Previously we discussed where beach sand comes from and how river dams have reduced the sand supply and caused beach erosion. The next question is "where is all that sand going?" To explain this, one must first come to understand the beach as a "river of sand".
The sand on a particular beach does not ‘belong’ to that beach, but continually moves along the coast. This is because waves that strike the shoreline at an angle create a current that runs parallel to the shore. Sand is churned up by the breaking waves and transported along the beach by this "longshore current". Even small waves transport sand, but any surfer knows how strong the longshore current can get when the waves are big. This natural transport of sand along the coast is called "littoral drift".
Since the majority of the waves in our area come from the west, sand on our beaches is transported from west to east. In Ventura County we receive sand that has been transported from places as far west as Point Conception; and the sand that is on Ventura’s beaches today will soon be in Oxnard.
Eventually all this sand makes it as far as Mugu Canyon, a submarine canyon that comes to shore near Mugu Lagoon. At this point the sand flows down into the deep ocean, forever removed from the coast. While it is hard to see the sand moving along the beach, oceanographers have actually filmed the "river of sand" flowing down submarine canyons.
Sand transport along the Ventura County coastline has been estimated to be as much as one million cubic yards per year. A dump truck may hold 10 cubic yards, so this is equivalent to 100,000 truck loads, or one dump truck load of sand every 5 minutes around the clock all year long! The "river of sand" requires a continual supply of new sand to maintain the beach width -- it’s no wonder that the dams on our rivers are causing our beaches to disappear.