A watershed is an area of land that holds water that eventually drains into a large body of water, like a lake, river, or ocean. The boundaries of a watershed are determined by the topography of the landscape. Mountains or ridges are the natural dividers of watersheds. The Continental Divide is an example of a large ridge that divides river systems that flow into the west (into the Pacific Ocean) from river systems that flow into the east (into the Atlantic or the Arctic Oceans).
A watershed has its beginning in an area known as its “headwaters”. This is the first main source of water for the watershed’s largest river. Often, the headwaters come from a lake or a glacier. The Continental Divide that runs through the Rocky Mountains contains many Headwaters and the Athabasca River has its headwaters in Jasper National Park, where it is fed by the Columbia Icefields.
As the river flows downstream, its volume grows as it collects water from the smaller rivers and streams that flow into it. Each of the tributaries that flow into the mainstem (the watershed’s biggest river) have their own watershed and headwater. These smaller watersheds are known as sub-watersheds. Each watershed contains several sub-watersheds. For example, the Athabasca River watershed contains the Pembina River watershed and the Clearwater River watershed. (Both the Pembina River and the Clearwater river flow into the Athabasca River.)
Lakes also have watersheds, which can be defined by the area of land which contains water that eventually flows into a lake. The Athabasca River watershed contains many lake watersheds. One example is the Baptiste Lake watershed near the town of Athabasca.
A watershed is a unit in the Global Water Cycle (Hydrological Cycle). The amount of water on Earth remains constant over time and moves around the globe in rivers, oceans, ice, and groundwater, as well as through humidity and clouds. All life on Earth has adapted to the Global Water Cycle and is dependent on water to sustain it.