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Tibble Fork Reservoir

Sediment Release August 2016

A $7.3 million dam-rehabilitation project on Tibble Fork Reservoir triggered an unexpectedly large release of sediment into the North Fork of the American Fork River on Saturday, August 20, 2016, after crews began draining the 10-acre lake. While federal and local officials anticipated some sediment transport downstream as the reservoir was being drained, the volume outpaced predictions as well as best management practices (BMPs) required under the project’s U.S. Army Corps of Engineer (USACE) National 404 permit to protect water quality.

Located in American Fork Canyon, Tibble Fork Reservoir is fed by the American Fork River, Deer Creek, and Tibble Fork Creek. The reservoir is a popular fishing, hiking, canoeing, and boating area and provides irrigation water for local irrigation companies. NRCS performed an assessment of Tibble Fork Dam in 2004 which concluded that reservoir’s 50-year-old earthen dam does not meet current NRCS and Utah State Dam Safety regulations and engineering standards for a high hazard dam. The dam rehabilitation project is designed to stabilize the structure to meet safety standards, prevent flooding, retain sediment, increase the storage capacity of the reservoir, and extend the life of the dam.

Prior to the sediment release, the American Fork River below Tibble Fork Dam met water-quality criteria for designated beneficial uses for infrequent primary contact recreation such as fishing, protection of warm-water and cold-water game fish, and agriculture. Above the reservoir, there was insufficient data to determine if the river’s water quality supports its beneficial uses.

Elevated metals concentrations from historic mining activities have been found in the reservoir’s sediment, but have not typically been found in the water itself. Upstream mines include the Live Yankee, Globe, as well as a number of other abandoned gold and silver mines. Estimates of existing sediment volume in the reservoir is approximately 151 acre-feet.

Project Plan and Environmental Assessment

According to the NRCS, dam construction required the total drainage of the reservoir to begin work on the dam embankment. The agency expected elevated sediment levels, but BMPs were to be in place to minimize sediment transport, and all discharges were to be in accordance with the Environmental Assessment (EA), applicable permits, and state and federal laws. Officials anticipated that any increased sediment in the downstream waters would be temporary and dissipate within 48 hours of release.

Water Quality Permits

The Clean Water Act (CWA) requires a 401 permit to ensure that the dam rehabilitation project does not violate state water-quality standards; both numeric and narrative. Certification for this project was obtained as part of the USACE Section 404 National Permit. The CWA also requires a Utah Pollutant Discharge Elimination System (UPDES) Storm Water General Permit for Construction Activities and the development of a Storm Water Pollution Prevention Plan (SWPPP).

Historic Mining Activities

American Fork Canyon was once a hub of gold, silver, and lead mining activity. Concerns about adverse effects to the canyon headwaters from mine drainage have led to mine-reclamation initiatives to improve water quality. However, the reservoir sediments still contain heavy metals from these earlier mining activities.

Tibble Fork Dam, in fact, was originally constructed to retain sediments and prevent downstream flooding. According to the EA, “a sediment survey of Tibble Fork Reservoir was conducted in 2010 to sample sediments for metals content. Five sediment samples and one duplicate sediment sample were tested for metals content. All six samples showed levels of arsenic (As) that exceed both the Reportable Detection Limit and the EPA Residential Regional Screening Level (RSL). Also, two samples indicated levels of lead (Pb) that exceed both the Reportable Detection Limit and the EPA Residential RSL. This suggests that further evaluation of the potential risks due to arsenic and lead exposure should be conducted. EPA RSLs do not address non-human health end-points, such as ecological impacts.”

The EA continues, stating that “(a) study conducted in 2002 by the Forest Service took fish tissue samples from five reaches of the American Fork River and found contamination from below Pacific Mine to below Tibble Fork Reservoir. The samples were high in arsenic, cadmium, zinc and lead. Elevated levels of heavy metals in fish were found to be from water and soils contamination associated with past mining operations. In June of 2002, a fish consumption advisory was issued for the North Fork of the American Fork Canyon. Since that time efforts have been made to clean up contamination associated with the past mining operations. There is currently no fish consumption advisory for fish in the American Fork River or Tibble Fork Reservoir according to the Utah Fish Advisories webpage. Water quality samples were also taken in and above Tibble Fork reservoir in the summer and early fall of 2000 that indicated high levels of arsenic. Other water quality constituents tested appear safe.”

RCS EA Appendix D: Table D-11. Water Quality Criteria and Sample Concentrations above Tibble Fork Reservoir

Contaminant (mg/kg)Freshwater Aquatic Life Acute ExposureFreshwater Aquatic Life Chronic ExposureHuman Ingestion of Water + FishSample Concentrations (mg/kg) (UDWRe 2011)
Cadmium3.9 (a)1.1 (a)10<1.0
Copper18 (a)12 (a)N/A< 0.012
Lead82 (a)3.2 (a)50< 0.003
Zinc120110 (a)N/A< 0.03

(a) Computed from hardness; 100 mg/l used. Criteria values taken from USFS 2002

Data and Analysis

Sampling Results

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