Dam Ecosystem Restoration Feasibility Study
The Baseline Conditions
F3 Report and other technical studies may be accessed at www.matilijadam.org.
The following is a
summary of the findings from the baseline conditions studies conducted
from June 2001 to October 2002:
An examination of the historic flood records on the Ventura River revealed
that Matilija Dam has never controlled peak river flows, and some localized
flooding continues to occur with the dam in place. Levees have reduced
the flood risk for much of the existing floodplain development.
Sediment: Surveys of the reservoir indicate that it contains approximately
six million cubic yards of sediment, including about 1.8 million cubic
yards of beach quality sand. While the sediments tested clean, much of
the beach sand is intermixed with fine sediment that is not suitable for
direct use for beach replenishment. Studies estimate that the reservoir
will completely fill with sediment by 2020, but will continue to trap
sediments until 2030, when some coarse sediment will begin to pass over
the dam. At that time the impounded sediments will total over nine million
Habitat: The sediment trapped by Matilija Dam has significantly
degraded the downstream river and coastal habitats. Surveys reveal that,
downstream of the dam, sediment-starved hungry water has eroded
the riverbed so that it now suffers a deficit of almost two million cubic
yards of sand, gravel and cobble. The historically broad floodplain with
many braided channels has become a single channel cut into the river bed.
Sediment reduction has exacerbated beach erosion from the mouth of the
river to the Ventura harbor. High quality habitat still exists in the
upper watershed, but remains disconnected from the river and coast. Arundo
donax, or giant reed, degrades the habitat value, and is spreading throughout
the reservoir and floodplain terraces downstream.
Dam: Structurally, the dam suffers from extensive Alkali
Silica Reaction, a chemical reaction that weakens the concrete.
Despite this problem, there are no structural modifications needed for
the dam to remain adequately stable for next the 50 years.