GL 107 Earth Systems Science
K. Hannula
Discussion questions for 10/19/07
1. Where and why is sediment deposited:
on a delta: sediment is deposited at the mouth of a river entering a lake or the ocean, because the velocity of the water slows to zero in the still water.
in natural levees: sediment is deposited on the banks of rivers during floods, because when water leaves the river channel, it slows due to friction (shallow water flowing over plants, buildings, etc.). When the water slows, it can't carry the largest sediment, and drops it.
along a meandering stream: sediment is deposited on the inside of curves, forming point bars, because water travels the most slowly there.
2. How does building a dam change erosion and deposition upstream of the dam? Downstream of a dam?
Upstream: sediment is deposited in the lake that forms, because the still water can not carry sediment.
Downstream: the river erodes its bed, because the water does not carry sediment, but the water is moving (has velocity and discharge).
3. Describe how a meandering stream's path changes through time. (Aside: the Animas River just north of Durango is a beautiful example of a meandering stream.)
Meandering streams erode the outside of curves, and deposit on the inside of their channels. The stream's course wanders across its entire floodplain, given enough time. Oxbow lakes form where two curves are eroded so much that they meet, stranding a curve of the river. (See Fig. 5.9, p. 124 in textbook.)
4. How are flash floods different from other floods? What areas are susceptible to flash floods, and why?
In flash floods, water levels both rise and fall quickly. Flash floods are more likely in places where water runs off (and does not soak into the ground): places with steep slopes, little vegetation, or rocky ground (or lots of pavement) are more likely to experience flash floods.