Studies of the distribution of suspended particulate matter in the waters over Hueneme, Redondo, Newport, and La Jolla submarine canyons off southern California are combined with analyses of recordings of canyon-floor currents to evaluate the influence of canyons on the seaward dispersal of fine, terrigenous sediment.
The canyon surveys were undertaken during the fall and winter months of 1971 to 1973 when suspended-sediment concentrations and composition over the mainland shelf are controlled principally by terrigenous-sediment supply. Consequently, particle concentrations at all levels generally increased toward the coast; highest values, exceeding 6 mg per 1, were present in the Newport and Hueneme areas because of the relative proximity of large rivers. The vertical distribution of suspended particulate matter is influenced strongly by the density structure of the water column on the shelf and slope, and sharply bounded, midwater, turbidity maxima are well developed over the shelf where sediment supply is comparatively large. Nepheloid layers were present and ranged from a few meters to >200 m in thickness with peak particle concentrations of 5 to 7 mg per 1 over the inner shelf and within the steep-walled, headward portions of each canyon.
Although a nepheloid layer is a characteristic component of suspensate distribution in all sampled areas, maximum particle concentrations were < 10 mg per 1 and therefore were far below those required to penetrate the water-column stratification and to produce significant density underflows. Nevertheless, computation by a modified Chézy formula of mean flow velocity due to suspended sediment above the canyon axes suggests that the slow, net downcanyon water transport demonstrated by Shepard and Marshall (1973) may be explained in part by the mechanism of turbid-layer flow. This conclusion is tentative, and the mechanism can only be evaluated adequately when more measurements of canyon currents and suspended sediment become available.