RIVER PROCESSESIntroduction to Watershed ScienceIntroduction to Watershed Science

Merritt CollegeMerritt College

Marc Epstein, InstructorMarc Epstein, Instructor

California River SystemCalifornia River System

Major Major California California

RiversRivers and and

TopographyTopography

Drainage Basin DefinedDrainage Basin Defined

Every stream has a drainage basin.Every stream has a drainage basin. A drainage basin is a spatial geomorphic A drainage basin is a spatial geomorphic

unit occupied by a river system. unit occupied by a river system. It is defined by ridges that form drainage It is defined by ridges that form drainage

divides; that is, the ridges are the dividing divides; that is, the ridges are the dividing lines that control into which basin lines that control into which basin precipitation drain.precipitation drain.

Drainage divides define a watershed. Drainage divides define a watershed.

California Drainage BasinsCalifornia Drainage Basins

Rivers and the Hydrologic Cycle

Most rivers and streams begin as a result Most rivers and streams begin as a result of precipitationof precipitation

Precipitation will infiltrate into the ground Precipitation will infiltrate into the ground water or flow on the surfacewater or flow on the surface

When sufficient water collects and flows in When sufficient water collects and flows in a uniform pattern, a stream is createda uniform pattern, a stream is created

Most of California rivers begin from the Most of California rivers begin from the melting of the snow packmelting of the snow pack

Stream FlowStream Flow

Stream Flow is potential energy converted Stream Flow is potential energy converted to kinetic energy by gravity.to kinetic energy by gravity.

Measure of Stream FlowMeasure of Stream Flow

Q Q == w w xx d d xx v v Q = DischargeQ = Discharge ww = channel width = channel width dd = channel depth = channel depth vv = stream velocity = stream velocity Expressed as cubic meters or cubic feet Expressed as cubic meters or cubic feet

per secondper second

Calculating DischargeCalculating Discharge

Compare the discharge of a stream whose Compare the discharge of a stream whose measurements at a gauging station are:measurements at a gauging station are:

Width = 65 feetWidth = 65 feet

Average depth = 3 feetAverage depth = 3 feet

Velocity = 4.2 feet/secondVelocity = 4.2 feet/second

If the stream is the previous question If the stream is the previous question passed through a more narrow channel of passed through a more narrow channel of only 40 foot width, and we assume the only 40 foot width, and we assume the same depth of 3 feet and the discharge same depth of 3 feet and the discharge remains constant, what will be the remains constant, what will be the increased velocity?increased velocity?

If the stream in the preceding question If the stream in the preceding question achieves a deeper 4.5 foot average depth achieves a deeper 4.5 foot average depth at the site of the 40 foot width what will the at the site of the 40 foot width what will the new velocity be?new velocity be?

SuspensionSuspension

Streams transport materialStreams transport material Carrying capacity increases with velocityCarrying capacity increases with velocity Dissolved Load – Chemical solutionsDissolved Load – Chemical solutions Suspended Load – Fine grained clastic Suspended Load – Fine grained clastic

particlesparticles Bed Load – Coarser materials dragged Bed Load – Coarser materials dragged

along the bed of the streamalong the bed of the stream

Velocity vs. Particle SizeVelocity vs. Particle Size

Drainage GeometryDrainage Geometry

CharacteristicsCharacteristics

Dendritic most common – From Greek Dendritic most common – From Greek word dendros meaning tree.word dendros meaning tree.

Small streams flow into larger ones.Small streams flow into larger ones. Local geology will determine the geometry.Local geology will determine the geometry.

Stream EvolutionStream Evolution

Streams and rivers have an evolutionary Streams and rivers have an evolutionary pattern from initial formation to discharge pattern from initial formation to discharge into the ocean.into the ocean.

Stream Stream AgeAge

Initial Stream DevelopmentInitial Stream Development

Youthful StreamYouthful Stream

Characteristics of Youthful Characteristics of Youthful StreamStream

Steep gradientSteep gradient Creates V shaped gorge or ravineCreates V shaped gorge or ravine

Mature StreamMature Stream

Characteristics of Mature Characteristics of Mature StreamsStreams

Reduced gradientReduced gradient Sediment load = Load carrying capacitySediment load = Load carrying capacity

Old Age StreamOld Age Stream

Characteristics of Old Age Characteristics of Old Age StreamStream

Little gradientLittle gradient Evidence of shifts in the river course is Evidence of shifts in the river course is

evident in the vicinityevident in the vicinity

Stream ErosionStream Erosion

The erosional work of a stream carves and The erosional work of a stream carves and shapes the landscape through which it shapes the landscape through which it flows.flows.

Types of ErosionTypes of Erosion

Hydraulic Action – Is the work of Hydraulic Action – Is the work of turbulence in the water running. Water turbulence in the water running. Water running causes hydraulic squeezing and running causes hydraulic squeezing and release of action to loosen and lift rocks.release of action to loosen and lift rocks.

Abrasion – Is caused when debris move Abrasion – Is caused when debris move along, mechanically loosens and lifts rocks along, mechanically loosens and lifts rocks through grinding and carving of the through grinding and carving of the streambed.streambed.

DepositionDeposition

Whenever a streams velocity decreases, Whenever a streams velocity decreases, its ability to carry material is reduced. its ability to carry material is reduced. Consequently, some of the suspended Consequently, some of the suspended particles begin to settle out.particles begin to settle out.

Meandering StreamMeandering Stream

Where slopes are gradual, stream channels Where slopes are gradual, stream channels assume a snakelike form weaving across the assume a snakelike form weaving across the landscape. landscape.

The outer portion of each meandering curve is The outer portion of each meandering curve is subject to greater velocity and erosive action subject to greater velocity and erosive action and can be the site of steep bank called a and can be the site of steep bank called a cut cut bankbank..

The inner portion of a meander is subject to less The inner portion of a meander is subject to less velocity and deposition forming a deposit called velocity and deposition forming a deposit called a a point barpoint bar..

Meander Curve FormationMeander Curve Formation

FloodplainsFloodplains

Floodplains – The flat low lying area along Floodplains – The flat low lying area along a stream that is subjected to recurrent a stream that is subjected to recurrent flooding. It is formed when the river flooding. It is formed when the river overflows its channel during times of high overflows its channel during times of high flow. When floods occur, the floodplain is flow. When floods occur, the floodplain is inundated. When the water recedes, inundated. When the water recedes, deposits generally mask the underlying deposits generally mask the underlying rock. rock.

Floodplain StructureFloodplain Structure

Floodplain TermsFloodplain Terms

Floodplain Terms DefinedFloodplain Terms Defined Natural Levee – On either bank of most streams, Natural Levee – On either bank of most streams,

develop as a by-product of flooding. When flood develop as a by-product of flooding. When flood water arrive, the river overflows it bank, loses water arrive, the river overflows it bank, loses velocity and spreads out, and drops a portion of its velocity and spreads out, and drops a portion of its sediment load.sediment load.

Stream Terraces – Formation is a result of a stream Stream Terraces – Formation is a result of a stream cutting its floodplain to a lower elevation base level.cutting its floodplain to a lower elevation base level.

River Delta – The mouth of a river is where it River Delta – The mouth of a river is where it reaches its base level. The river’s forward velocity reaches its base level. The river’s forward velocity rapidly decelerates as it enters a larger body of rapidly decelerates as it enters a larger body of water, resulting in deposition. water, resulting in deposition.

Examples of River DeltaExamples of River Delta

Sacramento Sacramento River River

FloodplainFloodplain

Floods and River ManagementFloods and River Management

Floods and floodplains are rated Floods and floodplains are rated statistically for the expected intervals statistically for the expected intervals between floods. A ten year flood is between floods. A ten year flood is expected to occur every 10 years, it has a expected to occur every 10 years, it has a 10% probability of occurring in any one 10% probability of occurring in any one year. A 50 or 100 year flood is of greater year. A 50 or 100 year flood is of greater catastrophic consequence, but it is less catastrophic consequence, but it is less likely to occur in any given year. These likely to occur in any given year. These events will occur randomly over time.events will occur randomly over time.

Calculating ProbabilitiesCalculating Probabilities

Floodplain and Flood MapsFloodplain and Flood Maps

Human Impacts on Stream Human Impacts on Stream ProcessesProcesses

Flood ControlFlood Control UrbanizationUrbanization

Flood ControlFlood Control Channelization – Process to straighten Channelization – Process to straighten

and deepen streams.and deepen streams. Reduce upstream floodingReduce upstream flooding Removes bank vegetationRemoves bank vegetation Increases stream velocityIncreases stream velocity Increases stream flowIncreases stream flow Promotes upstream erosionPromotes upstream erosion Increases downstream floodingIncreases downstream flooding Increases downstream depostionIncreases downstream depostion Reduces habitat for aquatic wildlifeReduces habitat for aquatic wildlife

Flood ControlFlood Control

Building Levees or FloodwallsBuilding Levees or Floodwalls Contain and accelerate stream flowContain and accelerate stream flow Increases water capacity to do damage Increases water capacity to do damage

downstreamdownstream Does not protect against unusually high Does not protect against unusually high

and power floodwatersand power floodwaters

Natural Flood ControlNatural Flood Control

Preserve existing wetlandsPreserve existing wetlands Restore degraded wetlandsRestore degraded wetlands

UrbanizationUrbanization

Covers land with buildings, asphalt, and Covers land with buildings, asphalt, and concrete.concrete.

Greatly increases land area which is Greatly increases land area which is impervious to water.impervious to water.

Increases urban runoff during precipitation Increases urban runoff during precipitation events.events.

Decreases groundwater recharge to local Decreases groundwater recharge to local streams. streams.

Urbanization MitigationUrbanization Mitigation

Create green islands.Create green islands. Storm drain retention basins.Storm drain retention basins.

Climatic Change in CaliforniaClimatic Change in California

Models indicate no change in precipitation amounts

Precipitation to be more rain and less snow

Reduced snow pack storage Increased frequency of flood events Less water available during the dry season