Abstract Thorough and accurate models of modern sand sources to the Santa Monica Basin, offshore southern California, are needed to facilitate the interpretation of sediment supply to the Hueneme–Mugu Fan, the largest submarine fan within the basin. Bathymetry and near-seafloor seismic-reflection isopach mapping of basin fill indicate that dominant sources of sand are canyons/channels that enter the basin from the northwest, which are likely fed via longshore drift by the Santa Clara River and Calleguas Creek. Sand within Calleguas Creek varies in composition across its drainage basin, ranging from more quartzofeldspathic in the northeast to more volcaniclastic in the southwest, ultimately producing sand with a compositional fingerprint distinct from that of the Santa Clara River and Santa Monica Mountains. The abundance of volcanic material and lack of metamorphic grains in downstream Calleguas Creek sand stand in stark contrast to the Santa Clara River’s relatively abundant metamorphic lithic fragments. In addition, Calleguas Creek sand can be further differentiated from sand derived from both the Santa Clara River and Santa Monica Mountains because both of these other sources have much higher proportions of plagioclase. The composition of late Pleistocene (<60 ka) sandy turbidites at Ocean Drilling Program Site 1015 on the distal Hueneme–Mugu submarine fan validates Calleguas Creek’s contribution of sandy sediment to this site in the Santa Monica Basin: out of 14 samples, four samples show compositions similar to the Santa Clara River sand, whereas five are similar to Calleguas Creek sand, and six exhibit mixed compositions. There is no indication of input to this distal environment from the southern Santa Monica Mountains. Trends in sand composition within the Santa Monica Basin can be related to alternating and/or mixing of sediment sources, possibly related to sea-level change, as well as frequency of floods/storms, earthquakes, and other destabilizing processes affecting offshore shelf-to-slope regions.

The rivers of Southern California deliver episodic pulses of water, sediment, nutrients, and pollutants to the region's coastal waters. Although river-sediment dispersal is observed in positively buoyant (hypopycnal) turbid plumes extending tens of kilometers from river mouths, very little of the river sediment is found in these plumes. Rather, river sediment settles quickly from hypopycnal plumes to the seabed, where transport is controlled by bottom-boundary layer processes, presumably including fluid-mud (hyperpycnal) gravity currents. Here we investigate the geographical patterns of river-sediment dispersal processes by examining suspended-sediment concentrations and loads and the continental shelf morphology offshore river mouths. Throughout Southern California, river sediment is discharged at concentrations adequately high to induce enhanced sediment settling, including negative buoyancy. The rivers draining the Western Transverse Range produce suspended-sediment concentrations that are orders of magnitude greater than those in the urbanized region and Peninsular Range to the south, largely due to differences in sediment yield. The majority of sediment discharge from the Santa Clara River and Calleguas Creek occurs above the theoretical negative buoyancy concentration (>40 g/l). These rivers also produce event sediment loading as great as the Eel River, where fluid-mud gravity currents are observed. The continental shelf of Southern California has variable morphology, which influences the ability to transport via gravity currents. Over half of the rivers examined are adjacent to shelf slopes greater than 0.01, which are adequately steep to sustain auto-suspending gravity currents across the shelf, and have little (<10 m) Holocene sediment accumulation. Shelf settings of the Ventura, Santa Clara, and Tijuana Rivers are very broad and low sloped (less than 0.004), which suggests that fluid-mud gravity currents could transport across these shelves, albeit slowly (~10 cm/s) and only with adequate wave-generated shear stress and sediment loading. Calleguas Creek is unique in that it discharges directly into a steep-sloped canyon (greater than 0.1) that should allow for violent auto-suspending gravity currents. In light of this, only one shelf setting—the Santa Clara and Ventura—has considerable Holocene sediment accumulation (exceeding 60 m), and here we show that the morphology of this shelf is very similar to an equilibrium shape predicted by gravity-current sediment transport. Thus, we conclude that a wide distribution of river-shelf settings occur in the Southern California Bight, which will directly influence sediment dispersal processes—both dilute suspended and gravity-current transport—and sediment-accumulation patterns.

Southern California, extending from Point Conception (34 1/2 °N) to the Tijuana estuary (32° N), has a varied and attractive coastline, with a moderate Mediterranean climate and a large human population. Rapid urban development has resulted in coastal modification for shipping, military, industrial, housing and recreational needs. Because of their flat topography and availability of water, the coastal wetlands have been the prime target for these modifications. According to Zedler (1982, p.l), the ‘disturbance has been so pervasive, taken such different forms, and had such different results in each wetland, that sorting out natural and unnatural features is extremely difficult’. Southern California coastal wetlands are small and discrete, confined to narrow river valleys, and separated by coastal hills, mountains, harbors and extensive urban tracts. There are about 30 wetlands, occupying a total area of about 12,500 acres (Fig 1) which represent about 10% of their extent prior to arrival of Europeans. The wetlands occur on intertidal slopes and along the mouth of creeks, and support a variety of salt tolerant plants, called halophytes. Poised at the edge of both land and sea, these habitats receive fresh water, sediments and nutrients from the watershed, and tidal water with its salts, minerals and sands from the sea. At some locations, coastal dunes enclose these wetlands and occasionally, sand bars form, cutting off tidal circulation. This has resulted in decimation of certain plant communities and dependant bird populations.

Abstract Continental shelves are the key interfaces between terrestrial sediment source areas and deep-sea depositional systems, promoting the transfer of sediment across continental margins. Work on shelves in the context of entire continental-margin sediment-routing systems has focused on their importance as capacitors of sediment during several to tens of thousands of years of post-glacial shoreline transgression and sea-level highstand. We demonstrate that the tectonically active Oceanside shelf offshore southern California has served as an efficient conveyor of sediment from land to the deep sea during millennia of significant climatic fluctuations. This conveyance is a result of littoral drift of sediment to canyon heads at narrow segments of the shelf. We compare insights from the Oceanside shelf to other shelves across the tectonically active Pacific margin of the United States, and demonstrate the importance of shelf width, climatic forcings and timescale of observation in assessing the role of shelves as sediment capacitors or conveyors.