Sand is primarily deposited on Carlsbad beaches from three
rivers: the Santa Margarita, the San Luis Rey, and the San
Dieguito. When it rains, the sand moves westward from the river
mouths to the ocean. Sand also moves north to south and south to
north with the changing direction of the wave-caused currents.
During winter, northern storms transport the sand southward.
During summer, southern storms move sand north. In addition,
wave-caused currents and tides continuously move sand from
shallow water to the shoreline and vice-versa. Manmade
structures, such as harbors and jetties, also affect the
movement of sand. And as the sea has slowly risen over thousands
of years, the beach has inevitably narrowed.