Willwood Dam Sediment Mobilization and Work Group Updates

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Wyoming Department of Environmental Quality

Willwood Dam Sediment Mobilization and Work Group Updates

Spring Sediment Mobilization at Willwood Dam: Work Group 2 (Wyoming Department of Environmental Quality, Wyoming Game and Fish Department, Willwood Irrigation District, United States Bureau of Reclamation, Wyoming State Engineer’s Office, and others) is coordinating a spring sediment mobilization beginning in approximately mid-March, as prescribed in the November 2017 Willwood Dam Operating Recommendations. The sediment mobilization is intended to help move some of the sediment that has been deposited behind Willwood Dam while also protecting the downstream Shoshone River fishery. The sediment mobilization can only occur if flows in the Shoshone River at Willwood Dam are not greater than approximately 800 cfs, so is contingent on flows being released from Buffalo Bill Reservoir. If executed, the sediment mobilization will likely result in elevated turbidity in the Shoshone River downstream from Willwood Dam for an approximately two week period. The mobilization effort will be closely monitored using real-time United States Geological Survey (USGS) equipment, described below, and on-site Work Group 2 staff.

United States Geological Survey (USGS) Monitoring: In 2017, the Wyoming Department of Environmental Quality contracted with the USGS to install streamflow, turbidity, and acoustic equipment 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. The sensors were kept in place until December 2018, but were removed to avoid damage from freezing temperatures. It is anticipated that the sensors will be reinstalled in mid-March 2019. 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 in 2018. 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.

Buffalo Bill Reservoir and Dam Operations: Flow releases from Buffalo Bill Dam are expected to increase by approximately 400 cfs over the next few days to help accommodate increased inflows to Buffalo Bill Reservoir. Releases have been at winter levels, with flows in the Shoshone River at the Cody gage of approximately 410 cubic feet per second (cfs). As of March 1st, the United States Bureau of Reclamation (USBR) estimated that snow conditions in the watershed were approximately 105% of the 30-year average. USBR expects flows to stay at approximately 810 cfs at the Cody gage until the end of March and to increase further during April.

Willwood Dam Fall Drawdown: Between approximately October 1st and October 28th, the Willwood Irrigation District (WID) lowered the water level behind Willwood Dam below the bottom of the canal gates. Lowering the pool elevation is necessary each fall to prevent damage to the canal gates that may occur during freezing air temperatures. As described in a September 28, 2018 listserv notice, WID started dropping the pool elevation earlier in 2018 to help minimize the release of sediment during sensitive spawning periods for brown trout and mountain whitefish. The Shoshone River downstream of the dam had lower turbidity during the drawdown in 2018 than in 2017 and the duration of elevated turbidity was much shorter during 2018 than in 2017.

Operating Recommendations: Work Group 2 has drafted revised Operating Recommendations for Willwood Dam along with a Response to Comments received on the November 2017 Draft Operating Recommendations. These documents are awaiting final review by the Willwood Executive Committee and will be released soon.

To help improve the operating recommendations, Work Group 2 coordinated with fisheries biologists at the University of Wyoming to develop and submit a proposal for the Water Research Program to synthesize the current state of knowledge of sediment effects on fisheries and evaluate metrics and approaches for assessing sediment levels in rivers. The project will also compile current best management practices for managing sediment behind dams. The project was approved for funding during the 2019 legislative session and should commence in the next few months. To help provide some baseline information for the study, WDEQ and the Wyoming Game and Fish Department (WGFD) have begun to identify trout spawning habitat downstream of Willwood Dam. These sites will be used to help evaluate the impact of sediment releases on the downstream fishery.

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 is scheduled for summer or fall of 2019 and will supplement samples collected at the same sites in August 2018. Attempts at representative biological sampling in fall 2017 were unsuccessful due to higher than normal streamflows. Biological sampling results from 2018 are expected in spring 2019.

Shoshone River Fishery: 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 in fall 2018 indicated that there has been a slight increase in the abundance of trout greater than six inches over the last three years; there were 442 trout/mi in 2016 (following the Willwood sediment release), 585 trout/mi in 2017, and 763 trout/mi in 2018. The trout population primarily consists of stocked rainbow and cutthroat and stocking rates have increased each year between 2016 and 2018. The abundance of brown trout, the only species of trout not stocked in this section of river, was 148/mi in 2018 and has not changed substantively from 146/mi in 2016 and 125/mi in 2017. The biomass of brown trout increased slightly from 109 pounds/mi in 2017 to 148 pounds/mi in 2018.

Wyoming Water Development Commission Study: Work Group 2 has had multiple discussions on pursuing a Wyoming Water Development Commission study that would assist the Willwood Irrigation District in evaluating the structural integrity of the dam and identifying maintenance priorities. A study could potentially evaluate options for managing the sediment that has accumulated behind the dam as well as alternative water delivery methods. Since a considerable amount of data on the amount of deposited sediment behind Willwood Dam as well as estimates of the annual sediment load into and out of the dam are still being collected, the group agreed that it would be best to pursue funding for a study once this data has been collected. The next application deadline for a Level I or Level II study is March 1st, 2020. Work Group 2 plans to continue discussions on the scope of a potential study.

Work Group 3: Work Group #3 has continued to meet regularly, working at each meeting to collaboratively draft a watershed plan that will identify potential sediment reduction projects upstream of Willwood Dam. Group members are compiling available information for the sub-watersheds of major tributaries that includes, but is not limited to, watershed characteristics, sediment contributors, recommendations, and information needs. Work Group #3 will provide public comment opportunities once a draft of the watershed plan is complete. As part of outreach efforts, the group hosted a public event in June 2018 that included presentations by Natural Resource Conservation Service, DEQ, WGFD, USGS, and landowners in the morning and a field tour in the afternoon. The field tour included examples of successful sediment reduction projects implemented in the watershed. The event was well-attended and provided a forum to discuss opportunities for future sediment reduction projects.

Tributary Monitoring: In coordination with Work Group 3, WGFD and DEQ have continued to collect sediment data on tributaries upstream of Willwood Dam in support of watershed and sediment load analysis. Fixed sampling sites have been established on Sulphur Creek, Sage Creek, Dry Creek (aka Homesteader Creek), Dry Gulch, Idaho Creek, and Cottonwood Creek. At each site, flow, total suspended sediment samples, and bedload samples have been collected using USGS sampling protocols (Edwards & Glysson 1999). A total of 83 samples were collected and submitted to the USGS for analysis. Sediment rating curves and total sediment contributions from each tributary will be calculated in coordination with DEQ.

Questions regarding these efforts can be directed to David Waterstreet at [email protected] or 307-777-6709.