Aparticular river has a very small watershed that drains into it. how would building a large shopping complex over that watershed affect the river's water supply

The water supply is affected because the flow has changed. It has to travel around the large complex where you can lose a lot of water or even pollute the water depending on where it flows through.

Answer volcanic eruptionsseismic waves from earthquakes;

Ineed you to put up a picture of the map so i can you as much as possible.