How do trees improve water quality?

The thing is that the roots of the trees hold on to the soil,

now we know about soil erosion..when it takes place the water gets mixed up with the soil and the quality decreases...the trees prevent this and hence keeps it clear.

Also trees play a major role in transpiration and that leads to rain,

and we all know that rain is one of the purest forms of water.

so i guess these are the ways in which trees improve water quality.