Physical Geography 102
Fluvial Processes II

Stream Erosion

Uplands - undissected divide between stream systems
Precipitation on this area will begin to erode it
Headward Erosion - the deepening and widening of the channel uphill - results in making the valley longer
Stream head - the stream's point of origin

Downcutting and Lateral Erosion

- also known as downward erosion
- results in a deepening of the stream channel and valley
- removal of material from valley floor
Lateral Erosion
- results in widening of the valley
- removal of material from valley walls

Base Level

The amount and rate of downcutting and lateral erosion are dependent on the base level of the stream
Base Level is the lowest theoretical level to which a stream may erode
- for most streams, base level is sea level
- variations in bedrock and topography will often result in a temporary base level above the ultimate sea level

Erosion to Base Level

When a streams elevation is well above its base level, downward erosion will be greater than lateral erosion
When a stream's elevation is close to its base level, lateral erosion will be greater than downward erosion

Downcutting Streams

Valley develops a "V" shape - steep valley walls
Usually found in mountainous regions or newly uplifted areas
Valley bottoms will be narrow; no flood plain
Moderate to steep gradient
Rapids and cascades are common
The run of the valley tends to be relatively straight

Lateral Erosion Streams

As downward erosion continues in V-shaped valleys, the valley bottom will approach base level or a temporary base level
Downward erosion will slow and lateral will become more important
V-shaped valley begins to give way to a flat-bottomed valley
Valley floor is broad and flat, with a wide flood plain
Upland areas are dissected and eroding
Shallow gradient
Occurs in plains, platform areas of the craton and intermontaine regions

Eventually the upland areas and divides are completely removed - results in a peneplain


Uplift of the land during any stage of stream evolution will result in the stream moving above its base level
Downcutting will become dominant

Stream Morphology

Stream Bed - floor or bottom of the stream channel
Mouth - end of a valley or stream - usually end in a lake or ocean
Waterfalls, cascades and rapids
- areas of steep gradient (up to vertical)
- stream crosses resistant rock layers or structures
- debris temporarily disrupts flow
Bars - embankments of sand and gravel deposited on the stream bed
- Channel bars - linear sand bars which occur within the stream flow
- Point bars - sand bars which develop on the inside bank of a meander loop
Meander - a sharp, loop-like bend in the stream channel
Oxbow Lake - cut off meander loop
Flood Plain - areas that a stream overflows during periods of excess discharge

Stream Types

V-shaped Valley - straight streams
Flat-Bottomed Valley
- Braided Streams - multiple channel streams - indicate that the stream is overloaded with sediment
- Meandering Stream - winding, meandering channel