The reason that less erosion occurs along riverbanks where trees grow is due to their root systems. The root systems of the trees set up an underground network that help hold the soil in place. With this root system in place, the soil is more protected from the water and wind and less erosion takes place. Also, since the trees have large leaf systems covering the ground, less of the rainfall hits the ground directly near the trees. As a result, there is less water travelling across the soil near the trees and less erosion occurring there as well.
♦ River acts as one of the best medium for erosion. It collects the sedements, silt, sand particles etc from riverbanks and on their way of travel deposits those things to newer places. This is how large plains are formed.
♦ eosion can only occur on those places where soil particles aren't tightly packed together. This is what treees do!
♦ trees have long roots and spreaded root structure over an area. The fibrous, and dense root system holds the soil paticles at a place and didn't allow them to move by any other external force.
♦ as roots hold the particles at a place river water is unable to erode of carry them into it.