Situated at 7,733 feet (2,357 m) above sea level, Yellowstone Lake is the largest high elevation lake (above 7,000 feet / 2,134 m) in North America. It is roughly 20 miles (32.2 km) long and 14 miles (22.5 km) wide, with 141 miles (227 km) of shoreline and a surface area of 132 square miles (342 km2). Yellowstone Lake freezes over completely every winter in late December or early January, with ice thicknesses varying from a few inches to more than two feet. The lake usually thaws in late May or early June. Yellowstone Lake remains cold year-round, with an average water temperature of 41Â°F (5Â°C). Because of the extremely cold water, swimming is not recommended. Survival time is estimated to be only 20 to 30 minutes in water at this temperature.
Yellowstone Lake has the largest population of wild cutthroat trout in North America. How a Pacific Ocean fish was trapped in a lake that drains to the Atlantic puzzled experts for years. Scientists now believe that Yellowstone Lake once drained to the Pacific Ocean via Outlet Canyon and the Snake River, and that fish swam across the Continental Divide at Two Ocean Pass. Lake trout, an illegally introduced, exotic species, is now found in Yellowstone Lake and threatens the existence of the native cutthroat trout.
The lake currently drains north from its only outlet, the Yellowstone River, at Fishing Bridge. The elevation of the lake's north end does not drop substantially until LeHardy Rapids, so many consider those rapids to be the actual northern boundary of the lake.