The following materials are 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.
Beaches are dynamic landforms altered by wind and waves in a continual process of creation and erosion. Seasonal cycles of sand deposition and loss dramatically affect the appearance of beaches from summer to winter. Wide and gently sloping in summer, they become steep-fronted and narrow in winter, and cam vanish overnight, stripped of sand by violent storm waves. Most of the sand removed from winter beaches is deposited in offshore sandbars and is returned to the beach during the mild summer months by gentle swells that push the sand to the exposed shore. River sediments are the source of 80 to 90 per cent of beach sand; some beaches are built to great widths by sediments washed to the sea by episodic floods, gradually eroding until the next major flood replenishes the sand.
Beach formation begins as eroded continental material--sand, gravel, and cobble fragments--is washed to sea by streams and rivers. Two separate processes result in the deposit of this sand and sediment on the shore. Most sediment is suspended in sea water and transported along the coast by the longshore current, a stream of water flowing parallel to the beach that is created by the action of waves breaking at an angle to shore. Longshore transport can deliver up to a million cubic yards of sediment annually to a single beach. In the second process, sand deposited onshore by the longshore current is then oscillated by waves breaking onto and receding from the beach. This continual onshore-offshore movement gradually pushes the sand along the beach edge. both the longshore transport of sediment along the coast and the movement of sand by waves along the foreshore are a part of the process called littoral drift.
The California coastline has been divided into geographic segments called littoral cells, that incorporate a complete cycle of beach sediment supply, sand transport by the longshore current, and eventual permanent loss of sand from the littoral cell. The five types of littoral cells along the California coast are each characterized by a different littoral process determined by the geographic features unique to the cell type. One type of cell is defined by a long stretch of coastline that begins at a headland and terminates in a submarine canyon, such as at Mugu Canyon in Venture County and La Jolla Canyon in San Diego County; another cell type consists of a large river delta bounded on either side by rocky headlands, such as at Humboldt Bay; a third type of littoral cell is defined by a crescent-shaped by downcoast of a promontory, like Half Moon Bay in San Mateo County; and a fourth type of cell consists of a rocky headland downcoast of a beach where waves break in a line parallel to the shore, as at Ten Mile Beach in Mendocino County. Finally, lagoons and closed bays with restricted tidal flow create a fifth type of littoral cell, such as Bolinas Bay in Marin.
Apart from littoral cell type, there are characteristic differences between Northern and Southern California beaches, depending upon the directions of prevailing wind and upon local coastal geology. Along California's north coast, cove or pocket beaches are common where the granitic and basaltic rock that composes the sea cliffs has been sculpted by prevailing northwesterly winds and battered by high energy waves over millions of years. In Southern California, beaches often consist of long ribbons of sand interrupted by widely separated rocky points. The bluffs of easily eroded shales and sandstones that edge the coast here continuously crumble away, creating on even coastline over time.
Some beach types are found along both Northern and Southern California coasts. Narrow cove beaches like those at Laguna Beach in Orange County form where the coast is composed of conglomerate rock and hard sandstone; even when exposed to direct wave attack this rock type is highly resistant to erosion. The narrow beaches formed within there coves often lose all their sand during winter storms, exposing the underlying cobbles, as at Boomer Beach, south of Point La Jolla in San Diego County. Barrier beaches and sand spits are also present along the coast at river mouths, bays, and lagoons; examples are Silver Strand Beach in San Diego, Zuma Beach in Malibu, and beaches at the Smith, Salinas, Pajaro, and Santa Maria River mouths.
Beaches vary in color according to the mineral content of the sand, which is also a clue to the origin to the eroded sediments that make up the sand supply. eroded shale cliffs create the charcoal gray beach sand at Shelter Cove in Humboldt County. North of Humboldt Bay, the coarse sands of Agate Beach are multicolored agates that have been ground and polished by the surf. Ground quartz and feldspar mineral make up the white beaches of Carmel, while a few miles to the north in Sand City, amber colored sand indicates the presence of iron mineral. Close inspection reveals that white sand beaches are mosaic of pale quartz grains, pink green or white feldspar and fleck o black mica.
Beaches are inhabited by a variety of invertebrates and insects. In the surf zone, bivalve mollusk, crustaceans, and tube-building worms adapt to their environment of tide cycles and buffeting waves by burrowing to protect themselves from wave impact, temperature fluctuations, desiccation, and predation. The smooth shells of clams and other bivalve burrowers reduce friction when they tunnel through the fine sandy beaches of their preferred habitat. At low tide, water retained between the sand particles is filled with millions of microscopic diatoms and zooplankton upon which the buried bivalves feed, using long siphons that reach to the sand surface. Fine screens within the siphons filter out sand particles by allow the passage of water and suspended organic material that provide an abundant food supply for the filter- feeding bivalves. Razor clams, surf clams and coquina clams are common burrowers along California beaches. Pismo clams occupy a special niche in the surf zone of Central California beaches, well-adapted to the crashing surf by nature of their large, heavy shells, which act as anchors. these giant clams are dependent upon the high-oxygen content of the roiling surf to survive.
Inland from the surf zone, sand craves scavenge in the sun-dried kelp and bury in the sand, using their antennae to rake food particles to their mouths. Kelp flies, wrack flies, rove beetles, tiger beetles, and dune beetles ream the beach foreshore. The dry upper beach is inhabited by air-breathing pill buys and beach hoppers. Numerous beetle species inhabit the dunes, some burrowing in the sand during the day to escape predators and the heat.
The natural process of beach building and erosion has been altered by extensive development of the California coast. Prior to development, natural loss of sand from beaches, largely to dunes and submarine canyons, and natural sand supply, mostly from rivers and streams, were in rough balance. The damming of rivers alone has reduced half of the natural sand supply to beaches from Santa Barbara to Mexico. The natural balance of beach sand supply and loss has been altered by the construction of offshore breakwaters, groins, and jetties, which may divert sand from one location to another and change beach slope. In a few locations large-scale beach nourishment projects have created wide beaches that may last several decades or more before eroding away, but not all coastal areas are suitable for such projects; even where these projects have been successful, this solution to the problem of beach erosion is costly and impermanent.
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