Question

how would a river change if rainfall in a watershed doubled in one year?

Answer 1

it would overflow

Answer 2

The river might overflow, yes, but the effects on the river basin are a bit more complicated. This kind of increase in rainfall could substantially increase the amount of runoff, which can cause a chain reaction in the ecosystem. There would probably be a huge increase in soil moisture which leads to erosion, etc. The hydrologic system is a delicate one. Hope this helps.