GEO101: Physical Geography
Lecture 17: Wednesday, March 25, 2009
Topics
1. Fluvial Processes and Landscapes
2. Streamflow Characteristics
3. Floods and River Management
Fluvial Processes and Landscapes
• Rivers are the "Earth's lifeblood"
• Most civilization occurs along major rivers and migrates with the change of rivers
• Functions of Rivers:
The artery for distributing fresh water
Redistributing mineral nutrients for soils and plant growth
Transporting weathering products: both mechanic particles and dissolved ions
Deposition of sediments
Power generation and transportation networks
• Fluvial Processes and Landscapes
1. Base Level of Streams
Stream base level: A imaged level below which a stream cannot erode its valley
The degradation of a landscape by weathering, erosion, and transportation will ultimately reduce the landscape down to the base level
2. Drainage Basins
Drainage basins are divided by ridges that form drainage divides
Drainage divides have different scales, from continental scale to kilometer scale
Drainage divides define watershed -- water-receiving drainage basins
3. Drainage Density and Drainage Patterns
Drainage Density: The total length of all stream channels in the basin divided by the area of the basin
-->High drainage density means that the drainage is highly dissected
Drainage Patterns: The arrangement of channels in an area
Drainage patterns are controlled by:
-->Slope
-->Differing rock resistance
-->Climatic and hydrologic variability
-->Topographic or geomorphic patterns
Seven common drainage patterns in response to different geological and climate conditions
Drainage patterns and their common occurrences:
Dendtritic: Gentle slope and relatively uniform rock types
Trellis: Folded mountain belt ?valley and ridge provinces
-->Capture: One river erodes through a drainage divide and captures the headwater regions of another stream
Radial: Domes such as a volcanic Mountain
Parallel: Steep slopes
Rectangular: Fault-controlled drainage divides
Annular: Structural domes
Deranged: Glaciated regions
Streamflow Characteristics
• Stream Discharge: The volume of water per second
Potential energy --> Kinetic energy
Q = wdv (Q-discharge; w-stream width; d-stream depth; v-velocity)
Question: if we keep building the levees to prevent floods along the river, what would happen to the velocity of the stream? (assume the discharge keeps constant)
• Exotic Streams
Yellow river, Nile River: discharge decreases with distance from source due to high evaporation rates in the arid area downstream
• Natural processes along streams
Weathering -- erosion -- transportation -- deposition
• Fluvial Transport
Two types of flows:
Laminated flow
Turbulent flow
Stream loads:
Suspended load
Solution load
Bed load
Bed loads transport:
Rolling
Traction
Saltation
• Abrasion among particles causes the decrease of grain sizes and increase of roundness downstream
• Stream types:
Braided Stream:
When sediment load is heavy relative to its discharge --> sediment deposition --> sand and gravel bars --> forming numerous small channels
Meandering Stream:
Erosion of river banks in one side and deposition at the other side --> point bar development
• Stream Longitudinal Profile
If the gradient of a stream increases, the stream will begin to erode its channel and/or its banks
• Problems of the Mississippi river delta:
Huge sediment supply cause subsidence
Oil pumping and pipelines cause subsidence as well
Major shift in the river's course now exists
New Orleans could become a "city without a river."
Floods and River Management
• Rating Floodplain Risk: Historical data, statistics and measurements
• Streamflow Measurement: 11,000 stream gaging stations (USGS)
• Urban Flooding: Urban pavement increases runoff --> reduce the lag time between storm peak and discharge peak --> behavior like deserts!! --> Harder to predict and take actions