Drainage Basins

A drainage basin, sometimes called a watershed, is an area where all surface water shares the same drainage outlet. 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 a river.

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 river to secondary rivers to streams which correspond to ocean basins, river basins, sub-basins, sub-sub-basins, and so forth. The boundary of a watershed is called a drainage divide.

In a drainage basin, water flows from high to low, from upstream to downstream. Basin 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 water, snow or hail).

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Wilberforce falls on the Hood River, Northwest Territories

Mapping Note

On this map, river flow (discharge) was the critical factor used to delineate basin boundaries. All rivers with an annual mean flow higher than 10 000 cubic feet per second were depicted. However, land areas containing river basins that did not meet this annual mean flow threshold were assigned to the generic category "seaboard". The major river basins are the rivers that directly flow into the ocean. Within each major river basin, up to three levels of component basins were identified. In addition, water diversions larger than 1000 cubic feet per second were mapped.

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