A stream can widen its valley through which of the following processes?

• A. Lateral erosion
• B. Vertical erosion
• C. Meandering
• D. Drainage basin expansion

Answer: (A)

Lateral erosion is the process by which the banks of a stream or river are eroded, causing the valley to widen over time. This occurs primarily during periods of high water flow when the force of the water is strong enough to dislodge and carry away sediment from the sides of the stream. As the water flows, it exerts pressure on the banks, leading to the gradual wearing away of soil and rock, which results in a broader valley.

In contrast, vertical erosion deepens the streambed rather than widening the valley; this process typically occurs in younger rivers flowing through steep gradients. Meandering refers to the patterns of curves and bends a river forms, but while it can influence the shape of a river's course, it does not directly indicate the widening of a valley. Drainage basin expansion involves the overall growth of the area that drains into a river but is not directly linked to the widening of the valley itself. Thus, lateral erosion is the specific process that contributes to the widening of a stream's valley.