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A watershed is an area where all surface water drains into the same body of water (river, lake, or ocean). Surface water consists of the tiny trickles of water flowing on the surface of the Earth that develop into larger streams and eventually combine to form rivers and lakes.
Rivers are organized into networks, each with its own recharge area upstream, and drainage channel and mouth downstream. Networks are ordered from ocean to main rivers to secondary rivers to streams, which correspond to ocean watersheds, river watersheds, sub-watersheds, sub-sub-watersheds, and so forth. The boundary of a watershed is called a watershed divide.
In a watershed, water flows from high to low, from upstream to downstream. Watershed recharge is a function of precipitation, soil and bedrock permeability, absorption of water in the soil by plant roots, and evapotranspiration. As part of the latter process, plants return moisture to the atmosphere by transpiration, and the water eventually returns to Earth in the form of precipitation (for example, as rain, snow or hail).
The easiest way to describe the network of rivers and lakes on a small-scale map is to show the watersheds. In Canada, there is a detailed hierarchy of watersheds, ranging from the largest (drainage into oceans and their equivalents), down to the smallest ramification.
Location and Main Rivers of Ocean Watersheds
The Pacific Ocean watershed drains the area west of the Rocky Mountains. The Fraser, Yukon and Columbia rivers are the largest rivers draining this region. It is separated from all other drainage areas by the continental divide. This is defined as the north-south line along the western Cordillera that separates rivers flowing ultimately into the Pacific Ocean from those flowing into other oceans.
The Arctic Ocean watershed is the area flowing directly into the Arctic Ocean or into the channels of the Arctic Islands. Hudson Bay, James Bay and Ungava Bay are considered to be part of the Arctic Ocean but, for most purposes, their drainage area is usually considered as a separate entity. The Mackenzie River dominates the Arctic Ocean watershed.
The Hudson Bay watershed is a huge area that captures about 30% of total Canadian runoff. Many of its river systems, such as the Nelson and Churchill rivers (of Manitoba), drain eastward from the continental divide to Hudson Bay. As well, many large rivers drain from the south and east into Hudson Bay or James Bay. The extensive areas of drainage into Ungava Bay and Foxe Basin are also considered to be part of the Hudson Bay drainage area.
The Atlantic Ocean watershed is dominated by the Great Lakes–St. Lawrence system, but there are other significant watersheds, such as those of the Churchill River (of Labrador) and the Saint John River in New Brunswick.
The Gulf of Mexico watershed occupies in Canada a small portion of southern Alberta and Saskatchewan. The waters drain south into the Mississippi system, which ultimately drains into the Gulf of Mexico. (The Gulf is part of the Atlantic Ocean but, because of the Mississippi, it is often studied as a separate entity).
Finally, parts of Alberta and Saskatchewan have closed watersheds or areas of internal drainage: these are river systems that do not drain into any ocean.
Mapping Note
On this map, the watershed hierarchy is based on the size of the drainage area, not on river flow (discharge) as was the criterion to map drainage basins. Furthermore, smaller watersheds were delineated in highly populated regions and watershed names are based on local water features. Some small water diversions were included and named. The watershed hierarchy used on this map can be aggregated to the Water Survey of Canada’s sub-basins: a system compatible with Canada’s water gauging stations. |
