Some farming practices can damage the vegetation cover, so the soil will be washed into the river easily.
Overgrazing
People want more food and money. They graze too many animals on the land and the pasture is eaten away quickly. Less vegetation cover results in soil washed into the rivers easily.
Overcultivation
When a piece of land has been used for farming for a long period of time, the soil may became so infertile that no vegetation can grow on it. The land is less fertile than before so the soil washed into the rivers more easily