When we plow the ground for farming, we increase the amount of erosion because the grasses and bushes and trees whose roots used to hold the soil grains together are no longer there. Every time it rains, the rain drops move the soil grains a bit. Go out and watch the stones during a heavy rainstorm. Every so often one of them will flip over when a raindrop hits it. If a raindrop can flip a stone that weighs about an ounce, it can easily move a much lighter soil grain Bare ground contributes much more soil to the streams than areas covered with grasses, and bushes and trees provide even more protection for the soil than grasses.
Forested areas undergo relatively little soil erosion, whereas bare ground loses a lot of soil.
In places that have not been stripped of their vegetation very little soil reaches the streams because it is trapped in the riparian zone. In these places, the water is clear. That is how riparian zones act as barriers. Some of the components of fertilizers dissolve easily in water. Nitrogen is an example.