The Lake You Cannot Swim In

Yellowstone Lake sits at seven thousand seven hundred thirty-three feet above sea level, making it the largest freshwater lake above seven thousand feet elevation in North America. From the shore at West Thumb, with the lake spreading away to the north and east, it looks cold and still and deep. The cold part is accurate. The still and safe parts are not.

In the nineteen nineties, research teams sent submersibles to the bottom of Yellowstone Lake and found something the surface gives no indication of: an active hydrothermal system. Fumaroles venting hot gas. Hot springs erupting from the lake floor. Explosion craters, some of them recent in geological terms, torn open by hydrothermal blasts beneath the water. The north arm of the lake sits inside the caldera boundary. Its floor is part of the same volcanic system as Norris Geyser Basin and Old Faithful.

The thermal gradient in the lake is severe and unpredictable. Cold water from snowmelt sits near the surface. Hydrothermal vents heat the water at depth. The gradient changes across distances of a few feet. A person who swam out from a cold shoreline and unknowingly moved over an underwater thermal vent could be fatally scalded before understanding what was happening. Swimming is prohibited across the lake. This is why.

Where you are standing, at West Thumb, the geyser basin comes all the way to the shoreline. There is a historic feature called the Fishing Cone, a small geyser vent sitting in a few inches of lake water. Early park visitors used to lower freshly caught trout directly from their fishing lines into the Cone and cook them without moving from the spot. The practice is no longer permitted. The lake is not as calm as it looks.