Mercury Contamination of Snake River Fish
Understanding and addressing toxic pollution in and downstream of three Hells Canyon reservoirs
Since the 1960s, it has been known that fish in and downstream of three Hells Canyon reservoirs are laden with mercury. The questions have been why mercury pollution is a particular problem there, how serious a problem it is, and what to do about it.
This is the story of the path of mercury to the Snake River; of its transformation in Hells Canyon into its most toxic form; of important recent scientific work to understand that transformation process; and of the work of the Nez Perce Tribe, Pacific Rivers, and Idaho Rivers United to ensure that meaningful action to address the problem begins soon.
Mercury pollution sources
Mercury pollution is a serious widespread problem today. It has many sources. They are natural and human; historic and current; and local, regional, and global. Today, thanks to atmospheric transport of mercury from sources around the world, it’s virtually everywhere in some amount and form. It’s in the food chain. It’s in all of us at some level.
Natural sources of mercury include volcanic eruptions and runoff from watersheds with mercury in their soil and water. Major human sources today include mining, coal burning, non-ferrous metals production, and cement production. Together these four major sources contribute about 85% of the new “unnatural” mercury being emitted around the world today. The remaining 15% comes from a long list of other human activities. All new sources contribute both to local mercury pollution in the short term and the pool of mercury circulating in the global environment over the long term.
This is the story of the path of mercury to the Snake River; of its transformation in Hells Canyon into its most toxic form; of important recent scientific work to understand that transformation process; and of the work of the Nez Perce Tribe, Pacific Rivers, and Idaho Rivers United to ensure that meaningful action to address the problem begins soon.
Mercury pollution sources
Mercury pollution is a serious widespread problem today. It has many sources. They are natural and human; historic and current; and local, regional, and global. Today, thanks to atmospheric transport of mercury from sources around the world, it’s virtually everywhere in some amount and form. It’s in the food chain. It’s in all of us at some level.
Natural sources of mercury include volcanic eruptions and runoff from watersheds with mercury in their soil and water. Major human sources today include mining, coal burning, non-ferrous metals production, and cement production. Together these four major sources contribute about 85% of the new “unnatural” mercury being emitted around the world today. The remaining 15% comes from a long list of other human activities. All new sources contribute both to local mercury pollution in the short term and the pool of mercury circulating in the global environment over the long term.
Geospatially distributed (total) mercury emissions to air from anthropogenic sources in 2015 (g/km2/a) from all sectors. Environmental Protection Agency.
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We have fewer major new human-caused sources of mercury in our region than in many other parts of the world. Nevertheless, we still have mercury in our waters. We have natural sources, including volcanoes and hot springs; significant past and present human sources, including mining; and airborne mercury from countless sources around the world today (see EPA map above). Most airborne mercury that doesn’t settle on water eventually makes its way into it. As long as water flows downhill that will be the case. |
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Transformation of mercury to methylmercury in lakes and reservoirs
Mercury ultimately makes its way into every aquatic ecosystem on Earth through the hydrologic cycle. It can be transformed into organic methylmercury in low-oxygen conditions. These conditions develop seasonally in many lakes and reservoirs due to decomposing organic matter. They become extreme in lakes and reservoirs that stratify in summer, creating very warm waters on top and very cold waters below. In those waterbodies, the cold bottom layer – the “hypoxic” zone – has almost no dissolved oxygen or life. Stratification is most pronounced in deep lakes and reservoirs during long, hot summers. While there is very little life in the bottom layer, some bacteria that actually thrive in such conditions convert inorganic mercury in sediments into organic methylmercury. This most toxic form of mercury remains largely trapped in the deep, cold hypoxic zone as long as the waterbody remains stratified. Stratification breaks down in late summer or early fall in what is commonly called the “fall turnover”. Cooler weather, wind, shorter days, lower sun angles, and longer |
Mercury cycling pathways in aquatic environments are very complex. The various forms of mercury can be converted from one to the next; most important is the conversion of inorganic mercury to methylmercury (MeHg), the most toxic form. Ultimately, mercury ends up in the sediments, fish, and wildlife, or returns back to the atmosphere by volatilization. Diagram from USGS Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon report.
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nights eventually combine to cause warm surface waters to cool and become dense enough to begin to sink.
It is then taken up in turn by plankton, zooplankton, and succeeding levels up the chain to fish, increasing in concentration with each step. It can bioaccumulate one million-fold or more as it works all its way up the food chain to large fish, birds, and other wildlife. The nature and degree of this phenomenon vary tremendously from one waterbody to the next. Many generate very little methylmercury. Some produce prodigious amounts. |
Health effects of methylmercury
There are several reasons organic methylmercury is even more harmful to ecosystems and people than elemental mercury. They include:
Children, infants and especially the unborn are extremely vulnerable to methylmercury. Because their brains and nervous systems are still developing, their exposure to small amounts can have serious long-term effects on vision, cognition, memory, attention, fine motor skills, and language.
In adults, moderate levels of mercury can lead to irritability, tremors, and problems with vision, hearing and memory. High levels can lead to dementia, cerebral palsy, deafness, blindness, sensory and motor impairment, and even death.
Fish consumption considerations
Fish contain essential nutrients and omega-3 fatty acids, and are low in saturated fat. They are the foundation of many of the world’s healthiest diets.
Many fish from many freshwaters are safe to eat regularly. Methylmercury accounts for a very large percentage of those that are not.
There is no “good” level of methylmercury in our bodies. Less is always better than more. However, for most people, concerns about consumption of fish with minute amounts of methylmercury are outweighed by their interest in the considerable health benefits of a well-balanced diet that includes fish.
Human health risks increase in proportion to the concentrations of methylmercury in fish eaten and the amounts of those fish consumed over time. Some populations are more at risk than others. Risks are more pronounced for individuals and groups whose diets and cultures are more dependent on fish.
Where methylmercury contamination cannot be reduced enough for people to eat fish safely, the public must be warned. Warnings from public health agencies take different forms. Some are general warnings for the general public. Others are specific warnings for particular populations, for consumption of particular species or sizes of fish, or for the frequency and amounts of fish consumed.
Warnings, however, are not enough. They should be calls to action. Every effort should be made to understand and then to reduce or eliminate methylmercury problems wherever possible, as quickly as possible.
The Snake River and its water quality upstream of Hells Canyon
The Snake River rises in Yellowstone National Park in eastern Idaho and northwestern Wyoming. Its headwaters support a healthy ecosystem and world-class trout fishing. However, the Snake’s character and condition change dramatically in its 1,078-mile journey west across southern Idaho, then north along the Idaho/Oregon border to its confluence with the Columbia River in southeast Washington.
The Snake is dammed, diverted, and dewatered at multiple points in southern Idaho for agricultural, industrial, and municipal use. It is partially replenished between dams by municipal and industrial discharges, urban runoff, irrigation return flows, and springs. These sources are hardly pristine.
Agricultural runoff and irrigation return flows are particular concerns because they are high in volume and generally high in nitrogen and phosphorus. Today, even some springs in the basin have high levels of nitrogen and phosphorus, because their groundwater sources have been polluted by surface activities over decades.
As the Snake turns north and approaches Hells Canyon it is joined by eastern Oregon tributaries that contribute similar pollution loads from runoff, irrigation return flows and other sources.
There are several reasons organic methylmercury is even more harmful to ecosystems and people than elemental mercury. They include:
- Toxicity: It is a powerful neurotoxin and endocrine disruptor with known neurological, cardiovascular, and reproductive health effects.
- Bioaccumulation: It accumulates rapidly in organisms and ecosystems.
- Bioavailability: Unlike elemental mercury, it is easily absorbed into the human body through ingestion, inhalation, and skin contact.
- Persistence: It does not break down easily in the environment or the human body, so it can have serious long-term effects on ecosystems and people.
- Exposure: Fish - especially long-lived predatory fish - from contaminated waterbodies accumulate high levels of methylmercury. Many are eaten by people around the world.
Children, infants and especially the unborn are extremely vulnerable to methylmercury. Because their brains and nervous systems are still developing, their exposure to small amounts can have serious long-term effects on vision, cognition, memory, attention, fine motor skills, and language.
In adults, moderate levels of mercury can lead to irritability, tremors, and problems with vision, hearing and memory. High levels can lead to dementia, cerebral palsy, deafness, blindness, sensory and motor impairment, and even death.
Fish consumption considerations
Fish contain essential nutrients and omega-3 fatty acids, and are low in saturated fat. They are the foundation of many of the world’s healthiest diets.
Many fish from many freshwaters are safe to eat regularly. Methylmercury accounts for a very large percentage of those that are not.
There is no “good” level of methylmercury in our bodies. Less is always better than more. However, for most people, concerns about consumption of fish with minute amounts of methylmercury are outweighed by their interest in the considerable health benefits of a well-balanced diet that includes fish.
Human health risks increase in proportion to the concentrations of methylmercury in fish eaten and the amounts of those fish consumed over time. Some populations are more at risk than others. Risks are more pronounced for individuals and groups whose diets and cultures are more dependent on fish.
Where methylmercury contamination cannot be reduced enough for people to eat fish safely, the public must be warned. Warnings from public health agencies take different forms. Some are general warnings for the general public. Others are specific warnings for particular populations, for consumption of particular species or sizes of fish, or for the frequency and amounts of fish consumed.
Warnings, however, are not enough. They should be calls to action. Every effort should be made to understand and then to reduce or eliminate methylmercury problems wherever possible, as quickly as possible.
The Snake River and its water quality upstream of Hells Canyon
The Snake River rises in Yellowstone National Park in eastern Idaho and northwestern Wyoming. Its headwaters support a healthy ecosystem and world-class trout fishing. However, the Snake’s character and condition change dramatically in its 1,078-mile journey west across southern Idaho, then north along the Idaho/Oregon border to its confluence with the Columbia River in southeast Washington.
The Snake is dammed, diverted, and dewatered at multiple points in southern Idaho for agricultural, industrial, and municipal use. It is partially replenished between dams by municipal and industrial discharges, urban runoff, irrigation return flows, and springs. These sources are hardly pristine.
Agricultural runoff and irrigation return flows are particular concerns because they are high in volume and generally high in nitrogen and phosphorus. Today, even some springs in the basin have high levels of nitrogen and phosphorus, because their groundwater sources have been polluted by surface activities over decades.
As the Snake turns north and approaches Hells Canyon it is joined by eastern Oregon tributaries that contribute similar pollution loads from runoff, irrigation return flows and other sources.
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One result of the Snake’s cumulative pollution loads is that it is now unsafe to swim in or eat fish from many sections of the river; in fact, Idaho now warns people not even to expose their pets to some sections. Our allies at the Idaho Conservation League have recently launched a long-term campaign to solve the Snake River’s serious pollution problems upstream of Hells Canyon.
Another result is that the Snake is primed for the creation of serious methylmercury problems as it enters the Hells Canyon reservoirs – problems that the Nez Perce Tribe, Pacific Rivers, and our allies at Idaho Rivers United are working to resolve. |
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Hells Canyon and its three Snake River reservoirs
Hells Canyon is a world wonder. Two thousand feet deeper than the Grand Canyon and spanning over 1,000 square miles, it is home to an astonishing variety of microclimates, habitats, and species.
Hells Canyon is a world wonder. Two thousand feet deeper than the Grand Canyon and spanning over 1,000 square miles, it is home to an astonishing variety of microclimates, habitats, and species.
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There are three dams on the Snake River in Hells Canyon. Licensed by the United States in the mid-1950s and built between 1958 and 1967, the reservoirs created by the Brownlee, Oxbow and Hells Canyon Dams span about 90 river miles. Together they comprise the Hells Canyon Complex, a hydroelectric project operated by the Idaho Power Company.
The bottom sediments of these reservoirs contain mercury from natural sources in the Snake River Basin, from human sources in Idaho and Oregon that include numerous legacy mining sites, and from watershed deposition of airborne mercury from sources around the globe. From the mercury sequestered in their bottom sediments, the three Hells Canyon reservoirs generate a great deal of mercury’s most toxic form. |
Methylmercury production in Hells Canyon reservoirs
How much? As early as 2004, the Idaho and Oregon Departments of Environmental Quality (IDEQ and ODEQ) stated that fish tissue samples for methylmercury already exceeded Idaho and Oregon levels of concern by 52% and 80%, respectively.
How much? As early as 2004, the Idaho and Oregon Departments of Environmental Quality (IDEQ and ODEQ) stated that fish tissue samples for methylmercury already exceeded Idaho and Oregon levels of concern by 52% and 80%, respectively.
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Fourteen years later, a 2018 report by the ODEQ found that the amount of methylmercury found in fish tissue in and downstream of the Hells Canyon reservoirs far exceeded standards for protection of human health. It stated that the average level of mercury found in fish tissue samples in Brownlee Reservoir was eight times Oregon’s standard, and that downstream of Hells Canyon dam it was more than ten times the standard. Why? When the Snake’s high loads of phosphorus and other pollutants enter the still, shallow waters of the upper portion of Brownlee Reservoir, massive algae blooms develop and deplete dissolved oxygen levels. That jumpstarts the transformation of mercury into methylmercury. In the blazing summer sun in the depths of Hells Canyon, surface temperatures of all three reservoirs rise sharply. The reservoirs then stratify distinctly – and methylmercury production explodes in the deep hypoxic zone. Mercury levels that are too low to detect in water even with today’s state-of-the-art methods can still be high enough to cause very serious problems in the food chain. That is why Oregon and Idaho have both set additional water quality standards for concentrations of methylmercury in fish tissues. For contaminants such as mercury that are difficult or impossible to measure directly, fish tissue sampling is a proven and cost-effective method. Idaho’s current fish-tissue standard is 0.3 milligrams per kilogram (mg/kg). Oregon’s standard of 0.04 mg/kg is more stringent. This is because Oregon’s policy is to take into consideration the many individuals and groups who eat more fish and shellfish than the average person. Oregon’s standard is intended to protect people who consume up to 175 grams of fish per day on average (about 23 typical moderate fish meals a month), which is common for many anglers and especially for Tribal populations whose diets and cultures revolve around fish. Methylmercury concentrations in fish tissue samples in and downstream of the Hells Canyon reservoirs regularly exceed the standards of both Oregon and Idaho. When the fall turnover occurs in the reservoirs, rapid mixing of warm surface and cold, hypoxic deep waters allows methylmercury to enter the food chain. As it moves up the chain it reaches extremely dangerous levels in the predatory and long-lived fish of the canyon, including bass, catfish, and sturgeon. |
Algae bloom in the Brownlee Reservoir. Courtesy of Lorraine Backer / Centers for Disease Control and Prevention public health image library.
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Today, the Nez Perce reservation is a small fraction of the historic homelands of the Nimiipuu people.
Map from National Parks Service. |
Methylmercury in the water column is discharged through the dams and flows far downstream. Fish tissue samples have shown high concentrations at least as far downstream as the Snake’s confluence with the Salmon River, about 60 miles below Hells Canyon Dam. Samples taken between 2006 and 2019 showed that smallmouth bass 12 inches or longer had levels ranging from 7 to 19 times the Oregon standard. Large white sturgeon had levels as much as 75 times the standard. Such levels are cause for serious concern for all anglers in the region. For the Nez Perce Tribe, they are much more than that: they are an existential threat to their Treaty rights and way of life. Rights, interests and concerns of the Nez Perce Tribe The historic territory of the Nez Perce stretched from the Bitterroot Mountains on the east to the Blue Mountains on the west. It included large parts of what is today northeastern Oregon, eastern Washington, the panhandle of Idaho, and northwestern Montana. The Snake River in Hells Canyon is at its heart. The 1855 Treaty between the United States and the Nez Perce recognized and promised to protect certain rights necessary to maintain the Tribe’s culture and way of life, including the right to take and eat fish at all of the Tribe’s “usual and accustomed places.” Historically, over one million Pacific salmon and steelhead spawned and reared upstream of Hells Canyon. |
Nevertheless, the original federal license for the Hells Canyon dams required no fish passage. Over the last six decades, these dams have eliminated anadromous salmon and steelhead runs above them. They have also fragmented the habitats of Snake River migratory fish including sturgeon, rainbow trout, and endangered bull trout.
The dams have also modified the temperature regime of the Snake River downstream. Summer river temperatures there are lower than before. Fall temperatures are higher, regularly reaching levels harmful or even lethal to returning salmon and steelhead.
A third insult to the rights of the Nez Perce is that methylmercury produced behind the dams has poisoned the resident fish of Hells Canyon. It has harmed their people, birds and other wildlife that depend on eating fish from the river.
Over more than six decades, these effects of the Hells Canyon dams have shattered the promises of the United States to the Nez Perce Tribe. It’s time to renew those promises.
Clean Water Act action
It’s also time to renew our commitment to the Clean Water Act’s overall goal to “restore and maintain the chemical, physical and biological integrity of the Nation’s waters” for the benefit of all people.
The Clean Water Act was passed by Congress in 1972. By that time the mercury contamination problem in and downstream of the Hells Canyon reservoirs was already known.
The dams have also modified the temperature regime of the Snake River downstream. Summer river temperatures there are lower than before. Fall temperatures are higher, regularly reaching levels harmful or even lethal to returning salmon and steelhead.
A third insult to the rights of the Nez Perce is that methylmercury produced behind the dams has poisoned the resident fish of Hells Canyon. It has harmed their people, birds and other wildlife that depend on eating fish from the river.
Over more than six decades, these effects of the Hells Canyon dams have shattered the promises of the United States to the Nez Perce Tribe. It’s time to renew those promises.
Clean Water Act action
It’s also time to renew our commitment to the Clean Water Act’s overall goal to “restore and maintain the chemical, physical and biological integrity of the Nation’s waters” for the benefit of all people.
The Clean Water Act was passed by Congress in 1972. By that time the mercury contamination problem in and downstream of the Hells Canyon reservoirs was already known.
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Pacific Rivers, Idaho Rivers United and our allies have long urged state and federal agencies to take a much harder look at the methylmercury problem and other problems that are either caused or exacerbated by the Hells Canyon Dams. In 2019, we took an additional step – to court. Idaho Power Company’s federal operating license for the Hells Canyon Dams expired in 2005. Since then, while seeking a new 50-year license, the company has been operating under a series of annual licenses issued by the Federal Energy Regulatory Commission (FERC). |
The enormous Hells Canyon dam completely blocks fish passage.
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Section 401 of the Clean Water Act (CWA) requires states to certify that proposed FERC licenses will not violate their water quality standards. Oregon and Idaho standards for temperature and methylmercury are violated persistently and significantly in and downstream of the Hells Canyon reservoirs.
The CWA requires that a Total Maximum Daily Load (TMDL) of pollutants be established for waterbodies where state water quality standards cannot be met through typical regulatory control of one or a few direct pollution sources. The TMDL sets an upper limit for the pollutants in question – a sort of “pollution budget” – that then drives decisions about management changes necessary to bring problems under enough control for water quality standards to be met and maintained.
Under the CWA, a state certification of a proposed FERC license is to provide reasonable assurance that state water quality standards will be met. In 2019, the Oregon Department of Environmental Quality (ODEQ) issued a CWA Section 401 certification for the three Hells Canyon Dams. It was inadequate. It failed to specify how much methylmercury would have to be reduced or a clear timeframe for reductions. It also failed to provide reasonable assurance that violations of the temperature standard would end.
Pacific Rivers and Idaho Rivers United petitioned for judicial review of the ODEQ certification. Our petition was later consolidated by the court with a similar one brought by the Nez Perce Tribe. Negotiations with ODEQ, the Oregon Department of Natural Resources (ODNR), and Idaho Power ensued.
Agreement
In 2021, Pacific Rivers, Idaho Rivers United, and the Nez Perce Tribe entered into a joint Settlement Agreement with Oregon. The Agreement calls for changes in dam operations to provide water temperatures safe for salmonids downstream; for reintroduction of spring Chinook salmon and summer steelhead in Pine Creek above Hells Canyon Dam; and for the establishment of a methylmercury TMDL for the Snake River and its reservoirs.
As part of the Agreement, the State of Oregon and Idaho Power Company contributed a combined $1.5 million toward further scientific studies of the Hells Canyon methylmercury problem. Those studies, which commenced in 2014, are now being completed by a large interagency technical team led by the U.S. Geological Survey (USGS). The technical team formally includes representatives of the Nez Perce Tribe. Important papers already published by the technical team are listed below with other sources for this article.
Pacific Rivers and Idaho Rivers United representatives serve on the advisory team. Our representative is longtime Board member and attorney Bryan Lessley.
The science team’s specific findings and analysis of potential management scenarios will inform regulatory and management decisions for the Snake River upstream of, in, and downstream of Hells Canyon. Their more general insights will have implications for many waterbodies with methylmercury problems across the country and around the world.
Next steps
We expect that within the next few months USGS will complete its predictive model for methylmercury management scenarios in and downstream of Hells Canyon. Our Settlement Agreement calls for ODEQ to:
- Release a draft Snake River Methylmercury TMDL for public comment within three months after receiving the USGS model, and
- Finalize the TMDL within eight months after receiving the model.
After the TMDL is finalized, regulatory and management actions by ODEQ, ODFW and Idaho Power Company will begin. After that, monitoring, evaluation, and adaptive management activities are to be conducted regularly in the decades ahead to ensure that steady progress is made toward the Nez Perce Tribe’s long-term goal. That goal is spelled out in our joint Settlement Agreement:
“…Treaty-reserved aquatic resources continue to inhabit the Snake River within and downstream of the [three-dam] Complex and are safe to support Treaty-reserved rights to harvest and consume fish within and below the Complex no later than the year 2045 and at levels at least as protective as Oregon’s human-health criteria reflecting a per-capita fish consumption rate of 175 grams per day at a risk level of 10-6.”
Finally, after more than 50 years, meaningful action toward this clearly defined goal is to begin soon. We will stay engaged in the Snake River process until all the agreed-upon actions are taken and all agreed-upon goals for the Snake River and the Nez Perce Tribe are met.
As always, Pacific Rivers is working today at the intersection of justice, science, policy, and law to protect and improve Northwest rivers for the benefit of fish, wildlife, and all people.
FOR MORE ON THE TOPICS OF THIS ARTICLE:
Mercury Pollution: The Global Context (US EPA) https://www.epa.gov/international-cooperation/mercury-emissions-global-context
US EPA Fish and Shellfish Advisories and Safe Eating Guidelines https://www.epa.gov/choose-fish-and-shellfish-wisely/fish-and-shellfish-advisories-and-safe-eating-guidelines
World Health Organization Fact Sheet: Mercury and Health https://www.who.int/news-room/fact-sheets/detail/mercury-and-health
Snake River Water Quality in the Spotlight (Idaho Conservation League) https://www.idahoconservation.org/blog/snake-river-water-quality/
Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon (USGS) https://pubs.usgs.gov/fs/2016/3051/fs20163051.pdf
Seasonal Dynamics and Interannual Variability in Mercury Concentrations and Loads through a Three-Reservoir Complex (Environmental Science and Technology) https://pubs.acs.org/doi/epdf/10.1021/acs.est.9b07103
In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir (Environmental Science and Technology) https://pubs.acs.org/doi/pdf/10.1021/acs.est.2c03958
Mercury Pollution: The Global Context (US EPA) https://www.epa.gov/international-cooperation/mercury-emissions-global-context
US EPA Fish and Shellfish Advisories and Safe Eating Guidelines https://www.epa.gov/choose-fish-and-shellfish-wisely/fish-and-shellfish-advisories-and-safe-eating-guidelines
World Health Organization Fact Sheet: Mercury and Health https://www.who.int/news-room/fact-sheets/detail/mercury-and-health
Snake River Water Quality in the Spotlight (Idaho Conservation League) https://www.idahoconservation.org/blog/snake-river-water-quality/
Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon (USGS) https://pubs.usgs.gov/fs/2016/3051/fs20163051.pdf
Seasonal Dynamics and Interannual Variability in Mercury Concentrations and Loads through a Three-Reservoir Complex (Environmental Science and Technology) https://pubs.acs.org/doi/epdf/10.1021/acs.est.9b07103
In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir (Environmental Science and Technology) https://pubs.acs.org/doi/pdf/10.1021/acs.est.2c03958
Settlement Agreement between Conservation groups, the Nez Perce Tribe, and Oregon DEQ
