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.