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Chapter 17: Fluvial Processes and Landforms

Physical Physical GeographyGeographyNinth EditionNinth Edition

Robert E. Gabler

James. F. Petersen

L. Michael Trapasso

Dorothy Sack

Fluvial Processes and Landforms

Fluvial Processes and Landforms

• Fluvial geomorphology– Study of flowing water

as a land-shaping process

– Stream• General term for natural,

channelized flow

– Interfluve– Floods– Long-term effects

17.1 Surface Runoff

• Surface runoff

• Infiltration capacity

• Interception

• Amount of runoff depends on:– Intensity and duration of

storm– Surface features– Infiltration & evap.– Deep soil, soil type, slope

17.1 Surface Runoff

• Surface runoff– Sheet wash

(unchannelized)• Rills• Gullies• Ephemeral flow

– Perennial flow– Intermittent flow– Base flow

17.2 The Stream System

• Small perennial streams join to make larger perennial streams– Tributaries– Trunk stream


• Drainage Basins (or watersheds)– Expanse of land from

which it received runoff

– Drainage area– Subbasins– Drainage divide

• Continental divide


• Drainage Basins (or watersheds)– Source– Stream order

• First-order stream• Second-order stream• Third-order stream• Mississippi River (10th

order stream)


• Drainage Basins (or watersheds)– Mouth– Exterior drainage– Interior drainage– Base level


• Drainage Density and Patterns– Drainage density (Dd)

• Length of channels per unit area

• Highly erodible and impermeable rocks tend to have higher Dd

• Slope and vegetation affects Dd


• Drainage Patterns– Dendritic– Trellis– Multiple channels– Centripetal– Rectangular– Deranged


• Drainage Patterns– Transverse

stream– Antecedent

stream • Examples:

Columbia River, Cumberland Gap

• Also called superimposed

17.3 Stream Discharge

• Amount of stream discharge (Q) depends on:– Recent weather– Drainage basin

• Size• Relief• Climate• Vegetation• Rock type• Land-use

• Q = Volume of water in a given cross section per unit of time


Ten Largest Rivers of the World


• Q = wdvw = width

d = channel depth

v = average stream velocity

17.4 Stream Energy

• Kinetic energy

• Stream gradient

• Channel roughness– Friction– 95% of energy is

consumed in overcoming friction

17.4 Stream Energy

• Stream load

• Stream competence and Stream capacity– Both increase in

response to small increases in velocity

– If velocity doubles, sediment load may go up 6-8 times

• Graded stream

17.5 Fluvial Processes

• Stream Erosion– Fluvial erosion– Degradation– Aggradation– Corrosion

• Also called solution

– Hydraulic action– Turbulence– Plunge PoolsQ: Why do deep plunge pools form

at the base of most waterfalls?


• Stream Erosion– Abrasion (more powerful

than hydraulic)– Potholes

• Originate below waterfalls, swirling rapids, structural weakness

– Attrition– Headward erosion


• Stream Transportation– Solution

• Minerals that are dissolved in water

– Suspension• Finest solid particles

carried

– Saltation• Particles that are

heavier and “bounce” along stream bed


• Stream Transportation– 3 Main Types of Stream

Load:• Dissolved load (Ions of rock

material in solution)• Suspended load (small

clastic in suspension)• Bed load (large particles

that saltate or move in traction along streambed)

– Relative proportion of these vary with drainage


• Stream Transportation– Relative proportion of these

vary with drainage• Humid regions

– Higher rates of weathering– Suspended loads– Muddy river– e.g. Yellow River, China

• Arid regions– Limited weathering– Bed load


• Stream Deposition– A decrease in stream

velocity will reduce its load through deposition

– Bar (accumulation of sediment, channel bend)

– Alluvium (fluvial deposits)

• Characteristic of sorting and/or rounding


• Stream Deposition– Natural levees– Floodplains– Vertical accretion– Lateral accretion

Q: What would the river floodwaters leave behind is flooded homes after the water recedes?

17.6 Channel Patterns

• Straight channels– Exist for short distances

• Braided river– Coarse sediment input

is high– Downstream of glaciers

• Yukon River, Canada• Brahmaputra River,

Tibet

17.6 Channel Patterns

• Meandering channels– Most common in

humid climates (e.g. Missouri River)

– May swing back and forth across valley

17.7 Land Sculpture by Streams

• Idealized river– Gradient diminished

downstream– Does not always occur

• e.g. Mississippi River

• Longitudinal Profile– Actual stream gradient

from source to mouth– Upper, middle and

lower


• Features of the Upper Course– Usually flows on

contact with bedrock– Steep gradient high

above its base level– Erosion creates steep

sided valley, gorge– This is called a V-

shaped valley


• Features of the Upper Course– Differential erosion– Many spill from lake to

lake (e.g., Niagara Falls) or gorges


• Features of the Middle Course– Moderate gradient– Moderately smooth

channel– Cut bank– Point bar– Lateral migration– Floodplain good for

farming but a flood hazard


• Features of the Lower Course– Minimal gradient– Low stream energy– Lateral shifting of channel– Large depositional plain– Natural levees– Alluvial plain


• Features of the Lower Course– Meander cut-offs– Oxbow lakes– Artificial levees

• Raised level of channel (e.g. Yuba river, CA)

• Flooding is a high risk

• Yazoo streams

17.8 Deltas

• Deltas– A stream flowing into a large body of water– Current expands in width, reducing flow velocity– Sediment may begin to settle out– Distinctive landform, a Delta forms– Slow going process– Distributaries– Example:

• Ganges River• Mississippi River

17.8 Deltas

• Deltas

17.9 Base-Level Changes and Tectonism

• Base Level change– Due primarily from

climate change (glaciers advancing, sea levels decrease)

– Drop: downcutting and rejuvenated stream

– Rise: deposition

• New Uplift– Entrenched

17.9 Base-Level Changes and Tectonism

• Stream Terraces– Older, higher valley

floors preserved– Caused by varying:

• Base-level• Stream equilibrium• Tectonism

Q: How many terraces can you identify in this photo?

17.10 Stream Hazards

• Flooding is a significant risk– Stream channel can

withstand 1 or 2 year flow

– 5, 10, 100 year flood overflows the channel

– Olivehurst, CA (Feather River)


• Stream Hydrograph– Record of changes in Q over time– Used to indicate how high/fast water level is


• Stream Hydrograph– Rising limb– Peak flow– Receding limb– Recurrence interval

Q: Why would such a time lag occur between the rainfall and rise in the river?


• Stream Hydrograph– Urbanization and

suburbanization• Increases

impermeable cover• Amount and rate of

runoff increases

Q: What features of the urbanized landscape shown here enhance runoff?

17.11 The Importance of Surface Waters

• Streams– Historical

• Settlement and growth via Mississippi River• Exploration• Power for mills

– Inexpensive transportation– Hydroelectricity– Irrigation water– Alluvial soils produce excellent farmland– Source of food and water

• Reservoirs– Artificial lakes

impounded by dams– Flood control– Store large amounts

of water to make available during dry seasons or drought

• Tennessee River• Lake Mead• Willamette River, OR


• Lakes– Inland water– Most hold surface water

temporarily along stream systems

• Lake Superior• Lake Victoria

– Closed basins (salty)• Caspian Sea• Dead Sea• Great Salt Lake


• Lakes– Formation:

• Most are products of glaciation• Rivers, groundwater, volcanism (e.g. Crater lake)

– Sedimentation and other processes lead to the destruction of most lakes

– Importance:• Recreation• Affect weather (moderate temperature/lake effect)• Water supply• Fishing


17.12 Quantitative Fluvial Geomorphology

• Objective analysis of fluvial systems

• Used by scientists including:– Climatologists– Geomorphologists– Hydrologists– Soil scientist

• Provide better understanding and improved prediction of water supply, floods, soil erosion, and pollution.

Physical Geography

End of Chapter 17: Fluvial Processes and Landforms

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Education

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