THE LAND CONSERVANCY OF SAN LUIS OBISPO COUNTY

Restoration Project: Fish Passage Site 1

Prefumo Creek at Highway 101

Overview/History

This project was among the most challenging fish passage projects we have attempted. This barrier prevented steelhead migration into the Prefumo Creek drainage area, or roughly 8 miles of habitat.

The Problem

Prior to our work, the culvert had a wide and flat bottom. This spread the water out and during lower flows, the water depth was too shallow for fish to swim through. By the time flow became deep enough during rain storms for fish to swim in, it was running 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 and rocks prevented fish from even getting close enough to try a jump.

Project Approach

Our design approach needed to address water depth and velocity inside the culvert and vertical jump height. To address the issues inside the culvert we built a wall down the middle to concentrate low flow, and installed a series of concrete baffles. The baffles slow down the water and cause an increase in depth. They create backwater places within the structure for fish to rest on their way through. 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 to seal them and were built with fishways between each one.

Results

Now, instead of a four foot jump, there are four 1-foot jumps. When we saw adult fish upstream from the project following the construction, we all felt good about this project.

Project Partners

A rewarding part of this project was working directly with Caltrans, the owners of the culvert. While their permitting process can be very cumbersome to get through, their engineers and hydrologists understood and shared our desire to move fish past this structure. When working on public infrastructure, the stakes are high and everyone wants to get it right the first time. The process took perseverance on our part and hinged on some key advocates for the project within Caltrans.