Little Limestone Lake: A Beautiful Marl Lake in the Interlake Region, Manitoba

Marl lakes are those accumulating fine-grained bottom sediments including at least 25 % CaCO3. Visually attractive examples have higher proportions of CaCO3, with crystallites precipitating in the water to give it a rich and opaque duck-egg blue colouration. Such lakes are largely limited to recently glaciated carbonate rock terrains. Most are also shallow, with much or all of the water column being in the photic zone. Little Limestone Lake (lat. 53°47′N, long. 99°19′W) is the finest example that the author has seen. It occupies a shallow glacial trough scoured in a plain of flat-lying cyclothem dolomites. It is ~12 km long, 1–5 km wide, and rarely >7 m deep. Including bordering wetlands, it occupies ~45 % of the area of an elongated, narrow topographic basin. Recharge is through impoverished boreal forest with little soil cover; it discharges chiefly as springs and seeps along and below the shore. Mean annual temperature is ~1 °C, and precipitation is ~475 mm/year. Springs in the surrounding region show groundwaters to be simple bicarbonate composition, with TDS = 230–300 mg/L. Grab sampling throughout the lake found its waters degassed to 125–135 mg/L. Little Limestone Lake is visually spectacular because it is almost entirely groundwater fed, with a ratio of recharge area to lake area that is low. In contrast, nearby lakes are regularly flushed by channelled surface storm water and thus cannot maintain high densities of crystallites in suspension. The lake became protected as a provincial park in June 2011.