Unless they are interfering with your recreational access, leave trees and branches that have fallen into the water alone. They form critical habitat for aquatic organisms that fish and other aquatic life feed on, provide cover from predators for small fish, and they serve as a dock for turtles, kingfishers and other interesting wildlife.
On land, dead trees serve as homes for birds, food for woodpeckers, and create interest on your property. If you enjoy watching wildlife, leave some habitat in the yard.
