The 430 square mile Cow Creek watershed provides spawning and rearing habitat for several species of fish including Chinook salmon and Steelhead trout. The watershed has five major tributaries, including Clover Creek. The Millville Ditch diversion consists of a concrete dam to divert water approximately 300 yards downstream to an inverted siphon encased in concrete. The existing concrete structures were several decades old. Major channel incision at the structures impeded upstream fish passage to approximately 10 miles of potential fish habitat.
The diversion intake, at the time, was unscreened and did not meet current California Department of Fish & Wildlife (CDFW) fish screening criteria. Fish passage improvements for both upstream spawning passage and downstream outmigration were proposed by the CDFW and Federal fisheries resource agencies as a high priority project for enhancing both Chinook and Steelhead populations and viability in the upper Sacramento River Watershed.
The California Department of Water Resources (DWR) prepared a Preliminary Engineering Technical Report that included a 50% design of a preferred solution to eliminate the fish passage impedance. SAGE Engineers (SAGE) was part of a multidiscipline engineering team, led by Northwest Hydraulic Consultants, Inc., that was retained by Western Shasta Resource Conservation District to advance DWRs preliminary concept into a final design.
The erosion of Clover Creek at the diversion structures created two barriers to fish passage—one at the existing siphon and a second at the upstream diversion dam. The primary goal of the project was to eliminate these two barriers and open up 10 miles of historic spawning and rearing habitat for anadromous fish. The preferred solution consisted of constructing a fish ladder at both the siphon and dam sites. Other improvements included adding a fish screen to the diversion, replacing the siphon carrying diverted flows, and improving the stability of the existing diversion dam.
SAGE performed a geologic/geotechnical assessment of the project site to characterize the soil conditions and develop recommendations for design of the fish ladders and other improvements. Our assessment included performing a geologic site reconnaissance to identify surface features that infer subsurface conditions in and around the project site. The site reconnaissance was followed by geotechnical subsurface investigation that included five (5) exploratory test pits excavated up to 8 feet below ground surface. The soil conditions encountered included siltstone/shale bedrock that is easily erodible and susceptible to degradation.
SAGE collaborated with the owner, project stakeholders, and NHC to refine the 50% design concept and prepare construction plans, technical specifications, and an opinion of probable construction costs. Based on NHC’s hydraulic analysis, the existing siphon was largely filled with lean concrete and left in place as a hydraulic feature to control upstream flow velocities. A portion of the existing siphon was removed to accommodate the downstream fish ladder. A new siphon, encased in concrete, was constructed approximately 35 feet upstream.
The existing diversion dam still functioned to retain water for diversion to the siphon and was largely left in place. The right portion of the dam was removed to accommodate the upstream fish ladder. Voids beneath the dam were filled with grout to improve the structure’s stability and reduce the potential for ongoing undermining of the structure through seepage and erosion. The fish ladder included a high cantilever retaining wall along the right bank of Clover Creek. The retaining wall extended approximately 6 feet above grade to prevent debris during major storm events from spilling into the ladder. A fish screen structure was placed just upstream of the fish ladder exit. The fish screen incorporated mechanical brushes that operate under solar power.