Assessment of Sediment Quality and Volume behind Enloe Dam
The U.S. Geological Survey (USGS) completed a pair of scientific studies to determine the volume and quality of sediment stored behind Enloe Dam in north-central Washington.
The Enloe Dam, a 100-year-old structure on the Similkameen River in north-central Washington, stopped producing hydropower in 1958. As resource managers consider options for the dam’s future, they need reliable information about the sediment that had built up behind it over the last century, including its chemical makeup and the total amount that would be released if the dam were removed.
USGS completed a pair of studies to answer these questions. The work assessed:
- The element concentrations and grain size of sediment in the reservoir, and
- The volume of sediment trapped behind the dam.
Motivation
For decades, sediment from the Similkameen River watershed, home to both historical and active mining, settled behind Enloe Dam. Until this project, scientists lacked up-to-date information about:
- Whether those sediments contained elevated levels of metals such as arsenic, silver, copper, and others that could affect water quality and aquatic habitat downstream.
- How much sediment had accumulated behind the dam since the last estimate in 1972.
Understanding sediment chemistry and volume is essential for evaluating potential dam removal and planning safe, effective habitat restoration.
What the USGS Did
The USGS carried out field surveys in 2019 and 2020 to:
- Collect and analyze surface and subsurface sediments along the 1.6‑mile (2.6‑kilometer) reservoir.
- Measure the concentration of trace elements and metals in both fine-grained and coarser material.
- Map the top of the sediment layer (using acoustic bathymetry) and the underlying bedrock–sediment boundary (using continuous resistivity profiling).
- Compare new sediment volume estimates with historical measurements.
Key Findings
Sediment Quality
The first study examined concentrations of 41 trace elements in the sediment behind the dam. The study found that:
- Sediments were substantially enriched in several elements, including silver (Ag), arsenic (As), gold (Au), bismuth (Bi), cadmium (Cd), copper (Cu), manganese (Mn), antimony (Sb), selenium (Se), tin (Sn), and tellurium (Te) relative to average concentrations found in upper continental-crustal material.
- Mercury (Hg) and lead (Pb) concentrations were lower than typical upper-crustal averages, unlike many other elements.
- Arsenic levels were consistently above State sediment management standards and aquatic toxicity reference concentrations.
- Most elements were found at higher concentrations in the silt grain size fraction.
Subsurface sediments generally held higher concentrations than surface material.
Media
Sources/Usage: Public Domain. View Media DetailsSegments of sediment cores showing texture and composition with massive sands, banded charcoal fragments, and silty-organic material collected in the lower Similkameen River upstream from Enloe Dam (Enloe Reservoir) near Oroville, Okanogan County, Washington. [These images were adjusted to account for lighting and perspective differences. On the tape measures shown in the photograph, the upper values are in inches and the lower values are in centimeters.]From: Element concentrations and grain size of sediment from the Similkameen River above Enloe Dam (Enloe Reservoir) near Oroville, Washington, 2019, Figure 3.
Sediment Volume
The second study produced a new estimate of the sediment volume behind Enloe Dam. The study determined that:
- In 2020, the reservoir held 2.17 ± 0.04 million cubic meters of sediment.
- This is a substantial increase from the 1.37 million cubic meters measured in 1971.
- The findings reflect a long-term deposition rate of roughly 16,300 cubic meters per year.
- Sediment thickness was deepest (about 20 meters) near the southern, downstream end of the reservoir and shallower upstream.
This updated volume helps managers assess the distribution of sediment in the basin and determine how much sediment could be mobilized if the dam were removed.
Why These Results Matter
The results of these studies give land and water managers clear, science-based information to support decisions about:
- Environmental and public safety risks associated with sediment release,
- Restoration planning for the Similkameen and Okanogan Rivers,
- Habitat improvements for fish and wildlife,
- Long-term watershed management.
The U.S. Geological Survey (USGS) completed a pair of scientific studies to determine the volume and quality of sediment stored behind Enloe Dam in north-central Washington.
The Enloe Dam, a 100-year-old structure on the Similkameen River in north-central Washington, stopped producing hydropower in 1958. As resource managers consider options for the dam’s future, they need reliable information about the sediment that had built up behind it over the last century, including its chemical makeup and the total amount that would be released if the dam were removed.
USGS completed a pair of studies to answer these questions. The work assessed:
- The element concentrations and grain size of sediment in the reservoir, and
- The volume of sediment trapped behind the dam.
Motivation
For decades, sediment from the Similkameen River watershed, home to both historical and active mining, settled behind Enloe Dam. Until this project, scientists lacked up-to-date information about:
- Whether those sediments contained elevated levels of metals such as arsenic, silver, copper, and others that could affect water quality and aquatic habitat downstream.
- How much sediment had accumulated behind the dam since the last estimate in 1972.
Understanding sediment chemistry and volume is essential for evaluating potential dam removal and planning safe, effective habitat restoration.
What the USGS Did
The USGS carried out field surveys in 2019 and 2020 to:
- Collect and analyze surface and subsurface sediments along the 1.6‑mile (2.6‑kilometer) reservoir.
- Measure the concentration of trace elements and metals in both fine-grained and coarser material.
- Map the top of the sediment layer (using acoustic bathymetry) and the underlying bedrock–sediment boundary (using continuous resistivity profiling).
- Compare new sediment volume estimates with historical measurements.
Key Findings
Sediment Quality
The first study examined concentrations of 41 trace elements in the sediment behind the dam. The study found that:
- Sediments were substantially enriched in several elements, including silver (Ag), arsenic (As), gold (Au), bismuth (Bi), cadmium (Cd), copper (Cu), manganese (Mn), antimony (Sb), selenium (Se), tin (Sn), and tellurium (Te) relative to average concentrations found in upper continental-crustal material.
- Mercury (Hg) and lead (Pb) concentrations were lower than typical upper-crustal averages, unlike many other elements.
- Arsenic levels were consistently above State sediment management standards and aquatic toxicity reference concentrations.
- Most elements were found at higher concentrations in the silt grain size fraction.
Subsurface sediments generally held higher concentrations than surface material.
Media
Sources/Usage: Public Domain. View Media DetailsSegments of sediment cores showing texture and composition with massive sands, banded charcoal fragments, and silty-organic material collected in the lower Similkameen River upstream from Enloe Dam (Enloe Reservoir) near Oroville, Okanogan County, Washington. [These images were adjusted to account for lighting and perspective differences. On the tape measures shown in the photograph, the upper values are in inches and the lower values are in centimeters.]From: Element concentrations and grain size of sediment from the Similkameen River above Enloe Dam (Enloe Reservoir) near Oroville, Washington, 2019, Figure 3.
Sediment Volume
The second study produced a new estimate of the sediment volume behind Enloe Dam. The study determined that:
- In 2020, the reservoir held 2.17 ± 0.04 million cubic meters of sediment.
- This is a substantial increase from the 1.37 million cubic meters measured in 1971.
- The findings reflect a long-term deposition rate of roughly 16,300 cubic meters per year.
- Sediment thickness was deepest (about 20 meters) near the southern, downstream end of the reservoir and shallower upstream.
This updated volume helps managers assess the distribution of sediment in the basin and determine how much sediment could be mobilized if the dam were removed.
Why These Results Matter
The results of these studies give land and water managers clear, science-based information to support decisions about:
- Environmental and public safety risks associated with sediment release,
- Restoration planning for the Similkameen and Okanogan Rivers,
- Habitat improvements for fish and wildlife,
- Long-term watershed management.