How does gravity relate to a streams ability to erode and deposit material

Gravity plays a major role in relation to streams ability to erode and deposit material along the banks in the flat lands. The streams mostly come down to the plains due to the gravitational force. If this gravitational force did not exist, then it would not have been possible for the streams to come down from the mountains with all the eroded materials. These eroded materials start getting deposited along the banks of the plains through which the stream passes. These deposited materials form a fertile land for irrigational purpose.