Why does fast moving water lead to greater erosion then slow moving water?

Remember, faster moving water causes erosion more quickly. Slower moving water erodes material more slowly. If water is moving slowly enough, the sediment being carried may settle out. This settling out, or dropping off, of sediment is deposition.

Explanation:

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.