A watershed, also known as a catchment basin or area, includes all of the land that is drained by a watercourse and its tributaries. Watershed boundaries are defined by heights of land. Boundaries are set where a height of land causes water to flow away from the watercourse.
A watershed includes all water, whether flowing or standing, the processes, factors and natural cycles which affect it and all the organisms which live in and rely on the water for survival. The movement of water in a watershed is described by the hydrologic cycle.
Three different scales of a water ecosystem include:
- the bioregion, which is an area of particular physical characteristics that supports certain types of organisms. Ontario has three terrestrial bioregions: Mixed Wood Plains, the Boreal Shield and the Hudson Plains
- the watershed, for example the Great Lakes watershed and
- the watershed sub–basin or sub-watershed (land area drained by a single tributary), for example the Ottawa River
There are three primary watersheds in the province of Ontario:
- the Great Lakes draining to the Great Lake system into the St. Lawrence
- Hudson Bay draining north to Hudson Bay and
- the Nelson River draining west to Manitoba
These three primary divisions have been divided into 28 secondary divisions. Most secondary divisions are either large river systems or groupings of small coastal streams. Ontario's secondary watershed divisions range in size from 4,000 - 150,000 square kilometres.
The secondary divisions are further sub-divided into 144 tertiary divisions ranging in size from 700 - 31,000 square kilometres.
