The canopy of leaves will help control the water temperature. Trees use nitrogen and phosphous as nutrients. Scientists believe that planting trees will help the Chesapeake Bay by taking the nutrients out of the water. So importatnt is this task, the Commonwealth of Virginia has a goal to restore or reforest 610 miles of riverbacks by the year 2010. And if you plant native species, it'll provide food and shelter for frogs, raccoons, birds, foxes, and other critters.
Trees also help control the rate and volume of water and influence flood levels.