A watershed (also called drainage basin or catchment) is the land area where all precipitation drains to a common outlet—a stream, river, lake, or ocean. Watershed boundaries follow ridgelines and high points separating drainage directions. Watersheds nest hierarchically—small tributaries drain to larger streams, which drain to rivers, ultimately reaching oceans. Everything happening within a watershed potentially affects water quality and quantity at the outlet—development, agriculture, forestry, and pollution sources all impact downstream waters. Watershed-based planning recognizes these connections, managing land and water together. Major Canadian watersheds include the Great Lakes-St. Lawrence, Mackenzie, Nelson-Saskatchewan, and Fraser systems. Understanding watershed boundaries helps identify upstream influences on water resources and downstream impacts of local activities.