Chapter 16 – Running Water & Rivers

Distribution of earths water - Ocean

96.5%

Distribution of earths water - Freshwater

2.5%

Distribution of earths water - Saline lake & groundwater

1%

Basic processes (Hydrologic Cycle)

• Evaporation

• Infiltration 

• Runoff

• Transpiration 

Evaporation

Liquid water → atmosphere

Infiltration

Water soaks into ground

Runofff

Rainfall > absorption

Transpiration

Plants absorb & release water

Evapotranspiration

Process by which water is transferred from the land to the atmosphere by evaporation from the soil & other sources, & by transpiration from plants 

Greenland & Antarctic Ice Sheets

Meelting rapidly

Greenland melting

4x faster than in 2003

Ice loss

Contributes to sea level rise

Drainage Basin (Watershed)

• Area where all runoff flows into one stream system 

• Bounded by drainage divides 

Drainage Basin (Watershed) Example

Mississippi River Dranage Basin

Drainage patterns depend on

• Rock type

• Orientation of joints & faults 

• Topography 

Zones of River Production

• Sediment production (erosion)

• Sediment transportation

• Sediment deposition

Zones of River Production - Sediment Production (Erosion) 

• Headwaters

• Brocken, bedrock, bank erosion, channel scouring 

Zones of River Production - Sediment Transportation 

• Trunk strains 

• Sediment eroded = sediment deposited 

Zones of River Production - Sediment Deposition 

• Near mouth (large body of water)

• Energy drops → sediment dumped

Velocity slowest

along boundaries

Velocity highest

in center of straight channel

Factors that affect flow velocity

Channel width, depth, roughness 

Roughness

boulders, debris, irregular shape

Max flow

When stream is bankful

Bankful

Steam is filled right up to the top of its channel (not overflowing yet) 

• Max depth, least friction 

Longitudinal Changes 

Stream flow & shape change as land slope (gradient) changes

Longitudinal changs near headwaters

• Steep gradient

• High velocity

• High roughness

• Low discharge

• Small channel

• Erosion dominates 

Longitudinal changes toward the mouth

• Gradient flattens

• Channel smoother & meanders

• High discharge 

• Wide chance

• Transportation dominates 

Base level

Lowest point a stream can erode

Ultimate base level

sea level

Local base level

Lakes or resistant rock layers

Meandering streams

Intricately looping curves

Meandering streams require

Low gradient & soft sediment

Meandering streams form

wide floodplains

Meandering streams - Outside curve

Fastest → erosion (cut-bank) 

Meandering streams - Inside curve 

Slower → deposition (point bar) 

Oxbow Lakes

• Meander neck narrows & cuts off during flooding 

• Eventually fills with sediment, leaving an arc-shaped scar

Meander Neck

The narrow land between the 2 closest side of a meandering loop 

Natural Levees

• Raised areas adjacent to the channels

• Formed during floods

• Waiter spills over → slow → deposits coarse sediment

• Fine material settles across floodplain 

Natural levees after flood 

Water topped → back swamps 

Bank Swamps

Poorly drained, swampy areas on the floodplain behind natural levees

Stream features

• Waterfalls

• Rapids

• Alluvial fans

• Braided streams

• Deltas

Waterfalls

Large gradient change; rock strength change

Rapids

Turbulent flow over boulders + gradient shifts

Alluvial Flows

Fan-shaped deposits at steep slope base

Braided Streams

Sediment-chocked, multiple channels

Deltas 

Sediment deposited at mouth of river 

• Stream splits into distributaries

• Evolve over time 

Distributaries

Smaller channels that branch off from the main river as it flows across a delta toward standing water 

Cities on deltas 

Subsidence + flood risk 

Mississippi River has

7 deltas on 7,500 years

Engineers Prevent

channel migration

Channel migration

river moves sideways as it erodes on bank & deposits on the other

Gradient

How steep or flat the rivers path is

Steep Gradient

Fast water, move erosion (new headwater)

Flat Gradient

Slow water, more deposition (near mouth)

Mouth of River

End of a river where it empties into a larger water body 

Discharge

Volume of water moving past a point in the river each second

During Flood

• Discharge increases 

• Water spills onto floodplains

• Sediment drops out + builds levees

Cause of Flooding

• Torrential rain

• Saturated soils

• Rapid snowmelt 

• Dam failure 

Flood control

Expensive + ultimately futile 

Artificial Levees

• Send flood problems downstream

• Can be overtopped or undermined

• Past failures: 1993, 2005, 2019

Flood Hazard - Recurrence Interval 

• 100 year flood = 1% chance per year

• Severe flood one year doesn’t lower next years risk 

Government agencies (FEMA - Federal Emergency Management Agency)

• Create flood hazard map

• Maintain levees

• Regulate building in flood zones 

Human impacts on running water/rivers

• Urbanization

• Contamination

• Dams

• Water scarcity 

Impermeable surfaces

Hard surfaces that do not absorb water

Urbanization impacts

• Impermeable surfaces → less infiltration

• Storm surfaces → faster flooding, higher crest

Contamination impact

Sewage, metals, pesticides, road sals, oil, toxins

Dams Advantage  

Irrigation, power, recreation, flood control

Dams disadvantage/impact

Ecosystem loss, reduced nutrients, upstream flooding

Upstream flooding

Water backing up behind a dam & causing flood upstream

Water scarcity impact

• Overuse for irrigation/industry prevents ecological function

• Texas projected water shortage by 2030

Headwater

Source or start of a river in high ground

Flow

How water moves in a stream as it travels downhill

Flow varries on

Depth, width, channel shape/roughness, slope (gradient)

Trunk stream

Main river that carries most of the water & sediment in a drainage basin

Floodplain

Flat land beside a river that gets covered with water during flood

Deposition

The setting or dropping of sediment by a river when its energy decreases

Subsidence

ground sinking over time