Matilija Dam is a 168-foot-high arched concrete dam, originally constructed by the Ventura County Flood Control District (now Watershed Protection) in 1947 to provide water supply and flood control within the Ventura River watershed. Structural concerns required lowering of the dam crest in 1965 and 1977 (original dam height was 198 feet), which, combined with reservoir sedimentation, has rendered the dam obsolete. The accumulation of approximately 8 million cubic yards of sediment negatively impacts aquatic and terrestrial habitats along the river, and deprives Ventura County beaches of much needed sand and cobble. The dam also blocks the migration of endangered Southern California steelhead (Oncorhynchus mykiss) to critical habitat in the headwaters of Matilija Creek.
Matilija Dam no longer provides the intended water storage and flood control benefits, and poses an increasing risk and liability to Watershed Protection as its condition continues to deteriorate. Recognizing this, in 1998 the Ventura County Board of Supervisors adopted a resolution stating, in part, that it is “necessary and beneficial” to remove the dam; and soon thereafter the first of many studies related to dam removal was underway.
Those initial studies revealed that, both as part of dam removal and independent of it, downstream infrastructure requires upgrading to address current and future flood protection and water supply needs. For example, if Matilija Dam were to be left in place, projections show that its reservoir would completely fill with sediment by 2030, creating additional pressure on downstream infrastructure. Thus, under any scenario, existing levees and bridges as well as the Robles Diversion Facility require modification to accommodate increased sediment transport from the Matilija Creek headwaters to the Ventura River estuary.