Answer to Question 1
When a stream flows out of a narrow bedrock channel in a mountain range, it forms a fan-shaped mass of alluvium called an alluvial fan because the water enters a broad valley where there are no hard-rock valley walls to confine the flow. Alluvial fans form when the discharge of the stream does not change, but the flow is deeper in the upstream channel than it is where the water spreads out downstream as a shallow sheet. The shallower flow exerts less shear stress, and this means the stream power drops, so sediment abruptly deposits to form the fan. A delta is the landform produced by deposition of sediment where a stream enters a lake, reservoir, or sea. Deltas form when there is a drop in stream velocity as it reaches the standing body of water and this causes a drop in stream power which leads to deposition.
Answer to Question 2
The point at which a stream is no longer eroding or depositing sediment. Streams seek to erode to this point. Ultimate base level is sea level. Base level can change when the system changes. Sea level can impact baseline such that when sea level falls, base level falls and the stream must adjust to the new lower shoreline through incision. During sea- level rise, the stream adjusts to a higher shoreline position with deposition occurring in the lower part of the stream.
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