In a most basic sense, the sand on our beaches comes from the erosion of the land. In our area the beaches rely on sand transported from the mountains by the rivers and from eroding cliffs near the shoreline.
The addition of new sand to the beaches is seasonal, occurring during rainy periods when the rivers flow and sediments are washed into the ocean. The Santa Clara river is capable of depositing huge quantities of sand during floods, but very little during dry years. For example, 52.4 million tons of sediment were discharged during the 1969 floods; floods that ended 30 years of relative drought when very little new sand was added to the beaches.
Today the supply of new sand to our beaches has been greatly reduced by human activity. Over the past 50 years river sand has been restricted by dams in the watershed areas and mining of floodplain gravels by private industry. About 42 percent of the Ventura River watershed is blocked by dams (at Matilija, 1948, and Casitas, 1959) and 37 percent of the Santa Clara River watershed is dammed (Bouquet, 1934; Piru, 1955; Pyramid,1971; Castaic, 1972). These dams trap river sediments and starve the beaches of their natural supply of sand. In addition, the erosion of coastal bluffs along the Rincon coast has been eliminated by the construction of seawalls for Highway 101, removing the other primary source of beach sand for our area.
Presently our beaches are eroding because of the of the reduction in the sand reaching the coast. However, Matilija Dam is no longer useful as a water supply because it is completely full of sediments; sediments that should be in the Ventura River and on our beaches. If obsolete dams like Matilija were removed our beaches would receive a much needed renewal of their rightful supply of sand.