A river bend tends to sand out on the outside and clog up on the inside. Because the water does not flow evenly through the bend, a flow is created in the form of a corkscrew: the spiral flow. The spiral flow loosens the sand from the outer bend. This sand settles in the inner bend because the water there does not flow as fast. The inner bend, also known as a convex bend or a short bend, becomes shallower.
So where the water flows the fastest, the river wears out the strongest. The river’s the deepest there. This is true to a lesser extent if the soil is rocky.