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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
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Fluvial Processes and Landforms
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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
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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
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17.1 Surface Runoff
• Surface runoff– Sheet wash
(unchannelized)• Rills• Gullies• Ephemeral flow
– Perennial flow– Intermittent flow– Base flow
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17.2 The Stream System
• Small perennial streams join to make larger perennial streams– Tributaries– Trunk stream
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17.2 The Stream System
• Drainage Basins (or watersheds)– Expanse of land from
which it received runoff
– Drainage area– Subbasins– Drainage divide
• Continental divide
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17.2 The Stream System
• Drainage Basins (or watersheds)– Source– Stream order
• First-order stream• Second-order stream• Third-order stream• Mississippi River (10th
order stream)
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17.2 The Stream System
• Drainage Basins (or watersheds)– Mouth– Exterior drainage– Interior drainage– Base level
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17.2 The Stream System
• 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
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17.2 The Stream System
• Drainage Patterns– Dendritic– Trellis– Multiple channels– Centripetal– Rectangular– Deranged
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17.2 The Stream System
• Drainage Patterns– Transverse
stream– Antecedent
stream • Examples:
Columbia River, Cumberland Gap
• Also called superimposed
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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
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17.3 Stream Discharge
Ten Largest Rivers of the World
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17.3 Stream Discharge
• Q = wdvw = width
d = channel depth
v = average stream velocity
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17.4 Stream Energy
• Kinetic energy
• Stream gradient
• Channel roughness– Friction– 95% of energy is
consumed in overcoming friction
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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
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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?
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17.5 Fluvial Processes
• Stream Erosion– Abrasion (more powerful
than hydraulic)– Potholes
• Originate below waterfalls, swirling rapids, structural weakness
– Attrition– Headward erosion
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17.5 Fluvial Processes
• Stream Transportation– Solution
• Minerals that are dissolved in water
– Suspension• Finest solid particles
carried
– Saltation• Particles that are
heavier and “bounce” along stream bed
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17.5 Fluvial Processes
• 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
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17.5 Fluvial Processes
• 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
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17.5 Fluvial Processes
• 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
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17.5 Fluvial Processes
• Stream Deposition– Natural levees– Floodplains– Vertical accretion– Lateral accretion
Q: What would the river floodwaters leave behind is flooded homes after the water recedes?
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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
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17.6 Channel Patterns
• Meandering channels– Most common in
humid climates (e.g. Missouri River)
– May swing back and forth across valley
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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
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17.7 Land Sculpture by Streams
• 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
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17.7 Land Sculpture by Streams
• Features of the Upper Course– Differential erosion– Many spill from lake to
lake (e.g., Niagara Falls) or gorges
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17.7 Land Sculpture by Streams
• Features of the Middle Course– Moderate gradient– Moderately smooth
channel– Cut bank– Point bar– Lateral migration– Floodplain good for
farming but a flood hazard
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17.7 Land Sculpture by Streams
• Features of the Lower Course– Minimal gradient– Low stream energy– Lateral shifting of channel– Large depositional plain– Natural levees– Alluvial plain
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17.7 Land Sculpture by Streams
• 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
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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
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17.8 Deltas
• Deltas
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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
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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?
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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)
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17.10 Stream Hazards
• Stream Hydrograph– Record of changes in Q over time– Used to indicate how high/fast water level is
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17.10 Stream Hazards
• 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?
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17.10 Stream Hazards
• 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?
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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
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• 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
17.11 The Importance of Surface Waters
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• 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
17.11 The Importance of Surface Waters
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• 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
17.11 The Importance of Surface Waters
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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.
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Physical Geography
End of Chapter 17: Fluvial Processes and Landforms