Editor's note:Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

At the beginning of 2024, the water level at Nuphar Lake, near Norris Geyser Basin, was on the rise. By the end of the summer, however, the water had receded, and the shoreline was marked by a ring of dead trees. What happened to cause these changes?

Nuphar Lake is a small, non-thermal body of water—more pond than lake, really—adjacent to the entrance road to Norris Geyser Basin. It was mostly unremarkable until the past few years, when some interesting changes began occurring.

In 2021 and 2022, the lake level started to rise dramatically, by several feet (about a meter), and the water changed color from dark green to a bright milky blue. These changes were caused by the input of hot water from hydrothermal features on a ridge just above the lake. Those features normally sent water into the adjoining Porcelain Basin of Norris Geyser Basin—a plain full of colorful and active geysers and hot springs. When the hot water started to flow instead into Nuphar Lake, the water changed color to the milky blue that is typical of many hydrothermal ponds and pools, and the water level began to rise. By early 2024 the lake was threatening to overtop its edges and possibly flow into Porcelain Basin.

By the end of summer in 2024, however, the lake level had receded, and the water had returned to its previous deep green color. What happened?

An explosion happened. That’s what.

On April 15, 2024, a newly installed monitoring station in Norris Geyser Basin detected an explosion from the direction of the hydrothermal features above Nuphar Lake. Inspection of the area in May revealed a crater measuring about 10 feet (about 3 meters) across, surrounded by cracked and disrupted ground. Also, the nearby hydrothermal features were no longer emitting water—not into Nuphar Lake, and not into Porcelain Basin. Instead, only some vigorous steam vents and a pool of hot, but not boiling, water were present in the area.

High-resolution satellite images of Norris Geyser Basin showing the area of Porcelain Basin and Nuphar Lake (both images cover the same area). In the left image, acquired on April 2, 2024, springs on Porcelain Terrace are full of water, and warm hydrothermal water is flowing into Nuphar Lake from the area circled in yellow. This warm water kept the north part of the lake free of ice, the lake level is high, and the color is a milky blue. Boardwalks in the area appear as white lineations because they are covered in snow. In the right image, acquired on August 20, 2024, the lake level is lower, and the color is a deep green. The variations in lake level and color were caused by the cessation of thermal water flowing into the lake from nearby hydrothermal features—a change that was coincident with a hydrothermal explosion in the area on April 15, 2024. Imagery were processed by R. Greg Vaughan (USGS), and data were collected by WorldView-3 satellite and made available thanks to the NEXTVIEW End User License Agreement between Maxar (formerly DigitalGlobe, Inc.), which supports Earth science research and applications.

It appears that pressure had been building beneath the hydrothermal features above Nuphar Lake during the preceding years. This pressure resulted in an increase in hot water and steam emissions, but the plumbing must have become clogged and sealed in April. The explosion on April 15—similar to, but smaller than, the well-observed explosion in Biscuit Basin on July 23, 2024—broke that seal and disrupted the hot-water plumbing to the features.

Because hot water was no longer flowing into Nuphar Lake after the explosion, the lake level dropped over the course of the summer of 2024, and the milky blue color faded as silica settled out of the water and was not replaced. The changes are especially visible in satellite images of the area.

The formerly high level of thermal water still left its mark, though. Nuphar Lake is now surrounded by a ring of dead trees along the shoreline. These trees, some of which have already fallen into the lake, were killed when they were inundated by the silica-rich water, marked by telltale white stains on the tree trunks.

Photo of dead trees along the edge of Nuphar Lake. The white staining at the base of the trees is a telltale sign that the trees were immersed in thermal water containing silica/USGS, Mike Poland, September 1, 2024.

Hydrothermal activity in the area of Nuphar Lake seems to have calmed significantly since 2021–2022, but the lesson of Yellowstone is one of change. The thermal areas of Yellowstone National Park are dynamic, so it would not be a surprise if the pools and vents above Nuphar Lake surged to life in the future and sent their hot, silica-laden water coursing into the lake once again.