A region or area over which water flows into a lake, reservoir, stream, or river. Ecologists define it as “an area of land that drains water, sediment, and dissolved materials to a common outlet.” It is essentially a water course that inextricably links living things within a wide geographical area. For this reason, the state of a watershed becomes a crucial environmental issue. A watershed provides water for drinking, recreation, and agriculture, and is a rich source of biological diversity that includes habitat for many threatened and endangered species–such as salmon and trout.