At 7,733ft altitude, the largest natural high-elevation lake in North America has 136sq mi of surface area, 110mi of shoreline and depths of nearly 400ft. Although the surface is frozen half the year, lake-bottom vents produce water as warm as 252°F. The Yellowstone River flows into the lake from the southeast, and exits at Fishing Bridge through the Grand Canyon of the Yellowstone, continuing 671mi to its confluence with the Missouri River. It is the longest river still undammed in the lower 48 states.