Question

How does gravity relate to a stream's ability to erode and deposit materials?

Answer 1

gravity pulls down the water, if there wasnt any water, the water would be floating in the air ;)

Answer 2

The stream's ability to erode and deposit materials is a prime example of how gravity works as these materials get pulled towards the center point of gravity due to them having weight. Without this - there wouldn't exist any erosion and any deposition of materials.