which form extensive networks across the country, are natural
drainage channels for surface waters. Surface waters
are received from two major sources: runoff and base flow.
Runoff is that part of precipitation that flows toward the
rivers or streams on the ground surface or within the soil
(subsurface runoff or interflow). Base flow is the part of
stream flow that enters the stream channel from groundwater.
Rivers in Canada flow into five ocean-equivalent drainage
basins: the Pacific, Arctic and Atlantic oceans, Hudson Bay
and the Gulf of Mexico. The drainage basin areas are separated
by a drainage divide or height of land. The individual river
system with the largest drainage area is the Mackenzie River,
with 1 805 200 square kilometres.
The discharge of a stream or river is derived from Canadian
water level measurements at the furthest-downstream gauging
station, and is converted to streamflow discharge in cubic
feet per second or cubic metres per second. The river in Canada
with the greatest annual discharge is the St. Lawrence River
at 9 850 cubic metres per second.