|
Algae bloom, courtesy of Lorraine Backer / Centers for Disease Control and Prevention Public Health Image Library
Fish and other aquatic life in the Snake River in and downstream of three Hells Canyon reservoirs are contaminated with mercury. Levels in many fish exceed state standards for safe human consumption. Fish-eating mammals and birds are also being contaminated. Levels of mercury in some fish are so high that they have been described as “swimming Superfund sites.”
This bad news is not new news. For more than fifty years, officials have known that we have a mercury problem in this portion of the Snake River. The questions have been why, how much, and what to do about it. Today’s good news is that plenty is now known to take action to solve the problem. In fact, so much is known now that inaction would be inexcusable. In this article, we review results of a series of recent scientific studies that have shed much light on Snake River mercury issues. Spoiler alert: This is not your typical pollution story. It’s not about a single polluter or pollution source. It’s about a complex interplay among water quality conditions, physical forces, biological processes and structures constructed by humans that together have created a serious problem that’s greater than the sum of its parts. It’s about a problem that must be solved, but that can’t be solved overnight. But it can be solved. Science has now provided ample perspective and data to do so. The Clean Water Act provides the framework to do so. We now know how to get started, measure progress, and proceed until the problem is solved. |
The Hells Canyon Complex consists of the Hells Canyon, Oxbow, and Brownlee dams and reservoirs. Map from USGS.
|
One of Pacific Rivers’ six strategic goals is Snake River fish safe for all to eat. Since it will take considerable time to reach it, we are adopting interim goals along the way. For the next two years, they are:
1. A much better public understanding of the nature, sources, current levels, and fate of mercury and other pollutants in and downstream of three Hells Canyon dams and their reservoirs.
2. An official pollution reduction target and action plan adopted in 2025 to make good progress beginning immediately in order to make Snake River fish safe for all to eat by 2045.
As a first step to increasing public awareness of this issue, we provided an article in last year’s Fall Free Flow that addressed ten topics:
1. A much better public understanding of the nature, sources, current levels, and fate of mercury and other pollutants in and downstream of three Hells Canyon dams and their reservoirs.
2. An official pollution reduction target and action plan adopted in 2025 to make good progress beginning immediately in order to make Snake River fish safe for all to eat by 2045.
As a first step to increasing public awareness of this issue, we provided an article in last year’s Fall Free Flow that addressed ten topics:
The diet of a river otter consists mainly of aquatic organisms, including plenty of fish. Photo by Jon Nelson.
|
1. Mercury pollution sources
2. Transformation of mercury into methylmercury 3. Health effects of methylmercury 4. The Snake River and its water quality upstream of Hells Canyon 5. Hells Canyon and its three Snake River reservoirs 6. Methylmercury production in Hells Canyon reservoirs 7. Rights, interests and concerns of the Nez Perce Tribe 8. Clean Water Act action 9. Agreement 10. Next steps |
Science Review
Over the past decade, scientists with the U.S. Geological Survey (USGS) and a number of partner agencies, universities and firms have conducted extensive research into Hells Canyon mercury pollution issues. Seven published papers listed at the end of this article have documented much of their work. Some were published within the last twelve months. In this article, we summarize these papers and our main conclusions by five major questions they helped answer:
We conclude with a look at the needed official actions and what Pacific Rivers will do to help bring them about in the next year.
Over the past decade, scientists with the U.S. Geological Survey (USGS) and a number of partner agencies, universities and firms have conducted extensive research into Hells Canyon mercury pollution issues. Seven published papers listed at the end of this article have documented much of their work. Some were published within the last twelve months. In this article, we summarize these papers and our main conclusions by five major questions they helped answer:
- How much of a problem is there?
- How does reservoir stratification influence fish mercury concentrations?
- How does methylmercury form, then move into and up the food chain?
- How do these processes unfold in the Hells Canyon Complex?
- Where are the pollutants in this system coming from?
We conclude with a look at the needed official actions and what Pacific Rivers will do to help bring them about in the next year.
|
1. How much of a problem is there?
Mercury sources, distribution, forms and health effects A 2016 publication “Mercury in Western North America” documented widespread mercury in the region. Some watersheds have much more than others, due to their widely varying geologic, mining and industrial histories. But all have at least some mercury today, thanks in large part to depositions to land and water of mercury emitted to the atmosphere by coal-burning power plants and other industrial facilities around the world. |
Geospatially distributed (total) mercury emissions to air from anthropogenic sources in 2015 (g/km2/a) from all sectors. Figure from US EPA.
|
Elemental mercury, commonly known as quicksilver, is pure mercury in its metallic form. Highly toxic, it is a liquid at room temperature that readily vaporizes into an odorless gas.
Inorganic mercury is created when mercury bonds with other elements, creating compounds such as mercury chloride or mercury sulfide. Some such forms can move easily through river systems in suspended sediments and then settle in lakes or reservoirs.
Methylmercury forms in some waterbodies, especially certain types of lakes and reservoirs. It is mercury’s most dangerous form. Reasons include:
Children, infants and especially the unborn are particularly vulnerable to methylmercury. Because their brains and nervous systems are still developing, their exposure to small amounts can have very serious long-term effects on vision, cognition, memory, attention, fine motor skills, and language.
In adults, moderate levels 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.
Mercury in fish in and below the Hells Canyon Complex
The Hells Canyon Complex (HCC) consists of three dams that were licensed by the United States in the 1950s.
The three dams of the HCC – Brownlee, Oxbow, and Hells Canyon – were constructed between 1958 and 1967. In the Hells Canyon Complex, mercury in various forms from multiple sources is transformed into methylmercury at very high rates.
Inorganic mercury is created when mercury bonds with other elements, creating compounds such as mercury chloride or mercury sulfide. Some such forms can move easily through river systems in suspended sediments and then settle in lakes or reservoirs.
Methylmercury forms in some waterbodies, especially certain types of lakes and reservoirs. It is mercury’s most dangerous form. Reasons include:
- Toxicity: It is a powerful neurotoxin and endocrine disruptor with known neurological, cardiovascular, and reproductive health effects.
- Persistence: It does not break down easily in the environment or the human body.
- Bioavailability: It is easily absorbed into the human body through ingestion, inhalation, and skin contact.
- Bioaccumulation: It accumulates rapidly in organisms and ecosystems.
- Exposure: Fish from contaminated waterbodies – especially long-lived predatory fish – can accumulate extremely high levels of methylmercury.
Children, infants and especially the unborn are particularly vulnerable to methylmercury. Because their brains and nervous systems are still developing, their exposure to small amounts can have very serious long-term effects on vision, cognition, memory, attention, fine motor skills, and language.
In adults, moderate levels 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.
Mercury in fish in and below the Hells Canyon Complex
The Hells Canyon Complex (HCC) consists of three dams that were licensed by the United States in the 1950s.
The three dams of the HCC – Brownlee, Oxbow, and Hells Canyon – were constructed between 1958 and 1967. In the Hells Canyon Complex, mercury in various forms from multiple sources is transformed into methylmercury at very high rates.
Mercury fish tissue content in and downstream of the Hells Canyon Complex in 2013. Figure from USGS.
|
A 2016 USGS report summarized what was known at that time. It included a graphic illustrating results from fish tissue sampling in 2013. Smallmouth bass from Brownlee, Oxbow and Hells Canyon Reservoirs, as well as from the Snake River downstream, were sampled. Almost all significantly exceeded Oregon’s state standard for safe fish consumption. Concentrations were highest – as much as 25 times the standard – just above Hells Canyon Dam. They remained very high – as much as 10 times the standard – at the downstream end of the study area at the Snake/Salmon River confluence, nearly 60 miles below Hells Canyon dam.
Smallmouth bass are usually chosen as the species for tissue sampling in the Snake because they are abundant, non-native fish that grow quickly and are caught and eaten by many people. However, we know that larger, longer-lived species such as catfish and sturgeon can accumulate even more mercury than bass. Some exceed Oregon’s standard well more than one hundred fold. |
Our conclusions: The mercury problem in and below the HCC is acute. It is a real and present danger to fish, wildlife and people. It must be brought under control. This is an ecosystem issue, a human health issue, and a justice issue.
2. How does reservoir stratification influence fish mercury concentrations?
|
Stratification is the development of distinct layers of different water temperatures in lakes and reservoirs during warm weather. Some stratify much more often and distinctly than others. Highly stratified waters have a warm layer with high levels of dissolved oxygen on top, a cold layer with much less dissolved oxygen on bottom, and a relatively thin layer in the middle where temperatures drop quickly. This middle layer – the “thermocline” – is well known to divers and anglers. It concentrates much of a waterbody’s aquatic life in summer, because temperatures above are too high for many species, and oxygen levels below are too low for almost all.
It has long been known that fish in some lakes and reservoirs that stratify have unusually high levels of methylmercury. Recent studies have explored where and how much. |
A layer of fish within the thermocline. Photo from Carpology.
|
|
One published just last December looked at a 536-mile section of the Snake River with more than a dozen reservoirs. (See map. The study area was between the red line above American Falls and the one well downstream of Hells Canyon Dam and the Salmon River confluence). It examined fish mercury concentrations in
|
Figure above and map below from “Reservoir Stratification Modulates the Influence of Impoundments in Fish Mercury Concentrations along an Arid Land River System”
Major mainstream dams on the Snake River.
|
The findings were eye-opening:
Our conclusions: It’s now very clear that dams on the Snake River make mercury problems worse in and below their reservoirs, especially if those reservoirs stratify. This needs to be taken into account when dams and reservoirs on the Snake and elsewhere are proposed, developed, managed, relicensed, or considered for removal. |
3. How does methylmercury form, then move into and up the food chain?
The primary way mercury becomes mobilized and then concentrated to dangerous levels in organisms is methylation – transformation into methylmercury – in aquatic ecosystems. This occurs in still or slow-moving water with high levels of nutrients and organic material that cause algae blooms. It also occurs in the low-oxygen zones of waterbodies that stratify in summer.
It has long been widely thought that methylation in waterbodies occurs mainly near the bottom, at the “sediment-water interface.” Recent studies have shown that it actually occurs throughout the water column in cold, low-oxygen zones in the HCC and elsewhere.
The primary way mercury becomes mobilized and then concentrated to dangerous levels in organisms is methylation – transformation into methylmercury – in aquatic ecosystems. This occurs in still or slow-moving water with high levels of nutrients and organic material that cause algae blooms. It also occurs in the low-oxygen zones of waterbodies that stratify in summer.
It has long been widely thought that methylation in waterbodies occurs mainly near the bottom, at the “sediment-water interface.” Recent studies have shown that it actually occurs throughout the water column in cold, low-oxygen zones in the HCC and elsewhere.
A duck cuts through dense algae bloom on the surface of the water.
Photo by Alexei Vinogradov. 
Osprey with fish dinner.
Photo by Mike Baird. |
It has also been thought that methylation is driven entirely by a small number of extremophile bacteria. Recent studies have shown that to be false as well; a wider range of bacteria than previously thought can generate methylmercury from various types of inorganic mercury in low-oxygen conditions.
In aquatic food webs, methylmercury can bioaccumulate in piscivorous (fish-eating) fish to concentrations more than one million times greater than that of the waters in which they swim. This is one reason that more than 80% of the fish-consumption advisories posted in the United States and Canada are partly or entirely attributed to mercury. Methylmercury is not only a threat to humans who eat fish. It is also a threat to wildlife that feed on fish, as well as to birds that feed on emergent insects with aquatic larval stages. Recent studies have shown high mercury levels in songbirds, shorebirds and bats that feed around waterbodies with methylmercury problems. Levels are very likely high in other fish-eating wildlife such as eagles, osprey and otters. Our conclusions:
4. How do these processes unfold in the Hells Canyon Complex? The pollution loads of the Snake River would be serious even if the Hells Canyon Complex did not exist. The HCC dams and reservoirs greatly magnify the Snake’s mercury problems. Hypoxic (very low-oxygen) conditions are created each spring as the Snake’s abundant loads of sediments, nutrients and organic material settle to the bottom of the upstream portion of Brownlee Reservoir, resulting in Brownlee’s infamous algae blooms. As spring progresses, a pocket of water with virtually no oxygen – an anoxic zone – develops. |
|
In the blazing sun and intense heat of Hells Canyon, its three reservoirs stratify distinctly, with a growing layer of warm water floating on the cold, low-oxygen, higher-density layer below. As the graphic here illustrates, the anoxic zone at the head of Brownlee grows to an immense volume in the deepest water of the reservoir all the way to Brownlee dam. Devoid of oxygen for months each year, this zone provides ideal conditions, space and time for bacteria to convert inorganic mercury into methylmercury.
Because almost all aquatic life is concentrated in the oxygenated layers above during summer, there is little uptake of methylmercury from the anoxic zone into the food web then. But when stratification breaks down in the fall, rapid mixing of the layers occurs, and prodigious amounts of methylmercury are released throughout the reservoir and into the food web. Much is first taken up by tiny invertebrates in Brownlee. A great deal also makes its way via suspended particles and microorganisms through Brownlee Dam to Oxbow Reservoir, then Hells Canyon Reservoir, and finally to the Snake River below. As it moves through the system each fall, it bioaccumulates to much higher concentrations in larger organisms such as crayfish and fish. Some of the highest concentrations in fish are found in Hells Canyon Reservoir and the Snake River below. Some long-lived fish-eating fish, including sturgeon, eventually become the aforementioned “swimming Superfund sites.” The larger the anoxic zone and the longer it lasts, the more methylation there is. The reverse is also true. One current estimate is that reducing the average volume and duration of Brownlee Reservoir’s annual anoxic zone by ⅓ would reduce methylation by ⅔ or more. Whatever the exact figures prove to be, limiting anoxia will surely be key to solving the Snake’s mercury problems. Our conclusions: To solve the mercury problem in and downstream of the HCC, we must reduce incoming mercury from the Snake River and its tributaries to the extent possible. But we must also reduce the other incoming pollutants that cause or contribute to the formation, size and duration of Brownlee Reservoir’s annual anoxic zone. Other measures may well be needed, but these two are clearly essential. 5. Where are the pollutants in this system coming from? |
Figure from USGS illustrating oxygen levels in Brownlee reservoir.
Cliff Swallow in Southeastern Oregon.
Photo by Vickie J. Anderson. 
White Sturgeon can live to over 100 years of age and have been around for 175 million years. Photo by ODFW.
|
Results from an in-depth study published in late 2023 shed much new light on this question. Detailed water quality and flow data for the Snake River were first compiled for two points:
1. The Snake’s entrance to Brownlee Reservoir, and
2. 101 miles upstream at Melba, Idaho.
Comparisons of the two datasets provided a crucial insight. Between those two points, the flow of the Snake increased by 70% – but inorganic mercury loads increased 500%.
1. The Snake’s entrance to Brownlee Reservoir, and
2. 101 miles upstream at Melba, Idaho.
Comparisons of the two datasets provided a crucial insight. Between those two points, the flow of the Snake increased by 70% – but inorganic mercury loads increased 500%.
HCC pollution sources study area.
Map from USGS. 
|
This begged the question: What could account for the highly disproportionate pollution impacts of the tributaries in this relatively short segment of the Snake River (which, after all, is hardly pristine even at Melba)?
The major tributaries to the Snake in this reach are the Owyhee, Boise, Malheur, Payette and Weiser Rivers. Relative to the Snake River at Melba, most had significantly higher concentrations of total suspended solids and inorganic mercury (up to 16- 27-fold, respectively). This accounted for much but not nearly all of the additional mercury and suspended solids at Brownlee. What could account for the rest? Twenty-two irrigation drains were then sampled during the irrigation season. Concentrations in irrigation drains, relative to the Snake River at Melba, averaged 45 times as much Total Suspended Solids and 65 times as much inorganic mercury. Our conclusions:
What is to be done? Efforts to clean up the Snake River must quickly expand and accelerate. Benefits will include hastening the day when Snake River fish are safe for all to eat. But they will not be limited to that. Many of the things we need to do to control mercury pollution in the Snake will also make its tributaries better habitat for fish and wildlife, safer for swimming and boating, better for fishing, and more beneficial to watershed communities’ economies and quality of life. Crucially, they will also begin to mitigate some of the many injustices visited on the Nez Perce Tribe since its treaty with the United States in 1855. |
The Clean Water Act requires state water quality agencies to adopt a Total Maximum Daily Load (TMDL) of pollutants for waterbodies where state water quality standards cannot be met through typical regulatory control of one or a few direct pollution sources – in other words, in situations like the one today in the Snake River in and below Hells Canyon. The TMDL sets an upper limit for the pollutants in question – a sort of “pollution budget” – that then drives decisions about management actions necessary for water quality standards to be met and maintained.
|
In 2019, the Nez Perce Tribe, Pacific Rivers and Idaho Rivers United objected to a draft state water quality certification required for the renewal of the federal operating license of the HCC. Our objection was that it did not provide the required assurance that state water quality standards for mercury and temperature would be met. In 2021 – after going to court over the issue – our three parties signed a Settlement Agreement with the state of Oregon that included the following specific goal regarding the mercury problems in and downstream of the three reservoirs:
“...Treaty-reserved aquatic resources continue to inhabit the Snake River within and downstream of the [Hells Canyon] 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.” |
The Nimiipuu, also known as the Nez Perce people of the Columbia Plateau, harvest fish along the Snake River. Photo by Shane Anderson.
|
That is a precise way to state our strategic goal: Snake River fish safe for all to eat. To be clear: by “all” we mean all people – especially those, beginning with the Nez Perce, whose health, cultures and/or livelihoods depend on a diet that includes Snake River fish. We also mean all wildlife of the Snake River and its environs.
With the needed scientific work now wrapping up, the Oregon Department of Environmental Quality must now prepare to issue a methylmercury TMDL for the Snake River in and downstream of Hells Canyon. It must be accompanied by a sound water quality management plan to make steady progress that begins in earnest right away.
Pacific Rivers will monitor this process closely and participate actively in the upcoming public comment period. We will insist on:
1. adherence to Oregon’s fish tissue standard for methylmercury;
2. commitment to its achievement no later than 2045; and
3. a water quality management plan designed to make consistent progress toward the goal.
These items are required and non-negotiable.
We will also advocate for:
1. a robust monitoring program to confirm that adequate progress is being made;
2. a firm commitment to adaptive management – using monitoring data to drive decisions about whether and when to take additional, expanded or accelerated actions to stay on track; and
3. annual progress reports to the public.
These items are good practices that will guide progress and provide accountability.
Making Snake River fish safe for all to eat will take considerable time. That’s all the more reason to get started now. Please stay tuned for updates and action alerts from Pacific Rivers as important decisions are made.
With the needed scientific work now wrapping up, the Oregon Department of Environmental Quality must now prepare to issue a methylmercury TMDL for the Snake River in and downstream of Hells Canyon. It must be accompanied by a sound water quality management plan to make steady progress that begins in earnest right away.
Pacific Rivers will monitor this process closely and participate actively in the upcoming public comment period. We will insist on:
1. adherence to Oregon’s fish tissue standard for methylmercury;
2. commitment to its achievement no later than 2045; and
3. a water quality management plan designed to make consistent progress toward the goal.
These items are required and non-negotiable.
We will also advocate for:
1. a robust monitoring program to confirm that adequate progress is being made;
2. a firm commitment to adaptive management – using monitoring data to drive decisions about whether and when to take additional, expanded or accelerated actions to stay on track; and
3. annual progress reports to the public.
These items are good practices that will guide progress and provide accountability.
Making Snake River fish safe for all to eat will take considerable time. That’s all the more reason to get started now. Please stay tuned for updates and action alerts from Pacific Rivers as important decisions are made.
KEY PAPERS SUMMARIZING STUDIES CONDUCTED
IN THE LAST DECADE ON SNAKE RIVER MERCURY ISSUES
Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon (USGS, 2016) https://www.usgs.gov/centers/idaho-water-science-center/science/mercury-cycling-hells-canyon-complex#publications
Mercury in western North America: A synthesis of environmental contamination, fluxes, bioaccumulation, and risk to fish and wildlife (USGS, Science of the Total Environment, October 2016) https://www.sciencedirect.com/science/article/abs/pii/S0048969716310245
Seasonal dynamics and interannual variability in mercury concentrations and loads through a three-reservoir complex (Environmental Science and Technology, July 15, 2020) https://pubs.acs.org/doi/10.1021/acs.est.9b07103
In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir (Environmental Science and Technology, 2022) https://www.usgs.gov/publications/reservoir-physical-processes-modulate-aqueous-and-biological-methylmercury-export-a
Metabolically diverse microorganisms mediate methylmercury formation under nitrate-reducing conditions in a dynamic hydroelectric reservoir (www.nature.com/isme, July 2023) https://www.nature.com/articles/s41396-023-01482-1
Reservoir Stratification Modulates the Influence of Impoundments in Fish Mercury Concentrations along an Arid Land River System (Environmental Science and Technology, 12/15/2023) https://pubs.acs.org/doi/10.1021/acs.est.3c04646
Mercury sources and budget for the Snake River above a hydroelectric reservoir complex (Science of the Total Environment, 1/10/24) https://www.usgs.gov/publications/mercury-sources-and-budget-snake-river-above-a-hydroelectric-reservoir-complex
IN THE LAST DECADE ON SNAKE RIVER MERCURY ISSUES
Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon (USGS, 2016) https://www.usgs.gov/centers/idaho-water-science-center/science/mercury-cycling-hells-canyon-complex#publications
Mercury in western North America: A synthesis of environmental contamination, fluxes, bioaccumulation, and risk to fish and wildlife (USGS, Science of the Total Environment, October 2016) https://www.sciencedirect.com/science/article/abs/pii/S0048969716310245
Seasonal dynamics and interannual variability in mercury concentrations and loads through a three-reservoir complex (Environmental Science and Technology, July 15, 2020) https://pubs.acs.org/doi/10.1021/acs.est.9b07103
In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir (Environmental Science and Technology, 2022) https://www.usgs.gov/publications/reservoir-physical-processes-modulate-aqueous-and-biological-methylmercury-export-a
Metabolically diverse microorganisms mediate methylmercury formation under nitrate-reducing conditions in a dynamic hydroelectric reservoir (www.nature.com/isme, July 2023) https://www.nature.com/articles/s41396-023-01482-1
Reservoir Stratification Modulates the Influence of Impoundments in Fish Mercury Concentrations along an Arid Land River System (Environmental Science and Technology, 12/15/2023) https://pubs.acs.org/doi/10.1021/acs.est.3c04646
Mercury sources and budget for the Snake River above a hydroelectric reservoir complex (Science of the Total Environment, 1/10/24) https://www.usgs.gov/publications/mercury-sources-and-budget-snake-river-above-a-hydroelectric-reservoir-complex
