Rivers

A river's watershed or drainage basin – the area supplying it with water – is separated from the watersheds of neighbouring rivers by higher lands called drainage divides. The map shows Canada's continental watersheds, one to each surrounding ocean: the Pacific, the Arctic, and the Atlantic as well as to Hudson Bay and to the Gulf of Mexico. Small watersheds combine to make up regional watersheds, which in turn join others to form continental watersheds. 

The world's largest rivers are shown in this figure. 

Sculpting the earth

As a swiftly flowing river, water can erode the underlying terrain. Where the river slope is flatter, the river slows down and deposits materials. This usually occurs in the lower reaches and especially near the mouth of the river, either at a lake or an ocean. A river can carve steep valleys, especially in higher parts of the drainage basin. In the lower parts of the basin, deposits may create deltas at the river's mouth.

The volume of water flowing in a river, together with the speed and timing of the flows, determines how a river shapes the surrounding landscape and how people can use its waters. Rainfall, snowmelt, and groundwater all contribute to the volume of flow, producing variations from season to season and year to year.

In Canada, most high flows are caused by spring snowmelt. This is the season when floods are most likely to occur. Rainstorms can also cause high flows and floods, especially on small streams. The effects of floods and storms can be much less severe on rivers with large drainage basins. The lowest flows for rivers in Canada generally occur in late summer, when precipitation is low and evaporation along with consumption by plants is high, and in late winter, when rivers are ice covered and the precipitation is stored until spring in the form of ice and snow.

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