,

the bed-load sediments edge closer to the dam where they eventually spill

downward at a specific “plunge point” to overlay the foreset bed of intermediate

grain sizes. The suspended silt and clay is then carried to the furthest reaches of the

reservoir where it falls to the reservoir floor and becomes the bottomset bed (see

figure 6). Over time these three strata will overlap one another to form a succession

of stratigraphic lenses that taper off at either direction.

Figure 10: Bed formation: Grain size decreases with distance from tributary. The delta will aggrade in successive layers, each of which thin out in both directions. Eventually, a thick deposit of fine sediment accumulates nearest the reservoir. This greatly reduces the storage capacity of the reservoir.

18

35

D) Influence of Turbidity Currents

As mentioned earlier, the water flowing into a reservoir will not closely

resemble the water within the reservoir. They differ from one another in

temperature, chemical composition, and suspended sediment. This creates density

variations that force the inflowing water above, below, or between the existing

water. Typically the presence of suspended sediment (and sometimes colder

temperature) increases the density of inflow so that upon entering the reservoir, the

water plunges below the surface and a distinct divergence line develops. 19 When the

water reaches to the downward sloping plunge point, it gains momentum and flows

onward until reaching the bottomset beds adjacent the dam. This continued flow of

sediment rich bottom water is known as turbidity current. The increase in velocity

amplifies the erosive energy of the current, allowing it to mobilize previously

deposited sediments and creative a positive feedback. (Figure 7). The location of

these bottomset deposits sometime overlaps the location of critical water outlets;

therefore, mitigation of turbidity currents is essential to the function of a dam. The

most persistent turbidity currents have been documented in Lake Mead, where

bottom flows extend the entire 160 km length of the reservoir.

19 Oehy, Ch D., and A. J. Schleiss. Hydraulics of Dams and River Structures. Taylor and Francis Group. 2014

36

Figure 11: Turbidity currents caused by density disparities facilitate the flow of sediment from the inflow point to the dam itself. As velocity increases on the downslope, previously deposited materials are eroded. This further increases the density of the current, and therefore velocity as well.

Healthy ecological balance in reservoirs is dependent on the proper mixing of

the epilimnion and hypolimnion throughout the year. If turbidity currents are stable

enough to exist year round, the natural processes of lake turnover will be thwarted.

Consequently, nutrient cycling is disrupted. The emergence of dissolved oxygen

gradients hinder primary production and often lead to eutrophic conditions. If the

region was not stripped of vegetation prior to flooding, standing trees will further

disrupt circulation patterns until bacteria decomposition levels the reservoir floor

(this can often take many years). To prevent the development of turbidity currents,

many management efforts have been employed. These strategies involve the

placement of obstacles along the reservoir bottom to dissipate energy and

encourage upstream deposition of sediment rather than bottomset deposition. 20

20 Oehy, Ch D., and A. J. Schleiss. Hydraulics of Dams and River Structures. Taylor and Francis Group, 2014.

37

E) Sediment Flushing

As a tactic for overcoming sedimentation problems and prolonging reservoir

life span, a number of strategies are employed to remove sediment loads from

within the reservoir. The three most widely accepted methods are flushing, dredging

and sluicing. Of these three strategies, flushing is favored due its high efficacy and

relatively low cost. Flushing is done by opening outlets at the bottom of the dam and

allowing water to drain at a rapid rate, a process known as drawback. 21The

increased flow within the reservoir causes deposited sediments to be rapidly

mobilized. The sediment then flows with the water through the outlets and

downstream, allowing the reservoir to be refilled with a greater storage capacity.

The environmental impacts on downstream ecosystems from flushing are

cause for acute concern. The concentration of suspended solids within the flushed

water is far above the endurable threshold for the stream.22 The infusion of

sediment-laden water into the river causes accelerated deposition. River

ecosystems rely on the nutrients contained in the suspended solids for healthy

functioning; however, this assumes that the inputs of sediment are regular, and

within a tolerable range. The altered chemistry and sediment load of flushed water

has been shown to cause die-offs in trout and salmon populations, especially among

juveniles. Particles below a certain size can traverse gill membranes and cause

inflammatory mucus production. The increased mucus slows the diffusion of oxygen

21 Dittrich, Andreas. River Flow 2010: Proceedings of the International Conference on Fluvial Hydraulics. Braunschweig, Germany, 2010. Giuseppe, Crosa; et al. Effects of Suspended Sediments From Reservoir Flushing on Fish and Macroinvertebrates in an Alpine Stream. Birkhauser Verlag. Switzerland. 200922 Kinoshita, Atsuhiko; et al. The Impact on Fish of Sediment Flushing From a Sabo Dam. International Congress. Matsumoto Japan. 2002.

38

between the outside water and the bloodstream. 23Additionally, channel widening

leads to the loss of riparian habitats, which eliminates the essential transitional

environments between rivers and floodplains. Flushing is more likely to transport

finer grained particles, as they are easier to mobilize. Downstream deposition of

these particles can bury benthic habitats that thrive only in coarse-grained riverbed

deposits.

X. DAM DECOMMISSIONING AND RIVER RESTORATION

A) Scope of the Problem

When a dam has become filled with sediment and can no longer perform the

functions for which it was built, it will then be decommissioned. Across the world,

many dams have been abandoned and left to deteriorate. This is extremely unsafe

both for downstream settlements and for the environment. Since 1999 there have

been 129 dam failures due to negligent abandonment within the United States

alone.24 This has incentivized a growing trend towards standardized dam removal

projects. The goal of these projects is to restore the natural river environment that

existed prior to flooding. While creating a reservoir environment is as simple as

damming a river and allowing the valley to flood, the process of reverting the

environment to its original fluvial properties is much more complex. It is generally

believed that the shift back to free flowing river will ultimately bring positive

23

24 Austin, Elizabeth M. Environmental and Social Implications of Dam Removal. University of Massachussetts. 2009.

39

environmental changes by reversing the damages inflicted during the life of the

reservoir. These positive changes are:

Deregulation of river flow regimes Shift from lotic to lentic ecosystem Changes in water quality (e.g., dissolved oxygen, temperature) Release of accumulated sediment Restoration of connectivity for migratory organisms25

Without question, these changes will boost the ecological vitality and

re-stabilize the morphology of the river valley. Sadly, even with the best-executed

plans, the re-establishment of equilibrium takes many years. During this time, a

plethora of entirely new environmental threats crop up. Management of these

hazards stipulates the consideration of complex system interactions. Understanding

dam-removal ramifications also demands examination of the social and economic

dimensions, both positive and negative.

B) Restoration of Flow Regime

Dams fragment the fluvial regime of rivers by dividing the system and

obstructing the flow of sediment. Dams also contribute to unnaturally regulated

peak flows and low flows. If a river has a large range between peak and low flow

water levels, as most large rivers do, a wide riparian buffer habitat will develop

between the lotic and floodplain regions. River organisms living in these three zones

have evolved internal clocks that anticipate annual regularity in flooding events. By

reducing peak flows, dams will shrink the extent of these three ecosystems and

curtail biodiversity. Removing dams restores the regularity of peak/low flow events,

25 Higgs, Stephen. The Ecology of Dam Removal. American Rivers. 2002.

40

which gradually restores the stream, riparian, and floodplain habitats to their

natural width.

C) Effect of Sediment Release

Perhaps the most important question when assessing the outcome of

decommissioned dams is the fate of the sediment that has accumulated in the

reservoir. When the river is allowed to resume its natural course, the ratio of

sediment that will be eroded to the amount that will remain in place varies due to

differences in discharge, slope and sediment volume/type. Observations have

shown that in most cases, half of the sediment will be eroded and half will remain in

place. Once a dam is removed, a new channel will form behind the dam causing the

banks to slump into a wider-profile. This facilitates the mobilization of sediment,

especially during storm events. The sudden release of this sediment triggers a

spectrum of problems for downstream environments that are ill-adapted to high

turbidity. The stratigraphic distribution of grain sizes on the floor of a reservoir also

represents a deviation from what a river would normally encounter. The infusion of

sediment-laden water will significantly decrease sinuosity downstream, causing a

shift from braided to single thread geometry. This is owed to decreased channel

gradients and increased average depth/width.

Mobilization of finer sediment occurs almost immediately. This can

overwhelm vertebrate species such as fish that cannot respire efficiently with high

suspended solids in the water. This sediment will eventually deposit as islands,

point bars, or beach material within the channel, or otherwise in coastal

environments as delta detritus. When deposited in the channel, the fine particles

41

will overly existing coarse deposits, which serve as vital spawning environments for

salmon. Mobilization of coarser material from within the reservoir requires the

increased energy of flood events. Once freed, these sediments form bars or rapids

along the channel, which are beneficial to the reemergence of benthic habitats as

well as fish. Clearly, the cost vs. benefit considerations of altered morphology are

complex, thus the nature of the outcome is highly debated.

D) Water Quality and Toxic Compounds

The composition of reservoir sediment can be a big contributor to downstream

water quality. Reservoir water differs from river or natural lake water in:

Oxygen content

Temperature

Nutrient load

Acidity

Supersaturation of gases

Salinity

Industrial contaminants26

During the life of a dam, inflowing nutrients from agricultural and urban runoff, as

well as contaminants from factories will precipitate out of solution and incorporate

with the floor deposits. In this way, reservoirs serve a beneficial role as a cleanser

for river water. However, when the dam is removed, these pollutants remobilize and

enter the river system in alarming high concentrations. These contaminants include

heavy metals, herbicides, pesticides, and radionuclides.27 Moreover, the dissolved

oxygen content of reservoirs is lower than rivers or natural lakes. As mentioned 26 Graf, William F. Dam Removal: Science and Decision Making. The H. John Heinz III Center For Science, Economics and the Environment. Pennsylvania. 2002.

27

42

earlier, this can disturb nearly every trophic level of river food webs. Evaporation in

reservoirs results in high salt concentrations and pH alterations. Release of this

water is hazardous to aquatic species. Another issue involves the release of

supersaturated gases from hypolimnic waters. The immense deep-water pressure

forces gases into solution. These gases then come out of solution in the

bloodstreams of fish causing “the bends”. Once released, nutrient loaded water

(nitrogen, phosphorous, potassium) from agricultural runoff further disrupts

downstream ecology. This has been shown to reduce populations of

macroinvertebrates including caddisfly and mayfly.

E) Additional Concerns

So far we have analyzed only the downstream effects, but it is also important

to consider the fate of lotic ecosystems that resided in the reservoir prior to

removal. While the restoration of natural flow regimes is a blessing to migratory

fish, non-migratory resident species may not profit in similar ways. The sudden shift

from a lake environment to a free flowing river is unendurable to resident species

that have evolved to a still-water environment. In fact, many cases have documented

the complete disappearance of reservoir species within weeks of restoration. Steps

to protect resident species are rarely taken. During reservoir life, a complex

shoreline ecosystem develops with specialized benthic fauna and flora, as well as

shoreline vegetation. Draining the reservoir reduces these environments to barren

mudflats. Secondary succession of these flats is a long and arduous process. Many

restoration efforts involve seeding the area with native species, yet ecologic

maturation still progresses slowly.

43

In higher latitudes during winter months, dams have blocked the

downstream flow of ice. In these cases, dam removal has drastically altered ice flow

regimes. The result is an increase in ice jams, channel erosion, scouring, and the risk

for flooding. The Edwards Dam in Augusta, Maine experienced severe ice jams for

two consecutive winters following dam removal. Risk assessment for altered ice

flows is rarely taken into consideration prior to dam removal.28

E) Sediment Management Strategies

Allowing the river to erode the existing sediment is the conventional method

for river restoration; however, as previously mentioned this method entails severe

environmental consequences. Two other strategies have been employed to manage

the accumulated sediment: mechanical sediment removal and stabilization.

Mechanical Sediment Removal

This alternative involves removing all or a portion of accumulated sediment

and transporting it to a permanent disposal site. Methods include excavation,

mechanical dredging, and hydraulic dredging. This method is by far the most

intrusive and most costly, making it an unflavored alternative. This can, however,

completely eliminate the negative effects of downstream sediment transport. The

trade-off is that downstream transport of this sediment could potentially strengthen

downstream environment environments. As mentioned earlier, mobilization of

coarse grain deposits translates to improved benthic habitat downstream. For this

reason, many dredging operations extract only the finer grained sediment, while

28 Austin, Elizabeth M. Environmental and Social Implications of Dam Removal. University of Massachussetts. 2009.

44

leaving the coarser detritus for the river to erode. Drawbacks to this method

typically revolve around disposal issues. Locating a disposal site can be difficult. If

sediments are contaminated, the risk for groundwater contamination at the disposal

site is high. This can effect water quality for surrounding communities, which brings

ethics into the equation.

Stabilization

Under this alternative, the sediments would remain in place within the

former reservoir, but their incorporation into the river system will be blocked. If

sediments are known to be contaminated, this method is usually favored. This is

done by constructing a channel through or around the deposited sediment, which

will remain stabilized into the future. This method is more costly than simply

allowing the river to erode, but less costly that mechanical removal. A drawback is

that because the channel is stabilized, the natural topography will not be restored.29

XI. ADDITIONAL IMPACTS

A) Reservoir Induced Seismicity

A now well-documented phenomenon related to large reservoirs is the

localized increase in tectonic activity at dam sites. This occurrence was first

documented in 1932 at Algeria’s Quedd Fodda Dam.30 Today, incidences of

reservoir-induced seismicity have been observed in over 70 separate locations

worldwide. The precise mechanisms that cause RIS are still unclear, but it is obvious

that the immense volumes of water present in large reservoirs exert considerable

29 Randle, Timothy J; Greimann, Blair. Dam Decommissioning and Sediment Management. American Society of Civil Engineers. 2014.30 McCully, Patrick. Silenced Rivers: The Ecology and Politics of Large Dams. Zed Books, London, 1996.

45

force on the underlying bedrock. The underlying crust in any reservoir is traversed

by fissures and micro faults, the prevalence of which depends on the geology of the

bedrock. The extreme pressure of the overlying lake forces water into these cracks.

The water then acts as lubricant, allowing the sheer stress capacity to be exceeded.

31 This causes the faults to slip, releasing the stored energy as an earthquake.

While most reservoir-induced quakes are minor, some have been strong

enough to result in deaths. The strongest recorded RIS was at the Koyna Dam in

India in 1967. This earthquake, which measured 6.3 on the Richter scale, resulted

the death of 180 people. RIS activities are thought to be responsible for a number of

dam failures throughout history. Earthquakes can lower the height of the

impoundment relative to the water level, leading to overtopping and downstream

flooding. Seismic risk is rarely assessed prior to construction, despite our ability to

easily identify high-risk bedrock.

B) Vector-borne Diseases

Reservoir construction has also been correlated with increased prevalence of

vector-borne diseases in tropical regions. This is owed to the creation of shallow

stagnant water bodies following inundation. These environments are prime

breeding sites for disease vectors, particularly mosquitoes and snails that carry

malaria and dengue fever. The incidence of food-borne pathogens is also higher

following the construction of dams. The tightly confined environment within

reservoirs facilitates the transmission of diseases between organisms. During

drawback events, diseases are transferred into the downstream system where a

31 Gupta, H.K. Reservoir Induced Earthquakes. Cochin University of Science and Technology, Indiana, 1976.

46

new population of hosts awaits. Dam construction along the Mekong River led to a

scourge of food-borne intestinal trematodes among local populations who regularly

ate raw fish from the area.32 Such instances have called for strict water management

protocols aimed at reducing the prevalence of vector-borne diseases.

Implementation of vector control in the Tennessee River Valley reservoirs was

largely successful.33

C) Greenhouse Gases

One of the biggest concerns involving the environmental impact of reservoirs

is the potential for an acceleration of climate change. All bodies of water naturally

emit greenhouse gases (carbon dioxide, methane, nitrous oxide). These emissions,

however, are typically in low concentrations and can be balanced by biogeochemical

cycling. When reservoirs are flooded, the newly submerged organic biomass dies,

triggering a spike in decomposition. As decomposers breakdown the organic

material, the stored carbon is converted to carbon dioxide and methane.34 These

gases then form bubbles at the floor of the reservoir, which eventually diffuse to the

surface where they are released into the atmosphere. Another way reservoirs

contribute to greenhouse emissions is through turbine diffusion. Hydraulic action at

turbines creates a localized change in water pressure and temperature, reducing the

solubility of gaseous compounds. This causes an immediate transference of

greenhouse gases from water to atmosphere.

32 Ziegler, Alan D, et al. Dams and Disease Triggers on the Lower Mekong River. National Institute of Parasitic Disease, China, 2013. Keiser, Jennifer, et al. The Effect of Irrigation and Large Dams on the Burden of Malaria on Global and Regional Scale. American Journal of Tropical Medicine and Hygiene, 2005.33 Cleare, Emily. An Introduction to Greenhouse Emissions from Hydroelectric Reservoirs: The What, the Where and the How.34

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The degree of greenhouse gas contribution is dependent on the location and

maturity of the dam. Dams in tropical areas will release extremely high GHGs due to

the high productivity of biological organisms. Of course, this is also a contributor to

eutrophication and oxygen depletion. Also, the rate of GHG being released will

gradually slow down as the dam ages, and the organic material is depleted.

Managing this issue calls for assessment of the GHG emission potential of dam sites

prior to construction. Additionally, the removal of vegetation prior to flooding

should become routine protocol.

XII. CONCLUSIONS AND RECOMMENDATIONS

The environmental impacts of man-made reservoirs are far too complex and

numerous to be accurately compressed into a single study. By it very nature, the

study of environmental science is subject to innumerable confounding variables; a

system of systems, so to speak. But even with our inherently limited understanding,

is it obvious that our conventional reservoir management practices are harming the

environment in ways it cannot withstand. These detriments can be summarized as

follows:

Alteration of flow regimes and normalization of vital flood events

Disruption of natural sediment transport

Changes to downstream/coastal morphology from altered sediment loads

Changes to downstream temperature and chemistry

Reduced soil fertility

Estuarine saltwater intrusion

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Decreased water quality from trapping and concentration of nutrients and

contaminants

Disruption of migratory fish routes

Eutrophication

Loss of biodiversity from unnatural ecologic conditions

Loss of habitat from inundation

Release of greenhouse gases

Growth stimulation of vector borne disease

Tectonic disturbances

In order to bring about positive long-term environmental change, while

continuing to rely on dams, it is imperative for management strategies to prioritize

the mitigation of environmental impacts. An effective reservoir management plan

will involve intervention in all three stages of dam life: construction, maintenance,

and removal. Before dam construction is even approved to launch, the potential

risks associated with specific locations must be thoroughly evaluated. A

standardized pre-construction protocol should mandate the following:

Assessment of geologic integrity

Chemical analysis of soil and vegetation for toxic compounds

Consideration of climate (flood regimes, ice flows)

Measuring inflowing sediment yield

Stripping of vegetation

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Transportation of resident fauna to alternate location

Identification of migratory fish routes, and appropriate construction of fish

ladders.

Effective environmental stability and sediment reduction strategies rely not only

on proper planning, but also on dedicated follow through. For best results, the

following up-keep procedures would need to be preserved for the entire life of the

reservoir:

Monitoring temperature, chemistry, and nutrient content of released water

to promote ecological suitability.

Regulating sediment yield in reservoirs through upstream soil conservation

efforts, sediment interception systems, sediment routing systems, and

sluicing to restore fluvial regularity and eliminate sedimentation.

Monitoring flushing events and downstream turbidity

Mitigating invasive species prevalence

Modifying drawback regimes to reflect the natural downstream flood cycles.

Monitoring contaminant influx and eutrophication

Implementing turbidity current prevention techniques

Monitoring ground water contamination; conjoining subsurface and surface

reservoirs

Employing vector disease intervention

Scientists are continuing to explore the environmental consequences of artificial

reservoirs. As more knowledge is assembled on this and other environmental issues,

awareness will circulate into the public spotlight, and eventually into high school

50

curriculums. As environmental education increases, and global value systems

continue valuing humanitarianism and environmental stewardship, awareness over

this issue will intensify. Already, numerous anti-reservoir activist groups have

cropped up from a seedbed of shared discontent. In August of this year, violence

broke out as the French activist group known as the “Zadistes” clashed with local

authorities over the construction of the Sivens Dam in France. During the mayhem, a

twenty-one year old protestor was killed by a riot control grenade.35 While such

extreme demonstrations are unnecessary, and arguably disparaging the overall

cause, the event still illustrates the imminence of future friction between the dam

industries and a restless youth.

With or without environmental activism, our reliance on dams for the continued

growth of society will likely prevent us from effecting a large-scale transition to

more sustainable alternatives in the near future. Our society, economy, and

infrastructure are simply too vulnerable to abandon these generators of energy and

reliable water. For meaningful big-picture reductions in global reservoir

dependence, our current mechanisms for irrigation supply, urban/industrial water

supply, and energy production must be reconfigured. In the mean time, routine

implementation of these and other management protocols will significantly reduce

the environmental harm associated with prolonged dam use. Because these changes

will demand larger monetary investments from dam proprietors, there will be

considerable resistance throughout the corporate sphere. For this reason, policy

35 Baume, Maia de la. In France, Dam is the Catalyst for a Flood of Young People’s Anger. The New York

Times, 2014.

51

change that mandates adherence must be enacted to strictly enforce responsible

practices.

XI. COMPLETE WORKS CITED

1. Baxter, R.M. Environmental Effects of Dams and Impoundments. Palo Alto, CA,

Annual Reviews, 1977.

2. The US Department of Agriculture. Stream Dynamics. Washington D.C., 2001.

3. http://www.internationalrivers.org/resources/wrong-climate-for-big-dams-2629

4. Istvanoviks, Vera. The Role of Biota in Shaping the Phosphorous Cycle in Lakes.

Water Resources Management Team of the Hungarian Society of Sciences, 2008.

5. Wrong Climate For Big Dams. International Rivers, California, 2011.

6. Manatunge, J. "Oceans and Aquatic Ecosystems.” Environmental and Social Impacts

of Reservoirs: Issues and Mitigation 

7. Toniolo, Horacio, and Gary Parker. 1d Numerical Modeling of Reservoir Sedientation.

IAHR Symposium on River, Coastal and Estuarine Morphodynamics. 2003

8. Dittrich, Andreas. River Flow 2010: Proceedings of the International Conference on

Fluvial Hydraulics. Braunschweig, Germany, 2010.

9. Kinoshita, Atsuhiko; Mizuyama, Takahisa; Fujita, Masaharu, and Sawada, Toyaki. The

Impact on Fish of Sediment Flushing From a Sabo Dam. International Congress.

Matsumoto Japan. 2002.

10. Giuseppe, Crosa; Castelli, Elena; Gentili, Gaetaon; Espa, Paolo. Effects of Suspended

Sediments From Reservoir Flushing on Fish and Macroinvertebrates in an Alpine

Stream. Birkhauser Verlag. Switzerland. 2009

11. Cleare, Emily. An Introduction to Greenhouse Emissions from Hydroelectric

Reservoirs: The What, the Where and the How.

12. Austin, Elizabeth M. Environmental and Social Implications of Dam Removal.

University of Massachussetts. 2009.

13. Higgs, Stephen. The Ecology of Dam Removal. American Rivers. 2002.

14. Graf, William F. Dam Removal: Science and Decision Making. The H. John Heinz III

Center For Science, Economics and the Environment. Pennsylvania. 2002.

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15. Randle, Timothy J; Greimann, Blair. Dam Decommissioning and Sediment

Management. American Society of Civil Engineers. 2014.

16. McCully, Patrick. Silenced Rivers: The Ecology and Politics of Large Dams. Zed

Books, London, 1996.

17. Gupta, H.K. Reservoir Induced Earthquakes. Cochin University of Science and

Technology, Indiana, 1976.

18. Ziegler, Alan D. Dams and Disease Triggers on the Lower Mekong River. National

Institute of Parasitic Disease, China, 2013.

19. Keiser, Jennifer, et al. The Effect of Irrigation and Large Dams on the Burden of

Malaria on Global and Regional Scale. American Journal of Tropical Medicine

and Hygiene, 2005.

20. Richter, Brian D; Thomas, Gregory A. Restoring Environmental Flows by Modifying

Dam Operations. The Nature Conservancy, 2007.

21. Degoutte, Gerard. Small Dams, Guidelines for Design, Construction and Monitoring.

Cemegref Editions and Engref (France), with French Committee on Large Dams.

2002.

22. Oehy, Ch D., and A. J. Schleiss. Hydraulics of Dams and River Structures. Taylor

and Francis Group. 2014

23. http://scholar.lib.vt.edu/theses/available/etd-01102003-162857/unrestricted/

%2807%29Lit_Rev_1.pdf

24. Gerritsen, Alida; Young, Benjamin. The Effects of Dams on Migratory Fish: A North

Country Case Study. St. Lawrence University, New York, 2008.

25. Butz, Stephen. Energy and Agriculture: Science, Environment, and Solutions.

Cengage Learning, 2014.

26. Cooper, C M. Effects of Abnormal Thermal Stratification on a Reservoir Benthic

Macroinvertebrate Community. The University of Notre Dame, 1980.

27. Baume, Maia de la. In France, Dam is the Catalyst for a Flood of Young People’s

Anger. The New York Times, 2014.

28. Osmond, D. L, et al. Watersheds. North Carolina State University, NC, 1976.

FIGURES

1. [http://www.sonicyouth.com/gossip/showthread.php?t=705]

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2. [http://web.gccaz.edu/~lnewman/gph111/topic_units/fluvial/fluvial2.html]

3. [http://blackpoolsixthasgeography.pbworks.com/w/page/22973907/Storm%20Hydrographs]

4. [http://www.bio.utexas.edu/faculty/sjasper/Bio213/aquahab.html]

5. [http://en.wikipedia.org/wiki/Dimictic_lake]

6. [http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/chemical_resources/

fertilisers_cropsrev3.shtml]

7. [http://www.usgs.gov/newsroom/article.asp?ID=3777#.VJH6FmTF8mU]

8. [Schwartz, Maurice. Encyclopedia of Coastal Science. Hutchinson Ross, 2006.]

9. [http://commons.wikimedia.org/wiki/File:John_Day_Dam_fish_ladder.jpg]

10. [http://jsedres.sepmonline.org/content/72/3/353.figures-only]

54