Running water: Rivers and Streams

The water budget and groundwater

The hydrologic cycle

Classification of Rivers

Braided Rivers

Meandering Rivers

The Niagara River

The 1997 Manitoba Flood

Weathering, transport and deposition

The Water Budget and Groundwater

Total Water on Earth 1,360,000,000 km3

Oceans and Seas 1,331,746,800 km3 (97.9%)

Glaciers and Ice Sheets 24,000,000 km3 (1.8%)

Groundwater 4,000,000 km3 (0.3%)

Lakes and Reservoirs 155,000 km3

Soil Moisture 83,000 km3

Vapor in the atmosphere 14,000 km3

Rivers 1,200 km3

Groundwater resides within the Earth.

The only water source for many areas (e.g., Walkerton)

Water filling void spaces in rocks and sediment.

Water table: the surface below which groundwater fills void spaces.

Zone of aeration: above the water table; voids filled with air.

Zone of saturation: below the water table; voids filled with water.

Effluent Rivers: water table rises to river bed (groundwater adds to the river)

Influent Rivers: water table is below the river bed (river adds to groundwater).

Effluent Rivers: water table rises to river bed (groundwater adds to the river)

The Hydrologic Cycle

From Lutgens and Tarbuck, figure 6-9

The constant exchange of water between all of the water reservoirs is termed the Hydrologic cycle.

The cycle is balanced over time.

The exchange between land and oceans is largely via rivers:

Of the 380,000 km3 of water in the cycle:

Land Oceans

 Precipitated: 25.3% 74.7%

Evaporated: 15.8% 84.2%

Net: +9.5% -9.5%

0.095 x 380,000 km3 = 36,100 km3 of excess water to the land.

99.7% of excess water to the land is returned to the oceans by rivers.

Total discharge into the oceans by rivers is 36,000 km3

Rivers deliver water and vast amounts of clastic sediment and material in solution to the world’s oceans.

Rivers

The ten largest rivers on Earth deliver 36% of all water that flows into the oceans.

The Amazon River delivers 15% of the world total.

6,300 km3/year

or

200,000 m3/second

A river’s discharge is the volume of water moving through a river over a given period of time.

A river’s sediment discharge is the volume of sediment moving with the water of a river over a given period of time.

River discharge depends on:

Climate

Relief

Geology

Drainage basin area

Orinoco 30% of Mississippi basin area; 190% of water discharge.

Tropical versus mid-latitude setting;more rainfall.

Ganges-Brahmaputra: 24% of Amazon basin area;

15% of water discharge;

185% of sediment discharge.

The Ganges-Brahmaputra drains the Himalayan Mountains (high relief, high rates of erosion)

20 billion tonnes of the products of weathering are carried annually by rivers to the oceans:

Approx. 16 billion tonnes of clastic sediment

Approx. 4 billion tonnes of dissolved material

Clastic sediment is transported in rivers as:

Bed Load: large particles that move in contact with the bed.

Suspension Load: fine sand, silt and clay that “floats” along with the water.

Over 90% of the total clastic sediment discharge into the oceans is as suspended load.

20 billionTonnes/yr = 8 km3/yr

= 800 km3/100yrs

= 8,000 km3/1000yrs

= 8,000,000 km3/1,000,000yrs

etc., etc., etc.

So why aren’t the continents flattened out by now?

Plate Tectonics: volcanism, igneous intrusion, thrusting and folding all build the continents.

Flow in Rivers

Discharge varies seasonally, daily and hourly.

Some rivers always have discharge.

Some rivers are dry over much of the year (ephemeral rivers).

Arroyos are steep-sided valleys produced by some ephemeral stream.

Hydrograph: a graph showing the variation in a river’s discharge with time.

Annual Hydrograph is a hydrograph showing variation in dischage over the course of a year.

Winter: accumulation of snow/ice cause lowest discharge).

Spring melt: maximum annual discharge.

Summer/Fall: rainstorms cause short duration discharge peaks.

Single storm hydrographs show the discharge of a river for a single rainfall event.

In a natural setting rainfall precedes the peak river discharge by Lag Time:

time required for rain to soak into the ground until it is saturated and then flows over the land surface into rivers.

Lag time can be hours to days, depending on area, relief and the nature of the drainage basin.

A natural surface behaves like a sponge, soaking rainwater into the ground; removing it from the surface.

Urbanization reduces lag time and increase the peak discharge.

Cities are vast, impermeable surfaces (water doesn’t soak into the ground) with human facilities to rapidly direct runoff to rivers (streets, storm drains, storm sewers).

Why does urbanization increase the risk of flooding?

Paving streets and parking lots, storm drains.

Greater runoff = higher storm peak.

Urban flooding

Flood Frequency Curves

Used to determine the risk of flooding by a given river.

Annual maximum discharge versus time between each occurrence of that discharge for a given river.

Data plot as a straight line which defines the Flood Frequency Curve.

e.g. 20,000 ft3/s every 1.2 years

50,000 ft3/s every 5 years

100 year flood:discharge with a recurrence rate of 100 years.

Curve shows the the probability of a given discharge occurring in a particular year.

20,000 ft3/s, 80% chance in a year

8 of 10 years 20,000 ft3/s will be exceeded.

100,000 ft3/s, 1% chance in a given year.

This is the 100 year flood for the Skykomish River.

Classification of Rivers

Braided Rivers

Multiple, interconnecting channels.

Relatively steep slopes.

Coarse sediment in transport, mostly bedload.

Discharge highly variable over time (not uniform)

Meandering Rivers

Single, sinusoidal (meandering) channel.

Relatively gentle slopes.

Sediment is fine-grained, mostly by suspension load.

Discharge is more uniform with time.

Braided Rivers

Channel system occupies the entire floodplain.

During high discharge the entire floodplain is covered forming a single channel.

Longitudinal bars migrate downstream during floods.

http://www.usra.edu/esse/ford/ESS205/fluvial/Rakaia1.jpg

Brahmaputra River, India

Gravel longitudinal bars on the Athabaska River

Classification of Rivers

Braided Rivers andMeandering Rivers

http://www.usra.edu/esse/ford/ESS205/fluvial/Rakaia1.jpg

During floods, longitudinal bars migrate down stream.

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

Migration of a longitudinal bar.

Longitudinal bar

The deposits of ancient braided rivers are characterized by horizontal beds of gravel with cross-strata dipping in the direction of longitudinal bar migration.

Meandering Rivers

The channel occupies a small part of the floodplain.

Morphological features include:

Levees

Point bars

Chute channels

Oxbow lakes

Meander scars

Straight channels evolve into meandering channels.

Thalweg:deepest part of a channel

Thalweg defines a line that meanders through a straight channel.

Erosion where the thalweg is close and deposition where the thalweg is farthest from channel margin.

As the thalweg alternates from side to side along the channel the sites of erosion and deposition also alternate along the channel.

o gDS

The boundary shear stress (o) at the floor of the channel determines whether or not erosion will take place.

Where is the density of water;g is the acceleration due to gravity;D is the water depth;S is the slope of the channel.

Where the water is deepest (i.e. along the thalweg) o is greatest and erosion occurs.

Over time the channel migrates laterally into larger amplitude, sinusoidal form.

http://www.wwnorton.com/earth/egeo/animations/ch14.htm

Click here for a flash animation of a meandering channel.

Channel terminology

Thalweg

Thalweg: the line defining the deepest part of the channel.

Riffle: regions of shallow, fast flowing water between meander bends.

Riffle

Pointbar

Point bar: the inside of a meander bend.

Pool: regions of deeper, slower flowing water at meander bends.Pool

Cut bank: the outside of a meander bend.

Cut bank

Pools

Cut banks

Riffles

Pointbars

During low flow, the river occupies only the main channel.

Time 2

During a flood, water levels rise and pass across the point bar via the chute channel.

The extensive floodplain of a meandering river is almost as characteristic as the sinusoidal channel form. Time 1

Chute Channels, the river is flooded to its banks.

Time 3Overflowing water runs down the slopes, away from the channel and covers the floodplain.

Organic-rich sediment yields excellent farmland.

Many large cities at risk of flood (e.g., New Orleans, Winnipeg).

Many cities are on the floodplains of rivers (rivers were very important for transportation).

Levees: linear mounds of sediment that parallel the meandering channel.

Normal flow: water and sediment are restricted to the channel.

Floods: water and sediment spread outward from the channel.

The deposits form the levees immediately adjacent to the channel.

Repeated floods build the levees and build the channel vertically.

The channel becomes perched above the floodplain.

Steep slope to the floodplain encourages erosion when the levees are breached.

Assinaboine River is 8 metres above the surrounding land.

Avulsion: rapid relocation of a meandering river to a new site on the floodplain.

Time 2. With flooding rapid flow down the slopes of a levee may erode a channel that leads out to the floodplain.

Water from the main river floods the side of the floodplain adjacent to the new channel.

Time 1. Normal flow conditions.

Time 3. A new channel is cut, parallel to the original channel.

Over time, the new channel takes more flow from the river while the old channel is abandoned.

Time 4. The new channel evolves into a meandering form.

Meandering channels migrate laterally due to the distribution of erosion and deposition.

Erosion on the outside of a meander bend.

Deposition on the inside of a meander bend.

Point bar: depositional body on the inside of a meander bend.

Flow strength diminishes along the point bar so that sediment is deposited.

The point bar “accretes” in the direction that the channel is migrating by erosion of the cutbank.

Scroll bars: the remnants of thetops of ancient point bars.

In ancient meandering river deposits point bars appear as long, low angle dipping beds of sandstone.

Such deposits are diagnostic of meandering river environments.

From Johnson and Graham (2004); Journal of Sedimentary Research, v. 74, p. 770-785.

Oxbow lakes form when two cutbanks approach each other.

Ongoing erosion of the cutbanks bring them progressively closer together.

If the two cutbanks intersect the channel is breached.

Flow in the main channel bypasses the meander and it’s entrance and exit become plugged with sediment.

The meander becomes isolated from the main channel and forms an oxbow lake.

Once isolated the abandoned lake diminishes in size, receiving water only during flooding of the floodplain.

Over time the lake fills with sediment and becomes vegetated to form a swamp.

Once completely filled a meander scar remains.

http://www.mhhe.com/earthsci/geology/mcconnell/streams/channel.htm

Oxbow Lake Formation

Niagara Falls was so well known among native North Americans that Jacques Cartier was told about it 80 years before the first Europeans visited the site.

The Iroquois name “Onguiaahra”, meant "the Strait".

The History of Niagara Falls

The current “natural” discharge of the river averages 5,760 m3/s (almost 500 million cubic metres per day).

(less than 1.5 million toilets)

Winds blowing east across Lake Erie have increased discharge to 9,760 m3/s.

Ice dams at the head of the river have reduced discharge to zero.

Today the discharge over the falls is controlled so that some of the flow is re-routed through turbines to produce electricity

50% of the discharge of the river is routed through turbines during summer daytime hours

75% of the discharge is routed around the falls during summer evenings and over the winter months (only 25% passes over the falls).

US Hydro capacity: 2,575,000 KW

Total capacity: 4,455,000 KW or 4,450 MW

Canadian capacity: 1,880,000 KW

Ontario’s peak consumption: 22,000 MW

Total US and Canadian hydro production at Niagara Falls is 20% of Ontario’s peak consumption

Niagara Falls exists because the Niagara River passes over the Niagara Escarpment.

About 12,500 years ago the Falls was located where the River intersects the Escarpment.

Through the erosive action of the water flowing over the Escarpment the location of the falls receded southward.

The position of the falls has receded to its present location by steady erosion that also formed the Niagara Gorge.

Recession of the falls took place at an average rate of 1.5 m/yr.

The current rate is 0.1 m/yr due to reduce flow over the falls and other measures to stabilize its position.

Prior to the last glaciation of the area the Niagara River flowed further west of its current position, passing through the escarpment at St. Davids.

The falls was a few kilometres downstream of its current position.

The old gorge passed out of the modern gorge at the Whirlpool.

The glaciers receded from the area 12,500 years ago.

Sediment from the glacier plugged the old gorge and the Niagara River followed its present course.

The waters fell over the Escarpment above Lewiston.

Initially discharge through the river was very large due to the glacial meltwater.

Recession rates of the falls were relatively rapid due to the large volumes of water flowing off the retreating glaciers.

By 10,500 years ago the falls had receded 2.5 km southward from the Escarpment.

Recession slowed markedly as discharge through the river was rapidly diminished.

As the glaciers retreated north of Lake Huron they left the crust “isostatically depressed”.

The weight of 2 km of ice had pushed the crust downward so that the land surface was lower than today.

Waters from Lakes Superior, Michigan and Huron flowed through the depressed region known as the Nipisssing Spillway.

The Niagara River only received discharge from Lake Erie.

Over the next 5,000 years the falls receded only 1.5 km southward

Over the next 5,000 years the falls receded only 1.5 km southward

… to a position just northeast of the Whirlpool

Over this 5,000 years the land surface to the north slowly rebounded and eventually shut off the Nipissing Spillway.

Discharge returned to the Niagara River and recession rates rose to levels that are comparable to today.

Within approximately 1,000 years the Falls reached the Whirlpool.

Within approximately 1,000 years the Falls reached the Whirlpool.

Just south of the Whirlpool the pre-glacial gorge remained plugged with sediments deposited by the glaciers.

Within a few days to a couple of weeks the river scoured through the soft sediment until it reached the location of the pre-glacial falls.

Subsequent recession of the Falls through the resistant bedrock was much slower and continued to today.

Eventually the falls will recede back to Lake Erie.

The lake may drain rapidly and eventually will only contain an extension of the St. Claire River.

Under natural conditions this could take place within 20,000 years.

The Manitoba Flood of 1997

Spring of 1997 saw unusually rapid melt of a thick snowpack over much of the Red River Basin.

Floodwaters rose sharply through April, peaking by mid-May.

Discharge at Fargo, N.D. were over 30 times normal flow for the river.

The Flood Recurrence Curve for Fargo indicated that this was the 200 year flood.

At Fargo the water height was almost 25 feet above low flow.

At Winnipeg the discharge was 138,000 cubic feet per second.

South of Winnipeg the flood waters spread out over the Red River Floodplain, forming a shallow lake up to 40 km across.

South of Winnipeg the town of Morris, Manitoba, remained relatively dry due to the construction of dykes.

Elsewhere single homes tried to keep the water out with sandbag dykes around their homes.

Dykes around the southern perimeter inhibited the floodwaters south of Winnipeg but the river flows through the city.

The diversion lowered the level of floodwaters by 15 feet in Winnipeg.

The city was saved by the 47 km long Winnipeg Floodway which redirected 59,000 cfs around the city.