This video shows how a rain garden at Bernheim Arboretum and Research Forest near Louisville, Kentucky slows down the flow of water from the forest's parking lot and helps prevent soil erosion. Since soil erosion is one of the causes of non-point-source water pollution, rain gardens help protect the water supply. The video defines "site flow," which is the movement of water across a site, and identifies several plants that thrive in the wet conditions of a rain garden.