a zoom into Lake McDonald. Smoke courtesy of California's wildfires. Per visitmt.com: Lake McDonald is Glacier National Park's biggest lake; ten miles long and 472 feet deep. Filling a basin gouged out by Ice Age glaciers, Lake McDonald is a classic glacial feature. This long fjord-like lake is surrounded by mountains on the north, south, and east with the Continental Divide, 14 miles away. The mountains provide a spectacular backdrop for the lake and also act as a rain block. Much of the blocked precipitation ends up falling on the McDonald valley. The result is a mild, damp climate.