This frigid 136-square-mile body of water near the heart of Yellowstone is the largest freshwater lake in North America above 7,000 feet. At its deepest point, it reaches 390 feet. The lake, averaging 41-degrees, is a crucial home to the largest population of wild (Yellowstone) cutthroat trout in North America, though the fish’s existence has been dramatically compromised by the illegal introduction of lake trout in the late 1980s. Park Service regulations require that all lake trout caught in Yellowstone Lake be killed and that cutthroat trout be released unharmed.