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.

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Just west of the short road into Norris Geyser Basin is a small body of water called Nuphar Lake.  It’s really more of a pond, about 175 meters (575 feet) long by 60 meters (200 feet) wide, and it fills the space between the current Norris Geyser Basin access road and an old roadbed—a road that was abandoned decades ago and that now serves as a trail along the east margin the Porcelain Basin area of Norris Geyser Basin.

The cold lake, named for a genus of aquatic plants that include the water lily, is generally unremarkable—a calm and non-thermal body of water on the edge of one of Yellowstone National Park’s most dynamic geyser basins.  Over the last few years, however, the lake has experienced some interesting changes.

The level of the lake fluctuates, rising and falling by a few tens of centimeters (1–2 feet) with the seasons and according to rain and snowfall amounts.  Between 2021 and 2022, however, the lake level rose significantly, causing the trail between Norris campground and the geyser basin to be closed due to flooding.  The color of the lake also began to change, from the typical dark green, to a milky blue/green, reminiscent of many of Yellowstone’s hot springs.

By 2023, the lake level had risen by several feet (more than a meter) relative to its 2021 level.  One of the Norris Geyser Basin temperature-monitoring stations along the lakeshore had to be repositioned as a consequence of the rising water levels.

What drove these changes at Nuphar Lake?

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Nuphar Lake has no significant thermal input from its bottom, despite the proximity to Norris Geyser Basin.  Rather, the changes are caused by water flowing into the lake from hot springs located to the north on a ridge above Porcelain Basin.  These springs normally flow southwest into Porcelain Basin, but on occasion the flow is to the south, into Nuphar Lake.  For example, Yellowstone National Park geologist Rick Hutchinson noted in 1971 that hot spring waters flowed into the lake, but only for a 2-day period in September of that year.  Thermal water also sometimes flows into the lake from features just to the east, occasionally impacting the color of the lake for short periods.

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The recent change in thermal water runoff has killed many of the trees between Nuphar Lake and Porcelain Basin in the past few years and altered the lake’s color—changes that are easily seen in satellite imagery.  And in the winter months of 2022 and 2023, the northern part of the lake remained ice free, which is clear evidence of thermal input from nearby hot springs.

In addition to the rising lake level, water also seeps under the old road bed, flowing underground from Nuphar Lake into Porcelain Basin.  This lake water is indicated by green moss-like vegetation that is growing in the seeps, and it also appears cooler than the surroundings in thermal imagery.  Similar seeps have been documented repeatedly in this area.  For example, in the 1990s there was so much water flowing into Porcelain Basin that it drowned many of the adjacent thermal features.  Incline Geyser, a powerful geyser on the floor of Porcelain Basin that was last active in the early 1990s, has occasionally served as a drain for some of this water.

The seeps are most vigorous in the spring and early summer, when groundwater and lake levels are high, and less active in the late summer and fall.

Will there be any consequences of this input of thermal water into Nuphar Lake?  It is possible that the lake will overflow its banks, adding cooler water to Porcelain Basin and quenching, at least temporarily, some of the thermal features in that area.  We probably won’t see any changes like that in the remainder of 2023 because water levels have historically decreased as the summer progresses.  Such changes remain possible, however, in spring or early summer of 2024, when the lake and groundwater levels will be highest owing to seasonal snowmelt.

It is also possible that the outflow from the hot springs feeding Nuphar Lake will change course once more, sending thermal water back into Porcelain Basin and returning the lake to lower levels and its usual deep green hue.

The recent changes at Nuphar Lake are yet another example of the dynamic nature of Yellowstone’s hydrothermal system, like the recent May-June thermal activity near Old Faithful and well-documented seasonal changes in chemical composition and pH of features at Norris Geyser Basin.  The one constant in Yellowstone’s geyser basins is change!

(Acknowledgement: Special thanks to M.A. Bellingham for her help in researching previous changes at Nuphar Lake noted in Yellowstone National Park and Geyser Observation and Study Association logs and notes, and for sharing her own observations of changes in the area over the past decade.)