A river emerges from a valley onto a flat plain,but the river itself lies in a narrow,shallow canyon as it crosses the plain,then plunges into a steep sided canyon as it flow past the plain.A geologist floating down the canyon headed for the whitewater in the canyon notices the river is flowing through lake deposits as it flows across the plain.What happened in this area?
A)The river is at base level throughout its trace and has been for a long time.
B)The river used to flow into a lake,which formed a temporary base level,but in recent geologic time the lakes dam broke,and the river is now downcutting in response to the change in local base level.
C)The river has cut a deep canyon that crossed through a lake,and this process has been continuous for a long time.
D)The geologist can't be right; this is an impossible scenario.

Question

Quizplus · Accepted Answer

The presence of lake deposits indicates that the area was once a lake, which acted as a temporary base level for the river. However, the dam of the lake broke in recent geologic time, changing the local base level and causing the river to downcut and carve a narrow, steep-sided canyon. The fact that the geologist is headed for whitewater in the canyon suggests that the river is currently experiencing turbulent flow, which is characteristic of a river that is actively downcutting. Therefore, option B is the best choice. Option A is incorrect because if the river has been at base level for a long time, it would not have been able to downcut and carve a narrow canyon. Option C is also incorrect because it implies that the river has been cutting through lake deposits continuously, which is unlikely given that the presence of lake deposits indicates a change in local base level. Option D is incorrect because the scenario is plausible and consistent with known geological processes.

A River Emerges from a Valley onto a Flat Plain,but