Trees: Water's guardian

As I was reading an article in the Indianapolis Star from the January 15th edition, I noticed a section talking about trees being cut down. Then it started to talk about how companies need to find new water sources because the river they were taking from wasn't being naturally purified anymore. Then the article started talking about how the upper layers of the forest take the nutrients from the rain water that make the water "dirty" and then trees also provide good protection along the banks to prevent erosion into the water. I never knew how the trees along rivers play a key role in society. One thing I plan on doing during the summer is helping restore the banks along the rivers around my community.