How and why does the depth of the river change, what does it depend on?

The depth of the river depends on several conditions:
– the amount (mass) of water carried by the river,
– the speed of the river,
– features of the soil on which the river flows.
The river creates its channel (the depression along which it flows) by washing out the soil. The greater the mass of the flow and its velocity, the greater the deepening is obtained. And, of course, if the soil on which the river flows is sandy, then it will be much easier to wash it out and the depth will be greater. And if it is clay or stony, then vice versa.