California Coastal Landforms
Introduction
The California coast is about 2,900 km long, which includes the San Francisco Bay and the offshore islands. Some 70% of the coastline is rocky or has cliffs, although pocket and fringing beaches are found commonly below the cliffs.
The rest comprises either sandy fringing beaches backed by dunes, as south of San Luis Obispo, barrier beaches fronting lagoons and wetlands, as near Eureka, or the wetlands around San Francisco Bay. As on the Oregon coast there are numerous islands and stacks in the coastal waters of northern California that are scattered residuals of hard rock left by the recession of cliffs cut into softer sediment. About 85% of the coast is actively eroding. Although long stretches of exposed west-facing shores remain largely natural, extensive stretches around San Francisco, Los Angeles, and San Diego have been much altered by development. Of the original 80,000 ha of wetlands (excluding San Francisco Bay), 52% have been destroyed by dredging and filling, and a further 40% have been moderately or severely damaged.
The coast embraces five geomorphic provinces, the Klamath Mountains, Northern Coast Ranges, Southern Coast Ranges, Transverse Ranges, and Peninsular Ranges. These provinces are structural units whose geology and relief reflect post-Palaeozoic interaction between the westward- moving North American lithospheric plate and subducting plates farther west. California’s structural framework began to emerge in late Jurassic and Cretaceous times as the Nevadan Orogeny caused the ancestral Kalmath Mountains, Sierra Nevada, and Peninsular Ranges to rise from shallow seas and an island arc along the western margins of the North American plate, accompanied by westward thrusting, volcanism, and batholith emplacement. Subduction of marine sedimentary and volcanic rocks in the deep eugeosynclinal trench along this plate margin, followed by uplift, produced the Franciscan Formation, a heterogeneous jumble of readily erodible greywackes, shales, and metamorphic rocks found throughout the Coastal Ranges. During late Cretaceous and Cainozoic times, great thicknesses of clastic sediment and submarine volcanic rocks accumulated in subsiding basins along the continental margin and strike-slip faulting became significant. Orogenic activity culminated in the uplift of coastal mountains from Miocene to mid-Pleistocene times.
Coastal lowlands were restricted to subsiding intermontane troughs (Ventura Basin), to pull-apart structures (Santa Maria Basin), or to the seaward ends of strike valleys (Eureka Basin). During Neogene times, the North American plate approached and then overran the East Pacific Rise, a complex spreading centre in the Pacific plate. As that portion of California west of the San Andreas Fault transferred to the western limbs of this spreading centre, the Peninsular Ranges mini-plate (including Baja California) accelerated northwest from 2 to 5 cm/a year causing the Transverse Ranges to rotate into their present east–west alignment. Reflecting the influence of en echelon transform structures in the subducted East Pacific Rise, extensive strike-slip faulting with locally intense thrust faulting came to dominate coastal zone structures and landforms.
Over the past century, tide gauge data show that the sea level has been rising at an average rate of 1.5 mm/a year along much of the coast (Hicks and Crosby 1974). Thus, despite ample evidence for Quaternary uplift and deformation, recent coastal changes must be viewed against the Flandrian transgression and a continuing sea level rise.
Tides, winds, waves, currents, tsunamis, mass wasting, sediment discharge, and human activity all influence the patterns of coastal erosion and deposition. Mean spring tide ranges are between 1.5 and 1.8 m along the coast maximum tides attaining 2.5 m. Tidal currents are strong through narrow straits. At the Golden Gate, the flood inflowing tide reaches 1.7 m/s and the ebb 2.3 m/s, augmented by the head of stream discharge through San Francisco Bay. At the mouth of Tomales Bay, the ebb current clashes with incoming swells to form dangerous 3-m sneaker waves over the bar. Prevailing winds reach the coast from the northwest, producing waves and determining dune orientation. North of Point Conception 30–50% of ocean swell arrives from the northwest and most others from WNW. South of Point Conception, changing coastal orientation, strong refraction, and offshore islands cause 70% of swells to pass up the Santa Barbara Channel from due west, while 80% of swells approach Los Angeles from the WSW. Here also, southerly swells set up by late summer hurricanes off western Mexico, by Southern Hemisphere winter storms, and by local winter depressions passing along more southerly tracks may cause erosion on south-facing beaches. Wave heights at Ventura average 1 m but range from 0.3–7.0 m (Orme 1982). The predominant northwest swells set up strong longshore currents, up to 2 m/s, and longshore drift from north to south, although northward drift may be favoured by shore configuration and reversing currents. The effect of the 1,000 km wide cold California Current (mean velocity 0.1–0.3 m/s; net discharge 11 million m3/s) is more climatic and ecological than geomorphic, but fog affects weathering and soil moisture on coastal slopes and the ocean current and its inshore counter current can transport fine sediment flushed out of coastal rivers. Tsunamis occasionally affect the coast, an example being the 1964 Alaska earthquake being followed by a series of up to 7 m waves on the coast at Crescent City.
Stream sediment discharge, so important for nourishing California’s beaches, reflects the availability of rock waste, the precipitation and runoff patterns, and the extent to which fluvial transport has been blocked by dams. Along the northern California coast, erodible rocks, steep slopes, and high precipitation and runoff combine to produce frequent landslides and high erosion rates. The Klamath, Eel and Russian Rivers together account for 77% of all fine-grained sediment discharged to the sea north of Point Conception.
Sediment yielded by the Sacramento-San Joaquin river system are mostly deposited before reaching the ocean. Farther south, Franciscan rocks and Cainozoic sediment are similarly erodible and prone to mass movement, but whether this debris reaches the coast depends on the winter precipitation regime, notably on the frequency of high magnitude storm flows. For example, most of the 9 million tonnes of sand delivered to the shore by the Santa Clara River between 1933 and 1938 arrived in 6 days of floods in 1938.
Further, sediment discharged by this river in the 1969 floods was 47.6 million tonnes, compared with 1 or 2 million tonnes in relatively dry years. The coastal sediment budget of southern California thus sees years of plenty (notably 1969, 1978, 1980 and 1983) and years of famine (the 30 years preceding the 1969 floods). Mass movement from sea cliffs and coastal slopes is also an important sediment source, notably where fractured or poorly consolidated rocks are found, but this source has been both aggravated and curtailed by road and other construction work, for example along the Malibu coast west of Los Angeles and on the Big Sur coast. In many places, for one reason or another, Pleistocene landslides and slumps have been reactivated in recent times, often through changes in groundwater hydrology.
Offshore California is represented north of Point Conception by a shelf and slope, and southward by a continental borderland. The relatively narrow northern shelf widens to 50 km off the Golden Gate but is dissected by several submarine canyons, notably Delgada, Noyo, and Bodega canyons off northern California and Sur, Lucia, and Arguello canyons farther south. The largest of all is the Monterey Canyon, which heads within 0.8 km of the shore but dissects the shelf and slope to a depth of over 3,500 m. A large lunate sand bar composed of terrestrial sediment lies off the Golden Gate. The continental borderland off southern California comprises a series of fault blocks and troughs, with some closed basins and rugged emergent islands. These structures trend east-west off the Transverse Ranges and northwest-southeast farther south, and their origins must be related to the Cainozoic behaviour of the adjacent land areas (Emery 1960). At least 32 distinct submarine canyons dissect southern California’s offshore area.
The Coastline of Northern California
South from the Oregon border the coast consists of low cliffs and bluffs fringed by grey sandy beaches, with a few islands offshore. Lake Earl is a lagoon north of Point St George, where the bluffs steepen and are undercut by low cliffs. The northern California cliffs and bluffs are cut in pulverised eugeosynclinal Franciscan rocks consisting of greywackes and shales, and though they are forested they are prone to frequent mass movement and rapid erosion. These rocks strike NW-SE and have been eroded into narrow valleys that follow the strike and open into bays separated by elongated ridges that end in headlands. On this exposed coast winter storms generate waves 4–7 m high at Point St George where the coast turns southeast, and Crescent City stands on a low plateau edged by cliffs, with a series of headlands and bays. The tsunami of 27 March 1964 sent four waves, the highest over 6 m, in through the town, demolishing 29 city blocks. Sandy barriers enclose Freshwater Lagoon, Stone Lagoon and Big Lagoon, separated by spurs of high ground. The lagoons have rushy marshes and are backed by steep slopes. At Trinidad the coast is steep and partly cliffed, and to the south the coast is fringed by numerous islets and stacks, consisting of hard chert, greenstone and sandstone eroded out of a matrix of soft mudstone in the Franciscan Mélange (Fig. 1.4.1). One of these harder components protrudes from a cliff of slumping clay at Luffenholtz, and will in due course become a headland, then an island. The steep coast ends as the Little River flows out at the northern end of a long sandy beach.
Humboldt Bay is a large coastal lagoon behind a dunecapped sandy barrier, with a central outlet bordered by jetties. The shores are partly marshy ( Fig. 1.4.2), and shoreline erosion is extensive, possibly as a result of the stabilisation and dredging of the entrance to allow ships access to Eureka. The River Eel carries large quantities of sand and gravel down to an estuary largely filled with shoals, but it has not yet built a delta, and the river mouth opens between paired spits. A long steep and cliffy coast, trenched by deep valleys incised into the coast ranges, extends south, past Cape Mendocino to Punta Gorda, then southeast to Point Delgarda and Cape Vizcaino. There are sloping cliffs and sandy beaches, as at Union Landing. Headlands end in lines of stacks near Westport and there is a wide coastal terrace which extends on to flat-topped islands at Brutal Point. The even-crested cliffs end at Ten Mile River ( Fig. 1.4.2) and a long sandy beach is backed by dunes, some drifting inland.
At Fort Bragg the cliffs return, fronting a coastal terrace, and there are again many rocky stacks. Glass Beach is unusual in that it is derived from broken glass, porcelain and other garbage dumped from the cliffs, a procedure that was halted in 1970. To the south, on the coastal terrace, is Mendocino, with cliffs in sandstone and clay and reefs with kelp at the mouth of Big River. There are beaches of grey sand and pebbles at the mouth of Little River, and the coastal terrace continues southward, backed by a steep rising slope. It was cut when sea level stood higher relative to the land, and rocky knolls that rise from the terrace were formerly stacks. Navarro River is one of several rivers that have incised deep steep-sided valleys across the coastal terrace. It has a small estuary, encumbered by sand spits with some lumber. At Alder Creek the cliffs pass inland as bluffs behind a sandy beach and dunes, which extend to the mouth of Hathaway Creek at Point Arena.
The sloping cliffs at and south of Point Arena are receding evenly, cut in soft shale over sandstone and rising to a broad coastal terrace. In places the basal sandstone forms shore platforms, usually coinciding with bedding planes, while interspersed outcrops of hard grey gnarled rock protrude as headlands and are eventually isolated as reefs and stacks. Inland, the Mendocino Range runs parallel to the coast, and there are segments where the coastal terrace fades out to a steep coast, as at Saunders Landing. Rivers deliver gravelly loads to the shore, and beaches are mixtures of sand and pebbles. South from Stewarts Point the cliffs are cut in yellow sandstones which show honeycomb weathering, and shore platforms are well developed near Walsh Landing.
Fort Ross stands on a coastal terrace that slopes gently seaward, incised by short narrow steep-sided valleys and ending in cliffs and coves ( Fig. 1.4.3). The coastal terrace is interrupted by a steep sector, the Sonoma Coast, but resumes to be crossed by the Russian River, which descends from a deep gorge cut through the recently uplifted Coast Range. The estuary is blocked by a barrier of grey sand ( Fig. 1.4.4), through which the river generally maintains an outlet that is enlarged during flood discharge. Near Peaked Hill, former stacks rise abruptly from the coastal terrace, which is backed by slopes marking a degraded cliff.
Shell Beach and Duncans Beach are wide in front of cliffs, but to the south the beach narrows and rapid cliff recession threatens houses at Sorano del Mar.
From Salmon Creek the sandy beach resumes, and becomes a barrier backed by dunes and curving out to Bodega Head. On the landward side is Bodega Harbour, with a narrow outlet at the western end of a curving sand spit, and the coast then runs close to the line of the San Andreas Fault along Bodega Bay. Dillon Beach is wide and sandy, receiving ocean swell, but Tomales Point, offshore, marks the beginning of the long narrow Tomales Bay, where wave energy is much reduced. There are successive headlands and bays along the coastline that runs southeast, past the salt marshes in the Chileno River estuary ( Fig. 1.4.5) and along the seaward slopes of the Balinas Ridge. Bluffs pass into cliffs on the more exposed promontories, south of Marconi, and at the head of Tomales Bay there is a marshy delta at the mouth of Olemo Creek.
The inner shore of the Point Reyes Peninsula is fairly straight, close to the San Andreas Fault, and consists of Mesozoic granite. Lateral movement along this fault is causing the Point Reyes Peninsula to move northwest along the coast at an average rate of about 5 cm/a year, much of the movement occurring during earthquakes, as in 1906 when there was sudden displacement of up to 6 m. The outer coast at Tomales Point is exposed to oceanic waves, and cliffs cut in red sandstone underlain by hard grey Sierran granite extend south past McLures Beach. The high energy Pacific swell generates major rip currents here.
Much of the west coast of the Point Reyes Peninsula has a wide sandy beach backed by dunes, forming a barrier that impounds Abbotts Lagoon in a gently incised valley. At the southern end, Point Reyes is a steep-sided ridge running west to east, and on the south coast is the broad curve of Drakes Bay, with cliffs cut in soft Pliocene sediment (Fig. 1.4.6) and beaches that become barriers and spits in front of lakes (Drakes Estero and the adjoining Estero de Limantour) impounded in rias.
Drakes Head is a cliffed promontory behind the barrier spits, and an outlet from the esteros has an ebb tide delta that refracts incoming waves. The coast steepens on the flanks of forested ranges down to Bolinas Point and Duxbury Point. U.S. Coast Survey records indicate that these points have been receding at about 0.5 m/a year.
The San Andreas Fault runs southeast from Tomales Bay along a valley that ends in Bolinas Lagoon, which is almost enclosed by a sandy spit.
The steep forested coast then continues to Point Bonita, on the northern side of Golden Gate, the strait at the mouth of San Francisco Bay.
San Francisco Bay
San Francisco Bay occupies a late Pliocene to mid-Pleistocene structural trough within the Coast Ranges. The trough has a Franciscan basement bounded by northwest- trending strike-slip faults and is largely filled with Neogene and Quaternary marine and fluvial sediment.
Continuing tectonism apart, this trough has been further deepened by fluvial erosion and invaded several times by Quaternary transgressions (Atwater et al. 1977). The bay is connected to the Central Valley through the Carquinez Strait, with a bedrock channel incised by the ancestral Sacramento River to 60 m below sea level, and to the Pacific Ocean through the Golden Gate, with a channel that is scoured to 104 m below sea level. San Francisco Bay is a natural sump for an 80,000 sq. km drainage area comprising the Sacramento-San Joaquin system. Some 200 years ago the bay waters covered 1,800 sq. km, but they have been reduced to 1,100 sq. km, and 70% of the bay is shallower than 4 m. This reduction is due in part to massive fluvial sedimentation during and after a period of hydraulic gold mining in the Sierra Nevada foothills in the latter half of the nineteenth century, and in part to extensive coastal land reclamation. Gilbert (1917) estimated that 1,816 million m3 of debris were eroded in the foothills by mining and natural processes between 1850 and 1914, of which 50% was deposited in the valley downstream, 48% reached the bay and only 2% reached the ocean. He also estimated that it would take 50 years (until 1964) for the upstream debris to reach the bay, after which the yield would diminish to about 6 million m3/a year, a figure that accords well with recent estimates.
Wave action within San Francisco Bay is limited by the short fetch, and breaker heights average at 0.3 m. Mean spring tide range is about 1.8 m, and tidal currents rarely exceed 1 m/s within the bay. The salinity averages 27–29‰, with dilution by inflow of about 570 m3/s from the Sacramento-San Joaquin River. The relatively low-energy shores have salt marshes around mean high tide level dominated by Spartina leiantha, Distichlis spicata, and Salicornia ambigua zones, upper tidal flats between mean high and mean low water and lower tidal flats below mean low water (Pestrong 1972). About 200 sq. km of marshes and mudflats are exposed at mean tide level.
The Coastline of Southern California
It is convenient here to consider Southern California as the coast, south from San Francisco. The Southern Coast Ranges extend 400 km southeast from the Golden Gate to Point Arguello and are dominated by massive deformation and faulting, strike valleys, and elongated ridges.
Between the San Andreas and Nacimiento fault zones, metamorphic and granitic rocks form the 60-km-wide Salinian basement. These rocks reach the coast in rugged slide-prone cliffs south of Pacifica.
Pillar Point is a southward projection bordering Half Moon Bay, which has been shaped by ocean swell refracted around the point and has a beach showing lateral gradation in grain size, coarsening and steepening as exposure to wave action increases southward (Bascom 1951). A generally steep coast with valleys incised into the ranges extends past Pigeon Point and Point Ano Nuevo, where the coast curves to Santa Cruz in Monterey Bay. Only one of the natural arches cut in Santa Cruz mudstone is still standing; the others have collapsed to leave stacks offshore. Late Cretaceous and Tertiary sediment reach the coast north of Monterey Bay in deformed marine terraces and eroding sea cliffs (Bradley and Griggs 1976). The terraces have been rising at 1.6–2.6 cm/century and long-term sea cliff erosion rates range from zero in resistant granodiorites to 0.3 m a year or more in softer Tertiary sediment, reaching 0.6 m/a year locally (Griggs and Johnson 1979).
Extensive sand dunes around Monterey Bay are explained in part by the abundance of sediment formerly brought down by the Pajaro and Salinas rivers.
For 150 km south from Monterey the Santa Lucia Range dominates the coast, often reaching 1,000 m within 2 km of the shore. Where Franciscan rocks reappear west of the Nacimiento Fault, landsliding is a perennial problem. South of Ragged Point, the mountains trend inland, emerged marine terraces reappear, and broad structural basins are veneered with extensive dunes of late Pleistocene and Holocene age, notably behind the 25 km of sandy beach between Pismo Beach and Point Sal. Sand for these dunes comes mainly from the Arroyo Grande and from the Santa Maria and Santa Ynez rivers. Cliff erosion threatens homes and roads at Cayucos, Avila Beach, and Shell Beach, despite partial shelter from northwesterly swells. In Morro Bay salt marshes are protected by a long barrier spit. In the hinterland is a series of transgressive dune formations (Orme 1990).
The Santa Ynez Mountains, which rise to 1,430 m extend 125 km east from Point Arguello to Ventura, and are a south-dipping homocline of mostly Tertiary marine and fluvial sediment. Quaternary uplift has produced much deformed marine terraces along the coast. Though protected by offshore islands, refracted westerly ocean swell have produced average erosion rates of 0.15 m/a year in the relatively soft Neogene sediment along the Santa Barbara coast. A small barrier-lagoon system survives at Carpinteria. The Oxnard Plain is a triangular lowland, formed mainly by deposition from the Santa Clara River and Calleguas Creek, where the Ventura structural trough plunges westward into the Santa Barbara Channel. The coastline once comprised a sandy barrier beach with low dunes backed by lagoons and marshes, breached by rivers during winter floods, but there has been much construction along the shore. Breakwaters and groynes have been built at Santa Barbara, Ventura and Port Hueneme, and there have been problems with interception of eastward longshore drifting of sand by the prevailing westerly swell.
About 750,000 m3/year of sediment moves along this coast in one of several littoral cells on the coast of Southern California, each with sand supplied mainly from rivers passing alongshore until it is lost into the head of a submarine canyon (Emery 1960). To the south of the Santa Barbara Channel the Channel Islands (Anacapa, Santa Cruz, Santa Rosa, San Miguel) are underlain by a metamorphic or granitic basement and mantled with thick late Cretaceous and Tertiary rocks, notably Miocene marine shales and volcanic formations.
The Santa Monica Mountains, which rise to 949 m, extend 50 km along the coast between Point Mugu and Santa Monica and are bounded to the south by the active Malibu thrust fault, the impact of which is seen in seismic activity, deformed terraces, fractured coastal rocks, and frequent mass movement. Much of the coast is typified by unwise housing development on the backshore or beneath crumbling cliffs, creating inevitable problems during storms and southerly swells. Access is restricted to many beaches, as at Malibu.
The Peninsular Ranges extend SSE for 200 km behind Los Angeles to the Mexican border. This mini-plate, comprising basement rocks, granitic plutons and post-batholithic sediment, has been diverging from the North American mainland at a mean rate of 6 cm/a year since it shifted on to the western limb of the East Pacific Rise some 4–5 million years ago.
Along the coast deformed marine terraces such as those on Palos Verdes ( Fig. 1.4.7), San Onofre Mountain and around San Diego, and active faults as in the Inglewood-Rose Canyon structural zone that outlines the coast between Newport Bay and San Diego, attest to continuing tectonic activity.
The beach fringed Los Angeles coast is heavily urbanised, and Long Beach is a port and industrial urban complex where coastal land subsidence has followed oil extraction. Barrier and lagoon systems were once common along this coast, but most have been modified, and some obliterated, by reclamation. Ballona Lagoon, once an outlet for the Los Angeles River, is now the marine and Newport Bay, until 1915 the outlet for the Santa Ana River, was heavily developed. Offshore is Santa Catalina Island, which has warped emerged marine terraces.
Much of the coast south of Newport is cliffed, between valleys where rivers supply sand to the shore, sometimes through small barrier lagoons. This is another littoral cell in which beaches have been described as ‘rivers of sand,’ fluvial sediment drifting southward along the coast until it disappears into a submarine canyon off San Diego.
Sand is also derived from receding cliffs, partly by marine erosion and slumping and partly by the effects of runoff. Erosion is a significant problem along much of this coast. Powerful ocean swell and storm waves arrive through windows between the offshore islands, coastal rock formations are often poorly consolidated and, owing to floodcontrol dams and channelization along most of the rivers, fluvial sediment supply has diminished in recent decades, depleting protective beaches. Cliff erosion is particularly rapid in front of Camp Pendleton, south of Oceanside harbour, near Encinitas and Del Mar, and at Sunset Cliffs, San Diego. Groundwater discharge weakens soft cliffs, causing exudations and triggering slumping. Inappropriate clifftop development has generally aggravated the problem, for example, through irrigation and lawn watering, increased impermeable surfaces, and poorly located culverts.
At La Jolla Scripps Pier has been used as a transect for measuring fluctuations in the beach profile, notably the cut and fill cycles that accompany alternations of winter storm erosion and summer shoreward drifting. These cycles can result in beach lamination, with layers of fine and coarse sediment deposited during accretion with phases of calm-weather fine sedimentation and rough weather winnowing.
Rocky shore outcrops show evidence of weathering by alternations of wetting and drying, producing platforms at the level of permanent saturation, as well as corrosion bysalt weathering. Boomer Beach, near La Jolla, is well known for its seasonal alternations. In winter storm waves scour away sand exposing a rocky and bouldery shore, but in summer constructive waves sweep sand back in from the sea floor to bury these beneath a sandy beach. To the south San Diego has an urbanised coast, and the former San Diego River outlet has become an important naval and commercial harbour, bordered by Coronado Strand, a built-over spit. The urbanised coast extends to the Mexican border.