Caspar Creek Bypass Pool and Chute Fishways

Location: Jackson State Forest, Mendocino County, California
Clients and Partners: CalFire, US Forest Service, 5 Counties Salmonid Conservation Program, California Coastal Conservancy, and Winzler & Kelly.


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Michael Love & Associates (MLA) were the fish passage engineers to improve upstream and downstream fish passage for adult and juvenile salmonids at flow measurement weirs on the North and South Forks of Caspar Creek. Caspar Creek is located in the Jackson Demonstration State Forest, which is a long established experimental forest that serves as an important research asset in studying forest watershed management practices. Stilling basins installed on both forks of Caspar Creek are used to assist in quantifying sediment and debris transport. The basins are controlled by concrete low-flow V-notch weirs, which are used for highly accurate measurement of streamflows.

Because both forks of Caspar Creek support coho salmon and steelhead trout, wooden fish ladders were constructed below each of the concrete weirs to accommodate adult fish passage. However, the redwood ladders only provided low-flow adult passage and no juvenile passage. Additionally, they leaked, presumably impinging and stranding juvenile salmonids during lower flows.

The project’s objective was to improve upstream and downstream fish passage for adult and juvenile salmonids at the two existing flow measurement weirs. The central hydraulic constraint of the project was to design and construct a fish passage facility that does not compromise the accuracy of the North Fork and South Fork flow measurement weirs. Additionally, the project needed to be designed and operated to meet strict performance criteria for juvenile salmonid passage set forth in a Biological Opinion by National Marine Fisheries Service.

MLA designed a new concrete 50-foot long, 8.5-foot wide pool and chute fishway, constructed in 2009. Each weir drops 0.5 feet and provides juvenile passage during flows as low as 0.25 cfs. The pools below each weir were sized to avoid excess turbulence while producing sufficient scour to transport fine sediment. To improve juvenile passage conditions during the dry-season, the fishway design also included four adjustable and removable low-flow weirs placed in the exit channel to increase forebay water levels, reducing the water surface drop across the flow measurement weir for juvenile salmonid passage. Two sluice gates allow dewatering of the fishway to facilitate cleanout and seasonal installation and removal of weirs.

The spillway consists of a single-cycle labyrinth weir with stainless steel gates to manually adjust crest heights. This type of weir provides increased spillway length within a narrow channel, thus minimizing forebay rise as streamflows. The fishway entrance is located close to the spillway to support fish attraction to the fishway.

A substantial design challenge was to minimize flow leakage to ensure that low summer flows pass through the fishway, allowing juvenile salmonid passage. Careful attention was paid to leakage rates of manufactured gates and actual seating of the gates during construction. Additionally, concrete cutoff walls were extended into the surrounding bedrock and sealed with bentonite and concrete.

The fishway includes a viewport structure and facilities for video monitoring for research into juvenile salmonid leaping abilities. A catwalk provides access to gate actuators, which were designed to actuate with minimal force, per OSHA standards.

This project was awarded The 2010 Outstanding Environmental Project of the Year, by the San Francisco Section of the American Society of Civil Engineers. The project also received a letter of commendation from the Five Counties Salmonid Conservation Program upon completion.