What effect does the drop in flow velocity have when a stream floods its banks?

When a stream floods its banks, the reduction in flow velocity is a critical factor influencing sediment transport and deposition. As the water spreads out over a larger area, its velocity decreases significantly. This decrease in velocity results in the river being unable to carry all the sediment that it has suspended in the faster-moving water. Consequently, the heavier and coarser particles are the first to settle out of the water column and deposit on the floodplain.

Coarser materials, such as gravel and sand, have a lower transport capacity compared to finer sediments when the flow velocity diminishes. Therefore, as the floodwater slows down, these coarser materials will settle down quickly, leading to their deposition. This process can reshape topographical features and contribute to changes in habitat and landforms in the area affected by the flooding.

The other choices either misrepresent the consequences of decreased flow velocity or pertain to factors not related to the deposition of coarse materials during flooding. Thus, the correct choice highlights a fundamental concept in fluvial geomorphology regarding sediment dynamics in response to changes in flow conditions.