River systems of Canada

The map shows the location of rivers, glaciers and the six physiographic regions of Canada. Rivers serve as the natural drainage channels for surface waters. The network formed by river systems receives water from two main sources: runoff and base flow. Runoff is that part of precipitation which flows toward the river on the ground surface (surface runoff) or within the soil (subsurface runoff or interflow). Evapotranspiration and flow to the groundwater are excluded. To understand the evolution of the pattern of river systems seen on the map it is necessary to assume that the physical processes which are in evidence today are the same as those which have operated throughout the earth's history. Given this assumption of continuity, it becomes apparent that river evolution may be attributed to a number of factors, including changing climate and precipitation, tilting of the earth's surface, glaciation and other geologic processes, ground cover, and man's activities. Possibly the most important factor is the geology of the area over which rivers flow. In Canada, glaciation has had a profound influence on river morphology because of the removal of soft rocks, creation of filled valleys, influence of glacier remains (eskers, moraines), and the overall topographic change resulting from glaciation. A drainage basin, often called a watershed or a catchment, is defined as that entire area which has a common outlet for its surface runoff. As a river system matures, smaller basins are incorporated into fewer but larger basins.