Glacial Flour

A very interesting phenomenon at Glacier Park is the bluish-green color of the water in small lakes. Several of the lakes in the park, which are feed from snow melt runoff in glaciated valleys, exhibit a brilliant aquamarine color in the water on primarily sunny days. This color hue is produced by sunlight reflecting off very fine-grained silt suspended in the water, frequently called glacial flour. As rocks were slowly carved away near the base of the glacier, a fine white powdery sediment was generated in the abrasion process, which still flows down out of the valleys thousands of years later. The most intense color typically appears where glacial valley streams flow into a lake. On cloudy days, a more whitish color predominates over the blue in these lakes. During our trip, we were impressed by the brilliant colors of St. Mary Lake, Grinnell Lake, Swiftcurrent Lake, Lake Josephine, and Avalanche Lake. Be sure to see the glacial flour exhibit at the St. Mary visitors center.