Why are meanders more common in the Northern Plains?

Correct answer: Due to low gradient and lateral erosion

The direct answer is B: low gradient and lateral erosion. A river flowing through mountains usually has a steep slope, strong downward cutting and a relatively narrow channel. In the Northern Plains the slope is gentle, so the river loses speed and has more opportunity to erode its banks sideways. The outer bank of a bend is eroded while sediment is deposited on the inner bank. Repeated unequal erosion and deposition make the channel curve into meanders. Option A is wrong because a hard mountain slope generally encourages vertical erosion and straighter, confined channels, not common wide meanders. Option B is correct because low slope plus lateral bank erosion produces bends. Option C is wrong because volcanic ash is not the controlling reason for ordinary river meanders. Option D is wrong because glaciers move as ice and do not explain the bends of alluvial plain rivers. The reasoning is low gradient, slower flow, side cutting, deposition, and increasing bends. Memory cue: steep mountains cut down; gentle plains cut sideways and meander.