Sources, Dispersal, and Clay Mineral Composition of Fine-Grained Sediment off Central and Northern California

Fine-grained sediment delivered to the continental margin of central and northern California is dominated by input from coastal rivers. Suspended sediment from northern California rivers (the Eel, Klamath-Trinity, Mad, Smith, and Mattole Rivers, and Redwood Creek) amounts to 81% of the sediment produced between Point Conception and the Oregon border. Steep and uns Table hillslopes combined with high rainfall and land use alterations produce average annual sediment yields up to nearly 2800 tons/km². Offshore circulation during periods of peak sediment discharge is dominated by the northerly flowing Davidson Current. LANDS AT imagery reveals large clockwise gyres that transport suspended sediment at least 100 km offshore in surface waters. Along the central coast the regional sediment yields are less than those along the northern California coast by an order of magnitude. Lower precipitation and numerous impoundments and diversions reduce sediment discharge. The Russian, Sacramento and San Joaquin Rivers do discharge significant volumes of suspended sediment, however. From Monterey Bay to Point Conception the climate is even drier, and sediment input declines further. Distinct sediment plumes on LANDS AT imagery, however, often extend offshore from Monterey Bay and also along the coast north of Point Conception. Based on the clay mineral composition of sediment samples from the continental margin, five distinct provinces were delineated. The composite clay mineral composition of the sediments taken from rivers draining into the individual provinces in general agree reasonably well with offshore compositions. Compositional differences existing off the central coast, however, are believed to be a result of relict Pleistocene sediments on the shelf which have not yet been covered by post-glacial sediment input.