Compared to most areas in the North American West, Glacier has lots of water. There are 131 named lakes in the Park, approximately 200 lakes are over five acres. Lake McDonald is the largest (10 miles long)and deepest (464 feet) body of water in the park.

Most of the low-elevation lakes are the remnants of long glacial valleys dammed at their outlets by end moraines. At high elevations, most of them exist as glacial "tarns" -- new lakes filling the bottoms of ice-scoured amphitheaters. They often have small end moraines at their lower ends as well, or a headwall waterfall which isolates them from fish migration. The fishless tarns have still developed complex ecosystems of smaller plants and animals. Depending on the level of nearby glacial activity, and therefore the amount of powdered rock they contain, these lakes have a milky brown, turquoise, sapphire blue or extremely clear appearance.

Glacier's water can be considered the headwaters of the entire continent. From Triple Divide Peak, a droplet can theoretically split 3 ways and eventually make it to the Pacific, Atlantic and Hudson Bay watersheds. By any standards, it is clean. Water quality can be significantly affected by air quality. The higher elevation tarns are sensitive to airborne pollutants because they get the entire winter load in spring, when the snow melts. In Glacier, the quality of air is going to effect the quality of the water.

Glacier's water is also clear. The temperature of most lakes never gets above 50 degrees Farenheit at the surface, so plankton growth is minimal. It is not unusual to spot details on the bottom of lakes beyond 30 feet.

In some areas, the lack of plankton growth and the clarity of water would be cause for worry -- pollutants like acid rain often kill the growth. Not here. These lakes are at the extreme low end of fertility, because of their low temperatures, extreme depth and the kinds of minerals supplied by surrounding rock. These sterile "oligotrophic" lakes are also an early warning system. They are easily altered by small amounts of pollution, and therefore serve as bell-weathers for our water conditions everywhere.