The lakes and streams of Glacier National Park were greatly influenced by the Pleistocene glaciations. Glacial action carved steep-sided, v-shaped watersheds into broad u-shaped valleys. In some instances the u-shaped valleys were dammed with terminal moraines creating the long finger lakes common on the park's west side. In the higher elevations, at the headwalls for the many valley glaciers, the plucking action of the ice scooped out depressions called cirques that eventually filled with water after glacial recession. These small alpine lakes, or tarns, are abundant in Glacier's high country. In other areas, a series of lakes, like beads on a rosary, were gouged out of the valley creating "Pater-noster" lakes. Numerous bog-like lakes exist today because of depressions created when blocks of ice became imbedded in the ground to eventually melt at lower elevations.
There are approximately 750 lakes (131 named) and about 1500 miles (2400 km) of perennial streams and rivers in Glacier National Park. The largest body of water is Lake McDonald at 10 miles (16 km) long, 1.5 miles (2.4 km) wide and 472 feet (144 m) deep. St. Mary Lake is a close second. On the west side of the park the Middle and North Forks of the Flathead River delineate park boundaries and have been designated parts of the Wild and Scenic Rivers System. On the east side of the park Divide Creek forms a small portion of the park boundary.
Water from Glacier's extensive system of lakes, rivers and streams drains into three major watersheds. West of the Continental Divide water flows into the Flathead River Basin, then into the Clark's Fork River and eventually the Columbia River System and the Pacific Ocean. East of the Continental Divide a portion of the drainage from Glacier enters the Missouri River and makes its way to the Atlantic Ocean via the Gulf of Mexico. The third major drainage is to the north via the South Saskatchewan River. A small portion of the Saskatchewan River System originates in Glacier National Park. Waters of the Saskatchewan River Basin drain directly into Lake Winnipeg with a small percentage (roughly 30% of the overall flow) continuing north via the Nelson River to Hudson Bay.
Water quality within Glacier is considered to be very good, but aquatic systems have been altered by past management practices. Today there are 17 species of native fishes and 8 introduced species. (Note: lake trout is counted in both "native – east of the divide" and "introduced – west of the divide.") Prior to the arrival of European people, native fish species were restricted to main drainages and tributary streams below waterfalls. Many drainages originally barren of fish were stocked by the National Park Service in order to provide for recreational fishing. The introduction of non-native species has had a negative impact overall for native populations. The Quartz Creek drainage located in the northwest region of Glacier is perhaps the best example of a pristine watershed from an overall water quality and species composition standpoint (including aquatic insects, etc.).
The aquatic ecosystem of Lake McDonald is similar to that of other large westslope lakes in Glacier. The cold, clear waters of Lake McDonald, fed by rainfall, snowmelt and glaciers, support relatively few plant and animal species. At the shoreline, there is little evidence of algae, weeds, or organic matter.
Despite the sparseness, this lake was once a healthy, balanced ecosystem which has since been radically altered. Between 1912 and 1971, the lake was stocked to promote fishing. Non-native kokanee salmon and lake trout competed for food, preyed on young fish and began displacing native populations. Because of competition from non-native species, native fish populations are declining.
Westslope cutthroat trout
Rocky Mountain whitefish
Lake trout (east of divide)
Shorthead (Spoonhead) sculpin
Peamouth chub Burbot (Ling cod)
Northern pike (Lake Sherburne)
Lake Superior whitefish
Eastern brook trout
Lake trout (west of the divide)
Although many lakes in Waterton National Park now support a population of one or more fish species, it was not always so. The Waterton and Belly River drainages were the only systems with natural fish populations. All other fishing lakes in the park have been artificially stocked. Fish are no longer stocked in Waterton Lakes National Park.
The tiny opossum shrimp (Myses relicta) is an interesting crustacean that is fed on by fish in the Waterton Lakes. It is a relic species, present before the Ice Age and then, as the glaciers melted, returning up the Missouri-Mississippi river system from the south. During the day it remains at the bottom of the lake in almost total darkness. At night, it migrates to the lake's surface, returning to the bottom by dawn. It spends almost its entire life in darkness.
Northern pike Ling (burbot)
British Columbia cutthroat trout
Eastern brook trout
Yellowstone cutthroat trout