Our native steelhead population, known as South-Central California Steelhead, is currently endangered. The population has dramatically declined from runs of 25,000 to less than 500 adult fish. Agricultural development and urbanization have made much of the spawning and rearing habitat inaccessible because of dams and other instream structures which block migration.
The Land Conservancy worked with regional partners to identify and rank the worst barriers that were threatening the steelhead’s existence. Once of these was a culvert under Highway 101 at Prefumo Creek which completely barred the passage of steelhead trout for decades. The attempt to remove this barrier resulted in one of the most challenging fish passage projects the Land Conservancy has attempted. In order to reduce this barrier and encourage the migration of steelhead trout into Prefumo Creek, we teamed up with Caltrans and initiated a restoration project
Prior to our work, the culvert was too shallow for fish to swim through. By the time rain storms made the culvert deep enough for fish to swim through, the water would flow too fast for even the strongest fish to swim against. Therefore, this barrier was considered both a depth and velocity barrier. It was also a vertical barrier because the outlet of the culvert was over 4 feet higher than the stream surface below, preventing fish from getting close enough to even try a jump.
Our design approach needed to address water depth and velocity inside the culvert along with vertical jump height. In order to mitigate the problem we built a wall down the middle of the culvert to concentrate low flow and installed a series of concrete baffles. The baffles slow down the water and cause an increase in depth thereby creating backwater places within the structure for fish to rest on their way through. Although this type of structure, in some areas, would simply fill in with sediment and be ineffective, it worked in this case because there is very little gravel moving in this system (gravel and sediment get settled out upstream in Laguna Lake). On the downstream end of the culvert, we re-arranged the existing rocks to form pools. The pools were grouted and sealed and included fish ways between each one.
Today, instead of a four foot jump, there are four 1-foot jumps that are easy for steelhead trout to swim through. When we saw adult fish upstream from the project following construction, we all knew it had been a success. By reducing the barrier in Prefumo Creek, we were able to enhance the habitat of the endangered Central Coast Steelhead trout.
- Avila Beach Trustee Council
- Questa Engineering Corp.
- Specialty Construction Inc.
- The Morro Group
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