Thermal and Optical Characteristics of Southern Indian Lake before, during, and after Impoundment and Churchill River Diversion

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Abstract
In 1976 Southern Indian Lake, a large, shallow, multibasin lake on the Churchill River in northern Manitoba, was impounded. The mean annual water level was raised 3 m over the historic mean level to divert 875 m3∙s−1 by gravity flow from the lake into the Nelson River drainage. Five years of observation beginning 2 yr before the year of impoundment and continuing for 2 yr after impoundment indicate that lake temperatures, light available for photosynthesis, and Secchi disk transparencies have all declined. Increased mean depths, loss of riverine heat input because of diversion, and radiative losses because of back-scattering from increased concentrations of suspended solids have caused a significant 1–2 °C cooling of the different basins of the lake and increased the natural south to north temperature gradient during the lake's heating period. Winter temperatures also decreased significantly in South Bay, the point of diversion, because of increased inflow from a cooler region, increases in light scattering caused by higher concentrations of suspended solids derived from shoreline erosion accounted for the most significant changes in vertical extinction coefficients and Secchi disk depths. Increased extinction of light and increased mean depth significantly reduced the mean water column light intensity of the mixed layer. These changes in the thermal and optical characteristics after impoundment occurred concurrently with significant changes in the impoundment's biological communities.