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How California Sand Dunes Form

Shaped by wind into curving ridges, coastal sand dunes are among the most dynamic and fragile natural formations. Their contours shift over time until hardy dune pioneer plants take hold in the drifting sand and create a stable landform. Even then, dunes can change form rapidly under the stress of storm waves and wind, or the traffic of human activity.


Offshore sandbars and sediment deposited at the mouths of rivers are the most important sources of material for dune building; sediment is carried buy longshore currents until a projection landform traps the particles and they are deposited on the beach by wave action. Dune formation begins when wind blows dry sand particles landward from the beach. Drifts accumulate around object, such as plants and logs, that interrupt the wind flow. With steady winds, the sand drift acts as a barrier to moving sand, and the drift gradually grows into a sizable mound. Until a dune is completely veiled and stabilized by plant cover, sand may be borne away by winds. 


Coastal dune fields form characteristic patterns. A common pattern along the Northern California coast is a series of parallel ridges perpendicular to the prevailing winds, called transverse ridges. The parabola-type dune field is a series of U-shaped dunes with the concave side facing the prevailing wind direction; this dune type is found at Pismo Beach in Central California. All dune fields consist of two or three sets of parallel dunes, with the most recently formed foredunes nearest the beach, and the older, usually vegetated and stabilized dunes farthest inland; the inland dunes may be as much as 18,000 years old. 


Deep-rooted succulent, matted plants such as beach strawberry, silver beachweed, and yellow sand verbena grow on the foredunes along with various dune grasses. The aggressive European beach grass, planted on California dunes in the 1930's, rapidly extends new shoots when half buried by sand drifts. The new shoots snag more sand, building and stabilizing the dune, and crowding out native species. The globose dune beetle, which cannot survive under European beach grass, is restricted to areas where native dune plants persist. 


Protected by the foredunes from salt spray and wind, wild buckwheat, yellow bush lupine, and purple-flowered beach lupine grow on the richer soils of the back dunes. At least tree rare insect species, the San Francisco tree lupine moth, the Pheres blue butterfly, and the Morro blue butterfly, lay eggs on the lupines. Wild buckwheat is a favorite food of the endangered Smith's blue butterfly larvae. 


Deer mice, California voles, and black legless lizards burrow into the sand dunes, seeking cover from predators such as the northern harrier. This white-rumped hawk hovers a few feet above land; concave disks on the sides of its head funnel sound into the ears, enabling the hawk to detect the slightest movement of its prey. Gray foxes and striped skunks eat insects and dune plants. Mule deer wander over the dunes, browsing on shrubs. 


Dunes shield low lying inland areas from violent storm waves. Sand eroded from dunes and beaches during winter storms usually forms a sandbar a short distance offshore. This sand is gradually returned to the beach during the calm summer season. A stable dune system can undergo some wave erosion without permanent damage. 


California's dunes were formed over thousands of years, yet today, dune erosion is outstripping sand deposition. Dams trap river sediments, depleting the sand supply, and coastal protective structures, such as seawalls, disrupt the natural recycling of sand from sandbar to beach. Coastal development has disturbed dunes at many points along the coast. Off-road vehicles, foot traffic, and horses cam damage dune plants, loosening the sands and leaving the dunes vulnerable to wind erosion and blowouts. 


Of the 27 dune fields in coastal California, the largest are the Monterey Bay dunes, covering about 40 square miles, and the 18 square mile Nipomo Dune complex, north and south of the Santa Maria River. Other major dune fields are located at Humboldt Bay and San Diego Bay.



Exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.