The plate contains four maps showing the Hardness of Surface Water, Total Dissolved Solids of Surface Waters, Turbidity of Surface Waters and the Chemical Quality of Typical Surface Waters of Canada. These water quality maps were based on data collected from 1970 and 1971 at national river monitoring stations. Only stations with five or more observations over the 2-year period were considered in the compilation. For stations with fewer than five observations and for areas where no data were available, data were extrapolated from surrounding areas. Interpretation is difficult for these maps because it is difficult to generalize on such a broad basis. Some of the provinces, as well as the federal government, have set down some criteria for various water uses, such as drinking water supplies, aesthetics and recreational water, shellfish culture, preservation of wildlife, industrial water, agricultural water, and the preservation of aquatic ecosystems. Since the water use categories are so varied, it is difficult to do interpretive analyses of waters on a national or basin scale except for a specific water use. The purpose of the water quality maps is to give the user a general idea of the concentrations of parameters across the country. To obtain these maps, the concentrations were averaged over fairly large areas and thus, within each area there may be large deviations from the values shown.