Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
PowerPoint Presentation Outline Notes
What is the most important agent shaping the landscape?
- Mass wasting
- Surface water runoff
- Rill erosion
Where do streams get their water supply?
- Surface water runoff
- Groundwater (base flow)
What is the most common pattern made by streams?
- The most common pattern is dendritic
- Streams form a branched network
- This is called a drainage system
What is a drainage basin?
- The total area drained by a river and its tributaries
- A drainage basin is defined by divides
- Divides separate one drainage basin from another
What is the ultimate limit of stream erosion?
- The limiting level for a stream is base level
- Sea level is base level for most streams
- Sometimes a lake forms a local base level
What is the work of streams?
What determines the ability of a stream to do work?
What determines the velocity?
- Gradient - Slope of the channel
- Channel shape and roughness
- Sediment load
- Discharge - Amount of water flowing past a point in a given unit of time.
How do streams erode?
- Hydraulic action or scour (sheer force of water)
What kind of erosion carved these potholes?
How do streams transport?
- In suspension (suspended load)
- In solution (dissolved load)
- By traction, rolling and dragging (bed load)
When do streams deposit their load?
- When the velocity decreases
- What would cause velocity to decrease?
- Reduced discharge
- Lower gradient
- Inside bend of a river
- Higher base level
- Increased sediment load
How does a stream change as it approaches sea level?
- Discharge increases as tributaries join the stream
- Width and depth of channel increase
- Sediment load increases but particle size decreases
- Velocity increases slightly
Streams show three stages of gradation from mountains to the sea:
Stream Stage 1 - Youth
Characteristics of Youthful Streams
- Steep irregular profile
- Waterfalls, rapids, and plunge pools
- River is deepening its channel
- Cutting into bedrock
- V-shaped cross-section
- Hard rock layers have not yet been smoothed away
- Potential Hydro Electric Power
Cutting Into Bedrock
Large Boulders as Bed Load
Stream Stage 2 - Mature Valley
Characteristics of Mature Valley Streams
- Valley widening begins
- Meandering Stream
Stream Stage 3 - Old Age
Characteristics of Old Age Streams
- Wide Alluvial Valley Stage
- Natural Levees
- Built by floods
- Depositional feature
- Parallel to the stream channel on both banks
Rivers drop their load when they enter the ocean and form a ________?
Mississippi River Basin
- Worst flood in U.S. history
- 17,000 sq miles
- Heavy rainfall in June and July
- Upper Mississippi Basin to St. Louis
- Artificial levees cannot contain floods
- Give false sense of security
Floods in St. Louis, 1993
Sometimes a river goes through a renewed period of erosion.
What causes rejuvenation?
- Increased discharge
- Decreased load
- Lower sea level
How Rejuvenation Affects Rivers
- Rivers cut down into bedrock to form entrenched meanders.
- Rivers cut down through their valleys.
How do people change rivers?
- Dam building
How does development change streams?
How do these changes affect rivers?
- Less infiltration so less ground water
- More runoff
- More flooding in form of flash floods
- Greater channel erosion
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