Yellowstone Lake is the largest body of water in the south of the Yellowstone National Park in Wyoming, United States. Some water sports are allowed on the lake. At the West Thumb of the lake there is a geothermal area, with fumaroles and hot springs right at the water's edge (see Geothermal areas of Yellowstone).

In recent years (as of 2004), the ground under the lake has started to rise significantly, indicating increased geological activity, and parts of the national park have been closed to the public.

There is a 'bulge' about 2000 feet long and 100 feet high under a section of Yellowstone Lake, where there are a variety of faults, hot springs and small craters. Seismic imaging has recently shown that sediment layers are tilted, but how old this feature is has not yet been established.

Lake Yellowstone on a fall morning

Geology

After the magma chamber under the Yellowstone area collapsed 600,000 years ago in its previous great eruption, it formed a large caldera that was later partially filled by subsequent lava flows (see Yellowstone Caldera). Part of this caldera is the 136 square mile (352 km²) basin of Yellowstone Lake. The original lake was 200 ft. higher than the present-day lake, extending northward across Hayden Valley to the base of Mt. Washburn.

It is thought that Yellowstone Lake originally drained south into the Pacific Ocean via the Snake River. 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. Therefore, this spot is considered the actual northern boundary of Yellowstone Lake.

In the 1990s, geological research has determined that the two volcanic vents, now known as "resurgent domes ", are rising again. From year to year, they either rise or fall, with an average net uplift of about one inch per year. During the period between 1923 and 1985, the Sour Creek Dome was rising. In the years since 1986, it has either declined or remained the same. The resurgence of the Sour Creek dome, just north of Fishing Bridge is causing Yellowstone Lake to "tilt" southward. Larger sandy beaches can now be found on the north shore of the lake, and flooded areas can be found in the southern arms.

Hayden Valley

The Hayden Valley was once filled by an arm of Yellowstone Lake. Therefore, it contains fine-grained lake sediments that are now covered with glacial till left from the most recent glacial retreat 13,000 years ago. Because the glacial till contains many different grain sizes, including clay and a thin layer of lake sediments, water cannot percolate readily into the ground. This is why the Hayden Valley is marshy and has little encroachment of trees.

Reference

• National Park Service - Geological Overview of Yellowstone Lake (adapted public domain text)

External link

• USGS answers questions about the 'bulge' under Yellowstone Lake.