Matilija Dam Ecosystem Restoration Program: Pre-Construction Removal Engineering and Design

This project includes preliminary engineering and design for the removal of the Matilija Dam on the Ventura River. Removal of the dam will restore fish passage to historic spawning and rearing habitat for southern steelhead in the upper watershed. It will also restore natural sediment transport downstream and improve sand replenishment at beaches along the coast. When the project is fully implemented, the Ventura River watershed and its related estuarine and ocean habitats offshore will more closely resemble historic conditions.

One of the major features of the Matilija Dam Ecosystem Restoration Project includes moving or re-contouring 6 million cubic yards of sediments that are now trapped behind the dam; 2 million cubic yards are to be slurried to a designated downstream disposal site, and the remaining 4 million cubic yards are to be recontoured into sediment storage areas as source for future natural erosion and transport downstream during storm events. The dam itself will be removed by controlled blasting in 15-foot increments, and a 100-foot wide meandering channel will be constructed through what is now the reservoir area behind the dam. The project also includes constructing a high flow sediment bypass system at a water diversion downstream; building levees along parts of the river channel to protect property from flooding resulting from expected increases in stream channel elevations in the first years after the dam removal; and building a recreation trail along the alignment for the slurry pipeline.

Matilija Dam is a 620-foot wide, concrete arch dam located inland of the coastal zone, about 16 miles upstream from the Pacific and just over half a mile from the confluence of Matilija Creek with the Ventura River. When the dam was built in 1948 (by the Ventura County Flood Control District), its height was 198 feet, but the dam has been notched twice (in 1965 and 1978) because of safety concerns, and it is now 168 feet high. Although it was constructed with a design reservoir capacity of more than 7,000 acre feet, significant sedimentation has reduced the reservoir’s capacity to less than 500 acre feet. Over six million cubic yards of silts, sands, gravels, cobbles and boulders are estimated to reside behind the dam, and the reservoir is projected to fill in completely by 2020 if the dam is not removed.