Question

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?

Answer 1

Since river used to flow into a lake, it formed a temporary base level, but in recent geologic time the lakes dam broke, and the river is now down cutting in response to the change in local base level.