Why does lateral erosion become more important in plain rivers than mountain rivers?

Correct answer: Because plains have low slope and slower flow

The direct answer is A: plains have low slope and slower flow. Mountain rivers usually flow down steep gradients, so they have greater vertical erosive power and cut downward into their beds. When a river reaches a plain, the slope becomes gentle and the flow generally slows. The river then has less energy for deep downward cutting and more opportunity to attack its banks sideways. This lateral erosion, together with deposition on inner bends, produces meanders and can later help form oxbow lakes. Option A is correct because it gives the physical reason for the increased importance of lateral erosion. Option B is wrong because plains are not defined by having only glaciers; glaciers are mainly high-mountain or polar ice features. Option C is wrong because lava flow is volcanic and is not the normal cause of river-bank erosion. Option D is wrong because plain rivers do contain water; water flow is essential for erosion and transport. The reasoning chain is steep slope in mountains, gentle slope in plains, reduced vertical cutting, increased bank cutting, and meandering. Memory cue: low slope means side cutting; side cutting means meanders.