Hebgen Lake Dam, built in 1914, has served a purpose beyond its initial function of controlling floodwaters, storing water for downstream users and generating electricity. It also has acted as a filter for mercury.
Mercury is a toxic element emitted from Yellowstone National Park’s geysers and hot springs, some of which flow into the Firehole and Gibbon rivers. The Firehole and Gibbon rivers join just inside the park’s West Entrance to create the Madison River, which flows into Hebgen Lake just outside the park’s boundary in southwestern Montana.
Because the water flow slows at Hebgen Lake, the mercury carried by the Madison River settles into the lake’s sediment. Some of it also changes into vapor — the process is called volitalization — and is released into the atmosphere. As a result of these processes, the water flowing from Hebgen Lake below the dam into the Madison River has much less mercury than the water flowing into the lake above.
“It’s interesting that mercury coming out of the park in the Madison River, almost 90 percent of it ends up in Hebgen Lake or volatilizes and goes back into the atmosphere,” said David Nimick, a retired U.S. Geological Survey scientist in Helena who was the lead author on a study of mercury in the river system.
He noted, though, that his study doesn’t show how much ends up in the lake or in the atmosphere.
Nimick’s paper detailing the results from a three-year study of the Madison and Missouri river systems — including seven tributaries and six lakes — tracked mercury concentrations in the waters, sediment and fish. The study was published in the Jan. 15 edition of the journal Science of the Total Environment.
The rivers are considered some of the top trout fisheries in the state and are world famous. Anglers flock to the waters in the summer to fly fish amid the beauty of the surrounding mountains and forests. Few probably realize that the waters and the fish are tainted, however slightly.
"Actually, I was very thankful," said Don Skaar, of Montana Fish, Wildlife and Parks, a co-author of the study. "Even though the Madison is largely a catch-and-release fishery, it was a relief to see the levels were pretty low."
Nimick and his fellow researchers, including Rodney Caldwell of the USGS and Trevor Selch of Montana Fish, Wildlife and Parks, took water samples at 38 sites along the Madison and Missouri from high up on the Firehole River all the way downstream to Hauser Lake, just outside Helena — a total of 235 miles. They also caught 294 rainbow and brown trout along the waterways, since mercury is known to accumulate in fish flesh.
The study showed that the Madison River carries about 7 kilograms a year of mercury into Hebgen Lake. That amount of mercury would be about the size of a softball. Sediment samples of the lake bottom showed some accumulation of the chemical, but Nimick said no one has done a large enough or deep enough sample to measure how much may have accumulated in the reservoir in the 98 years since the dam was constructed.
The study also showed a higher level of mercury in the flesh of trout caught in Hebgen Lake compared to those caught below the dam in the Madison River, the Missouri River and two of its reservoirs, Canyon Ferry and Hauser. The Madison, Jefferson and Gallatin rivers form the Missouri near the town of Three Forks. The Jefferson and Gallatin were also included in the sample.
Concentrations of mercury in fish in Hebgen Lake are high enough that the Montana Department of Fish, Wildlife and Parks long ago issued precautions to anglers who catch and eat fish from the study area. That’s not unique to Hebgen, though. FWP has also cautioned anglers about possible mercury concentrations in fish in the Madison and Missouri rivers as well as Canyon Ferry, Hauser and Holter reservoirs. In all, the state has issued consumption advisories for 53 waters in the state.
The study found that concentrations of mercury tended to be higher in brown trout than in rainbows. The exception was in the largest fish — 18 to 22 inches — of both species, where concentrations of mercury were similar. This was attributed to the fact that the larger fish are more predatory. As big fish eat smaller fish, they accumulate the mercury that the smaller fish contained in their flesh. The highest mercury concentration was found in a Firehole River brown trout, close to where geyser water enters the stream.
"I was actually surprised (the levels) weren't a little bit higher," Skaar said. "But on the other hand some of those fish in the Gibbon and Firehole weren't very big and weren't very old."
According to the study, “Mercury concentrations in 73 percent of the fish samples from streams within YP and 40 percent of the samples from Hebgen Lake were greater than the USEPA fish-tissue criterion. In contrast, none of the fish samples collected downstream from Hebgen Lake had concentrations greater than the criterion.”
The criterion referred to was developed by the Environmental Protection Agency, which found that .3 milligrams of mercury per kilogram of fish tissue should not be exceeded to ensure human health.
Mercury is particularly harmful to young and unborn children, affecting their nervous systems. But by eating small amounts, smaller fish that have had less time to build up mercury accumulations and by eating fish at fewer meals, the danger is substantially reduced. Consumption guidelines for the state’s waters can be found on the FWP website.
Although mercury levels are elevated in the upper Madison River, mercury is found in all waters at low concentrations. Much of it comes from coal-fired generating plants that release mercury into the atmosphere. It returns to the earth in rain and ends up in waterways. But the concentration is typically very low compared to that being released by Yellowstone’s geothermal features. Mercury is also released into waterways by mines and industry.
Before the USGS study, it was known that mercury levels were high in the Madison River system in Yellowstone, but little was understood about the waters downstream from Hebgen Lake.
Although the Yellowstone and Snake rivers also have their headwaters in Yellowstone National Park, Nimick said they have fewer geothermal features draining into them, and therefore lower concentrations of mercury as well as arsenic, another poisonous element found in geysers and hot springs.
After analyzing their data, the researchers also found that the level of mercury in the Madison River rises during the day and tapers off at night. They also noted that there are eight hot springs below Hebgen Lake Dam that may be adding small amounts of mercury and arsenic to the Madison and Missouri rivers downstream. Hot Springs Creek, which feeds Norris Hot Springs, had the second-highest concentration of mercury in samples collected.
“If I had to do this study over again, I would have done more sampling of the hot springs along the tributaries,” Nimick said. “Hot Springs Creek has very high concentrations of mercury, although the stream is very small.”
Overall, though, Nimick said the study conducted between 2002 and 2006 provides a good picture of a large river system and its distribution of mercury.