Big Wood River and South Valley Irrigators get Christmas gift

HAILEY, ID — The Big Wood River and south valley irrigators are receiving an extra gift this season: Trout Unlimited and the local flood control district have begun instream work on the Big Wood River just south of Bellevue to improve habitat and repair irrigation problems created in the 2017 flood.

Just before Christmas, the Glendale Restoration and Flood Mitigation Project began on the southern reaches of the Big Wood River. Trout Unlimited and Flood Control District No. 9 have received over $150,000 in grant monies and private donations to work in collaboration with agricultural irrigators to restore 1,250 feet of the Big Wood River. The Baseline Bypass Canal, the Glendale Diversion, and the Bannon Ditch were constructed in the 1920s to deliver water to approximately 6,300 acres of irrigated agricultural land south of Bellevue. Extreme high-water in the spring of 2017 severely eroded portions of the Bannon Ditch and required emergency instream work to protect the Bypass Canal. Following that flood, the altered river channel did not adequately deliver water to the three headgate structures, thus threatening the agricultural water supply. As a temporary fix, a 200-foot stretch of the Bannon Ditch that was obliterated was piped last summer.

“The Bypass Canal was created to ‘bypass’ the Glendale reach of the Big Wood River which loses a significant amount of water to ground infiltration,” said Kristy Molyneux, Upper Wood River Water Users Association Secretary. “Normally, we use loaders and excavators to move gravel around so that there is enough water in each of the canals and ditches. The flood damaged the Bypass Canal headgate and took out a section of the Bannon Ditch, so we were looking for a long-term solution to deliver water and protect structures.”

Aerial photographs from 1994 to 2014 show that the river channel has changed course and deposited gravel and sediment mid-channel, requiring annual instream work to deliver adequate water. During the flood of 2017, the river’s immense volume and velocity severely eroded the western streambank and destroyed 200 feet of the Bannon Ditch. The main force of the river was then focused across the channel directly at the Bypass Canal headgate, which required protection to prevent its complete destruction. After flood waters dissipated, an immense instream gravel bar remained and water delivery to the Bypass canal was impeded without major channel manipulation.

“Flood Control District No. 9 was interested in seeing a more comprehensive and integrated approach to water management at this location on the Big Wood,” said Dave Bell, Flood Control District No. 9 Chairman. “Grants from Idaho Water Resources Board and the Idaho Department of Environmental Quality are making it possible for us to address needs of water users in a way that also adds habitat and protects streambanks,” he added.

Sedimentation and erosion in this reach contribute to lower water quality for fish and other aquatic life, which was a concern for non-profit conservation group Trout Unlimited. Together, Trout Unlimited, Flood Control District No. 9 and the Upper Wood River Water Users Association are implementing instream treatments to mitigate the effects of flooding, ensure irrigation water delivery, stabilize streambanks, enhance riparian habitat, reduce the need for ongoing channel manipulations, and improve water quality. These measures will decrease future flood damage by dispersing flow, reducing velocity, protecting stream banks, and creating floodplain benches to absorb high flows.

A primary eastern river channel will be reactivated to direct flow towards the Bypass Canal, which delivers most of the irrigation water. Appropriate deflector structures and grade control will be created, and a riffle/pool sequence will be constructed to effectively transport sediment. Streambanks will be established with woody structures to reduce erosion and create aquatic and riparian habitat. These structures use root wads, tree clumps, woody shrubs, or logs to stabilize banks, and transplanted woody clumps will increase bank roughness. Floodplain benches will be created to absorb high flows, promote bank stability, and facilitate floodplain sediment deposition. The piped section of the Bannon Ditch will be protected and incorporated into floodplain and streambank stabilization.

“Collaborating with farmers and ranchers to find creative solutions benefitting both fisheries and producers’ bottom lines is a primary goal of Trout Unlimited,” said Keri York, Trout Unlimited Big Wood River Project Manager. “This project, in particular, achieves our goals of improving water quality, and floodplain and aquatic habitat, while finding water management solutions. We hope to do more projects like this to help resolve water quantity and quality issues in the Big Wood River.”