Will analyze sediment behind dam for toxins
Tens of thousands of endangered salmon could access high-quality habitat in the Similkameen River watershed if Enloe Dam were removed, but any plans to demolish the dam would have to address arsenic contamination in sediment that’s collected behind the dam under deep gravel.
At a community meeting on Enloe Dam in Tonasket last week, federal, state and tribal biologists and hydrologists, along with representatives from the Colville Tribes, presented conclusions from preliminary sediment analysis and studies of the potential for steelhead and spring Chinook to reach habitat in British Columbia. The presentation also included a tribal perspective on the Similkameen River and regional watershed.
With abysmally low steelhead return rates to the Okanogan watershed in recent years — just 87 native-origin adults survived this year — “The significance of removing Enloe Dam is probably the only chance these animals have to survive,” Colville Confederated Tribes Biologist Chris Fisher said.
Research on fish habitat and sediment sampling have ramped up in the past three years, since the Okanogan County Public Utility District (PUD), which owns Enloe Dam, decided in 2018 that it wasn’t cost-effective to re-energize the dam to produce power, Fisher said.
As people explore what to do with the dam, which hasn’t produced power for more than 60 years, questions have accumulated about what’s in the sediment, how much it would cost to remove the dam and clean up any toxins, and who would pay for it, Fisher told the 70 people who came to learn more about Enloe. The PUD was invited to provide an update on the dam-safety inspections they’ve been conducting, but declined, Fisher said. A PUD commissioner attended the presentation, he said.
It’s too soon to say what it would cost to remove the dam, since that would depend on the nature of the contamination and how to deal with it safely, according to the presentation.
There are about 2.8 million cubic yards of sediment behind Enloe Dam, an amount that would fill 294,000 cement trucks, said Andrew Spanjer, a hydrologist with the U.S. Geological Survey (USGS).
USGS started taking sediment samples of silt, clay and sand at different depths in the river in 2019. They obtained more than 100 samples in all, including material in the riverbed below the gravel and cobble, where the oldest sediment has settled, Spanjer said. There’s been concern about the potential for contaminants because of mining upstream of the dam in the late 19th and early 20th centuries.
USGS analyzed the samples for 54 trace elements and metals, including copper, arsenic and mercury, which can all be toxic at elevated levels. They didn’t find high concentrations of copper, cadmium, lead or zinc. Mercury was far below the level that would cause concern and require clean-up, Spanjer said.
But arsenic concentrations in a third of the samples of fine sediment — particularly in deeper areas — exceed the threshold for toxicity, Spanjer said. The fine sediment is a small fraction of the total sediment impounded behind the dam. Most samples of coarser material didn’t have high concentrations of arsenic, he said.
Despite the high levels of arsenic, the fine sediment is so deep and under so much clean gravel that it’s not getting into the Similkameen River system, meaning that fish and people are protected, Washington Department of Ecology Toxicologist Arthur Buchan said. There’s no concern unless the material is disturbed, he said. Ecology started its own sampling up and downstream of the dam at the end of last year.
Ecology is looking not only at what’s in the sediment, but also at whether the contaminants would bind to the sediment or leach into surrounding soils. This analysis will help determine if they’d need to take steps to prevent the sediment from traveling downstream if the dam were removed. If the contaminants bind to the sediment, it would be easier to remove safely, Buchan said.
In addition to looking at contaminants from historic mining, the researchers are monitoring the impacts of active mining upstream of the dam in Canada, Spanjer said. They expect that data to be available in 2023.
Cost of removal
There are still too many unknowns for a sound estimate of what it would cost to remove Enloe. Depending on whether all or some of the sediment could be spread on adjacent land — or simply be allowed to flow downstream — removing the dam could cost from $51 million to $3.3 million, said Mike Brunfelt, a fluvial geomorphologist with Inter-Fluve, a firm that specializes in river restoration that’s been involved in the removal of about 200 dams.
Brunfelt’s estimates were based on the assumption that the sediment is clean and safe and wouldn’t need to be taken to a special disposal site. If it’s contaminated, costs could increase five to 10 times, Spanjer said. Allowing that volume of sediment, even if clean, to enter the river system would present its own problems for the river and fish, biologists say.
About 1,520 miles of the total 3,420 miles in the upper Similkameen watershed in British Columbia is accessible to fish, National Oceanic and Atmospheric Administration Fisheries Biologist Morgan Bond said. There are six routes over Similkameen Falls that spring Chinook and steelhead could navigate to reach the upper habitat, Bond said.
The Canadian habitat could sustain 3.9 million spring Chinook and 9.8 million steelhead at the parr stage, during which young salmon adapt to the river in preparation for their journey to the ocean. The upper watershed also provides good habitat for redds, where salmon deposit their eggs. It would support 77,863 chinook redds and 203,310 steelhead redds, Bond said.
While survival rates for salmon returning from the ocean to their natal river to spawn may look grim — less than 1% of upper Columbia salmon typically make it back — that still translates into between 7,800 and 47,000 adult spring Chinook and 29,000 to 118,000 steelhead, Bond said. Steelhead are stronger and more athletic than spring Chinook and therefore typically more successful in traveling over falls, Brunfelt said.
Two members of Aboriginal Outfitters shared a cultural perspective on the river. The nonprofit was formed last year to raise awareness of the importance of land stewardship to a community that has felt disconnected from the river, Aboriginal Outfitters Executive Director Joy Abrahamson said. Water and its central role in sustaining life have always been central to Indigenous people, who maintain sacred rituals to protect water from pollution, drought and waste, she said.
The Colville Tribes voted to support removal of the dam in 2017. The Upper and Lower Similkameen bands in British Columbia also back dam removal.
The Washington Department of Fish and Wildlife is analyzing biological, management and legal issues connected with potential dam removal and restoration of the Similkameen River through a proviso in the state budget. Their findings are due to the Legislature by Dec. 1.
A video of the presentation will be available online. The link will be on the Methow Valley News website once it’s been posted.