Projects>Poindexter Slough Fishery Enhancement Project
Preliminary Feasibility and Engineering Plan
The Beaverhead Watershed Committee retained Confluence to survey and develop restoration recommendations for improving fish habitat and water quality along four miles of Poindexter Slough. At one time, Poindexter Slough was a natural side channel of the Beaverhead River, but is currently controlled by a headgate. Potential improvements identified were restoring appropriate width: depth ratios for riffles and pools, removing or isolating fine sediment deposits from the stream bed, and encouraging natural recruitment of willows and other woody riparian vegetation. Confluence assisted the Beaverhead Watershed Group in preparing a successful DNRC Resource Renewable Grant and Loan application, which was ranked in the top five to be funded by the Montana Legislature in 2011.
Final Engineering Plans and Construction Specifications
Confluence was extended a second contract with the Beaverhead Watershed Committee to produce final designs and construction specifications for the preferred alternative from the Feasibility Study. Project components included developing hydrologic design criteria, monitoring stream flows, surveying channel cross sections, longitudinal profiles, and irrigation infrastructure, producing construction-ready drawings and specifications, and preparing a flow management plan to the Watershed Committee, Montana Fish, Wildlife and Parks, and the Dillon Canal Company. Project goals include improving spawning habitat for rainbow and brown trout, restoring flushing flows to the Poindexter Slough channel, improving fish passage at irrigation withdraws, and improving recreational fishing on private and public lands managed by Montana Fish, Wildlife, and Parks.
Hydrology and Instream Flows
Confluence conducted synoptic flow sampling at 14 locations along Poindexter Slough to document the effects of drain tiles, groundwater, tributaries, and irrigation withdraws on the surface water hydrology of the system. Results of multiple synoptic flows were summarized and used to develop hydrologic design criteria and guide habitat restoration efforts. Discharges were measured using a Marsh McBirney digital velocity gage with results entered into a velocity-area spreadsheet.
Confluence compiled historic daily discharge records from the Clark Canyon Dam as well as the East Bench Irrigation Canal to develop yearly hydrographs for the Beaverhead River at the Poindexter Slough Headgate. A mean daily discharge hydrograph was developed from these hydrographs, including upper and lower 95% confidence intervals.
A hydraulic model was developed using HEC-RAS for the existing Poindexter Slough Headgate on the Beaverhead River to evaluate the capacity of the headgate through the developed hydrograph. This model was also utilized in the design for a new diversion structure to enable a flushing flow of 200 cfs down Poindexter Slough.
Poindexter Slough Hydraulics
Confluence developed a hydraulic model of Poindexter Slough and the Dillon Canal diversion based on 89 surveyed cross sections through the project reach. The existing conditions model was calibrated by adjusting friction values until modeled water surface elevations matched surveyed elevations at discharges measured during the synoptic flow sampling.
Sediment transport calculations were performed using HEC-RAS results to evaluate the ability of the channel, by reach, to maintain habitat features (pools, riffles) during various flow regimes. Also, the ability of the channel to mobilize spawning gravels during flushing discharges was analyzed.
A flushing flow discharge was developed based on hydraulic performance and sediment transport capacity of a reference reach. Design cross sections were developed for each reach using at-a-station hydraulics and sediment transport calculations to determine the appropriate channel size to match the reference reach sediment transport capacity.
In order to provide adequate flushing flows to the channel, a larger headgate structure was designed. The headgate was designed to provide the target flushing flow of 200 cfs down Poindexter Slough when flows in the Beaverhead River allow, with the capacity to increase flushing flows if necessary during larger events.
Dillon Canal Diversion and Headgate Design
A pin and plank check structure on Jefferson Slough diverts irrigation water to the Dillon Canal; however, this diversion permanantly backwatered 2,000 feet of the Slough, causing excessive fine sediment deposition and degrading aquatic habitat. A hydraulic model was developed to evaluate options for lowering the pin and plank and headgate structures and re-grading the backwatered channel segment to improve sediment transport and pool habitats.
The model was developed based on lowering the canal by 2 feet and run using various flow scenarios in Poindexter Slough. Confluence provided a design which included a larger headgate at the lower elevation and a narrowed design channel to increase available head while improving fish habitat through the reach.
Confluence assisted the Beaverhead Watershed Committee throughout the permitting phase of the project by submitting technical information necessary to obtain all required permits including:
- 124 Permit from Montana Fish, Wildlife and Parks
- 404 Permit from U.S. Army Corps of Engineers
- Beaverhead County Floodplain Permit with review by DNRC Water Resource Department
The County floodplain permitting required extensive hydraulic modeling to document the project’s influence on the 100-year base flood elevation throughout the length of Poindexter Slough, and beneath three railroad bridges, two Interstate bridges, and a fishing access bridge.
Project construction began in January, 2015, with Confluence providing oversight to ensure the project was built according to the project’s design and specifications. Oversight included staking all project reaches, inspecting elevations of the diversion structures and headgates, and ensuring the proper gradient throughout the re-graded channel segments.