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.
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. A secondary watershed is a subdivision of a primary watershed. Most secondary divisions are either large river systems or groupings of small coastal streams. Ontario’s secondary watershed divisions range in size from just under 4,000 square kilometer’s to 150,000 square kilometer’s (exclusive of the Great Lakes proper). A tertiary watershed is a subdivision of a secondary watershed. Tertiary divisions range in size from 700 square kilometer’s to 31,000 square kilometer’s. A quaternary watershed is a polygon feature that identifies one of the 1000+ subdivisions of the tertiary watershed divisions which comprise the entire Province of Ontario.
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).