What is the main physical reason for the slow flow of rivers in the Northern Plains?

Correct answer: Very gentle slope in the plains

River speed depends greatly on the gradient, or slope, of the land. In the Northern Plains, the land is broad and almost level, so the river channel has only a gentle fall. Water therefore loses the strong downward pull that it has on a steep mountain slope, and the current becomes slower. This physical feature is the main reason for the slow flow mentioned in the question.

A river may still carry a large volume of water in the plains, but a large discharge does not automatically mean a very fast current. The low gradient reduces the component of gravity acting along the channel and encourages deposition of alluvium in suitable places. Thus option A, very gentle slope in the plains, is correct. Lack of water is not the general reason, and sea depth or the closeness of mountains does not explain the river's normal inland speed.