Willwood Dam Fall Drawdown and Work Group 2 Update

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Wyoming Department of Environmental Quality

Willwood Dam Fall Drawdown and Work Group 2 Update

Fall Drawdown: Turbidity and suspended sediment concentrations may be higher than normal in the Shoshone River downstream of Willwood Dam between approximately October 10th and October 31st as the Willwood Irrigation District (WID) lowers the water level behind the dam in preparation for winter operations. Lowering the water level is necessary to prevent damage to the canal gates that may occur with freezing temperatures. The mid-October timeframe is intended to minimize potential impacts of sediment releases to the downstream fishery, particularly October 15th to November 15th, the fall spawning period for brown trout and mountain whitefish. WID is coordinating the drawdown with Wyoming Department of Environmental Quality (WDEQ), Wyoming Game and Fish Department (WGFD), Wyoming Water Development Office, Wyoming State Engineer’s Office, United States Bureau of Reclamation, and the United States Geological Survey (USGS), so that researchers from the University of Wyoming, USGS, WGFD, and WDEQ can collect data during this period.

Operating Recommendations and Response to Comments: Work Group 2 is releasing revised Operating Recommendations for Willwood Dam, dated June 2019, and Response to Comments for comments received on the November 2017 Draft Operating Recommendations. The updated recommendations were approved by the Willwood Executive Committee in July 2019 and take into consideration the public comments received on the previous draft, data and information that has been gathered on the Shoshone River and at Willwood Dam since the November 2017 draft was developed, and lessons learned while implementing the recommendations. The primary changes to the recommendations include (1) updated turbidity values for the fall drawdown and spring sediment mobilization periods based on the inclusion of additional field data; (2) details on how to determine compliance with the operating recommendations; (3) an allowance to modify the timing of the fall drawdown and spring sediment mobilization based on flow conditions, fishery concerns, irrigation demands, etc.; and (4) details on the flow conditions necessary for the fall drawdown and spring sediment mobilization. Work Group 2 plans to update the recommendations once the USGS and University of Wyoming Water Research Program study, described below, are completed.

University of Wyoming Water Research Program Study: Work Group 2 is working with University of Wyoming fisheries biologists and a Master’s candidate on a Water Research Program project that will (1) synthesize the current state of knowledge of sediment effects on fisheries; (2) evaluate metrics and approaches for assessing sediment levels in rivers; and (3) compile current best management practices for managing sediment behind dams. Data collection at sites on the Shoshone River downstream of Willwood Dam began in August of this year and intensive monitoring of sensitive spawning habitats is planned for the fall drawdown. The project will continue until approximately June 2022.

Bathymetric Survey of Sediment Behind Willwood Dam: WDEQ recently completed a second bathymetric survey of deposited sediment behind Willwood Dam. The survey will be used to (1) determine how flows and operations of Willwood Dam have changed the quantity and location of deposited sediment since November 2017 when a previous survey was completed, and (2) help determine how to best manage deposited sediment while also protecting the downstream fishery. 

United States Geological Survey (USGS) Monitoring: Work Group 2 continues to support the work of the USGS to provide streamflow, turbidity, and acoustic data at sites upstream (06283995) and downstream (06284010) of Willwood Dam on the Shoshone River. Turbidity and river stage for both sites is logged in real-time, transmitted on an hourly schedule, and available for public access on the USGS website using the links above. Sampling for suspended-sediment upstream and downstream of Willwood Dam began in October 2018 using bank-operated cableways that were installed at the two sites. The suspended sediment samples will be used to develop relationships between streamflow, acoustic signals, turbidity, and suspended-sediment concentrations so that suspended sediment concentrations can eventually be estimated in real-time and used to quantify sediment loads into and out of Willwood Dam.

Department of Environmental Quality (DEQ) Shoshone River Water Quality Monitoring: The DEQ’s Monitoring Program continues to collect monthly water quality samples at nine sites on the Shoshone River from Cody downstream to Lovell. Biological sampling (macroinvertebrates and periphyton) at all nine sites was conducted in September 2019 and will supplement samples collected at the same sites in August 2018. Biological sampling results from 2018 and 2019 will be evaluated beginning in Spring 2020.

Wyoming Game and Fish Shoshone River Fishery Monitoring: WGFD fish management crews have been collecting trout abundance and condition data on the Shoshone River downstream of Willwood Dam on an annual basis since the 2016 sediment release. Sampling for 2019 will be conducted in early October of this year.

Questions regarding these efforts can be directed to David Waterstreet, Watershed Protection Program Manager, Water Quality Division, Wyoming Department of Environmental Quality, at [email protected] or 307-777-6709.