Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Blaine McCleskey and David Roth, research chemists with the U.S. Geological Survey.
From June 10–13, 2022, several inches of rain fell in the northern parts of Yellowstone National Park, which was blanketed by a large snowpack at the time that was late to melt because of a cool spring. The large amount of rain combined with the rapid melting of snow created historic flooding in the Gardner, Lamar, and Yellowstone Rivers. The peak flow in the Gardner River during the 2022 flood event was provisionally determined to be 2890 cubic feet per second (CFS) compared to an average peak snowmelt flow of 800 CFS—nearly 4 times the average peak flow! Similarly, the peak flow during the 2022 flood event in the Yellowstone River at Corwin Springs, north of Yellowstone National Park, was provisionally determined to be 49,400 CFS, whereas the median peak during snowmelt is 12,000 CFS.
The Boiling River is a hot stream emerging from beneath a travertine ledge that flows for about 150 yards before discharging into the Gardner River. Downstream from where the Boiling River and Gardner River combine is a popular area for tourists to take a soak in warm thermal waters—one of the more popular swim holes in Yellowstone National Park. It is thought that a significant portion of the outflow from Mammoth Hot Springs becomes subterranean in the porous ground at several known sinkholes near the downstream end of the Mammoth terraces and then resurfaces as the Boiling River. Interestingly, the Boiling River discharge only increased to about 32 CFS from about 30 CFS during the storm. The Boiling River emerges high in the Gardner Canyon in an area that underwent a “Dramatic Shape Shift” as a result of the storm. The new streambed is narrower, and the main channel shifted to the far bank. Further north, the trail from a parking lot to the Boiling River was washed away.
The surficial geology in Gardner Canyon consists of unconsolidated materials from a large landslide along the western bank of the Gardner River, and cliffs consisting of shale, sandstone, and gravel layers on the eastern bank. This unconsolidated material is highly susceptible to erosion, and during the storm much of the banks, including parts of Highway 89, were washed away. Large amounts of debris made up of rocks and sediment were deposited downstream along the beds of the Gardiner and Yellowstone Rivers. Some of this debris was deposited on top of a river chemistry monitoring station located along the Gardner River just upstream from the Rescue Creek trailhead.
The monitoring site along the Gardner River is part of a network used to track hydrothermal activity in Yellowstone, measuring the amount of chloride in the major rivers to determine the amount of thermal water that is discharged from the region’s hot springs and geysers. Specific conductance, which measures the concentration of dissolved chloride ions in the water, is a proxy for chloride, and specific conductance probes are deployed at 10 monitoring sites in and around Yellowstone National Park, including the Gardner River site. The advantage of using specific conductance is that high-frequency measurements (every 15-minutes) are possible, providing the ability to capture changes in chemistry caused by short-term events like geyser eruptions and even flooding.
The changes at the monitoring site following the storm are amazing! Surprisingly a nearby tree and the monitoring site itself remained in place during the storm, providing a benchmark to measure the changes in stream morphology. Approximately 4 feet of debris now covers the site, burying the specific conductance monitoring equipment. The flow path of the stream also changed substantially. At the monitoring location, the river moved about 30 feet to the east and appears to be narrower and deeper than before. While this is a relatively small example, it shows how extreme events are the drivers of changes in the shapes and courses of rivers.
Despite being buried under several feet of rocks and sediment, the specific conductance probe collected data throughout the storm event and for several months afterward. Specific conductance, in conjunction with the river discharge, can provide some information as to the timing of events. Specific conductance is a measure of the dissolved salts in the river, and from June 10–12 the specific conductance decreased slightly, probably due to increasing water relatively to the salts from snow and rain. The specific conductance then suddenly increased by a factor of about 4 on June 13, coincident with the peak discharge. A large portion of the bank erosion likely occurred during peak flow, and during this time the sediment and dissolved salts substantially increased in the river! After the storm peak passed, the specific conductance decreased for a few hours before increasing again. By this time, the probe was probably buried under a large quantity of rocks and sediment. It is unclear why the specific conductance increased at this time, but one possibility is that the buried sediment surrounding the probe was saturated with water that was dissolving the salts held in the sediment.
The monitoring station has been relocated, and additional measurements and testing are being undertaken to develop a new specific conductance-chloride proxy for the Gardner River. Despite the flood, monitoring efforts to track changes in Yellowstone’s River systems continue!
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