Suspended sediment concentration

The map shows the concentration (annual load) of suspended sediments into rivers (in milligrams per litre). The load of sediment is lower in the Canadian Shield area and on the Arctic Islands. The greatest loads appear in the Prairie Provinces and in the Cordillera. The map also shows histograms of the mean monthly suspended sediment load in metric tons per day per square kilometre times 10-3 for ten rivers across Canada. Data are compiled from 1961 to 1970. Every year, about 300 million metric tons of sediment is carried by Canadian rivers to the oceans or across national boundaries. To appreciate the volumes involved consider that if the suspended sediment reaching the mouth of the Fraser River over a period of 1 year could be massed in a conical pile with 45-degree slopes, it would reach a height equivalent to a 100-story building (almost 300 metres). Transportation of soil particles from the land surface begins with the action of precipitation. The precipitation dislodges soil particles; the direction of movement of the soil particles depends on the prevailing winds and the land slope. If overland flow occurs, soil particles may also be loosened by the flowing water and carried towards stream channels. This process is called sheet erosion. Once the sediment load has reached a channel, it usually consists of two parts: suspended load, which does not spend any time on the bottom of the channel because of the stream turbulence (this includes wash load, which is very fine or colloidal material with a low settling velocity in still water); and bed load, which moves by rolling, sliding, or saltation (hopping) along the stream bed. Besides the above processes of soil break-up and transportation, other significant contributions are made by mechanisms such as shoreline erosion, wind, and glacial action. The average concentration on the map was prepared using data for the 10-year period 1961-1970, supplemented by shorter term records where available. To compile the map the sediment data were examined first, then they were related to drainage basin and stream runoff distribution between watersheds. In many areas of Canada, sediment data are limited and the sediment concentration zones could only be estimated.