I live near a river and every spring the water level rises quite high, it would flood if we hadn't built such a high bank around it. If it doesn't rain for a while, the water level drops.
It would be interesting if the areas of low elevation collected water after rains and the shores of the rivers changed. It would be dangerous to build your shelter too close to the river if it was raining a lot, you'd have to build on higher ground. Valleys between mountains would also flood suddenly if there was a lot of rain.