Which one of the following features is the result of erosion and deposition work of a river?

The correct answer is B, Oxbow lake. An oxbow lake forms when a river erodes the outer banks of a meander and deposits silt that seals off the loop, so it needs both erosion and deposition.

In its middle and lower course a river flows in wide bends called meanders. It cuts into the outer bank, where the water flows fastest, and drops sediment on the inner bank, where it flows slowly. Over time the neck of the loop narrows until, often during a flood, the river cuts straight across it. Deposition then blocks the ends of the old bend, leaving a curved, horseshoe-shaped lake beside the river. The lower Ganga plains have many such lakes.

A is wrong because potholes are round hollows drilled into the river bed by swirling pebbles, a purely erosional feature.

C is wrong because levees are raised banks built by silt dropped during floods, a purely depositional feature.

D is wrong because rapids form where a river erodes alternating hard and soft rock in its bed, an erosional feature.

Exam tip: pothole, gorge and rapid are erosional; levee, delta and floodplain are depositional; meander and oxbow lake involve both.