Short-term (1997--2000) and long-term (1928--2000) observations of river water and sediment discharge to the Santa Barbara Channel, California

Discharge into the Santa Barbara Channel was characterized with stream and oceanographic measurements, and analyses of remotely sensed satellite imagery. This discharge into the channel is ephemeral and highly pulsed. Over half of the discharge occurs during approximately 1% of the time (i.e. ∼3 days per year). Average annual runoff during 1928–2000 in the study area watersheds ranged between 2 and 23 cm. Runoff was largely a function of rainfall, and the steepest watersheds with large orographic effects on precipitation produce the greatest rates of runoff. The pulsed flood discharge is highly concentrated with suspended sediment (often >10 g L−1 ), and areas underlain by weak Plio-Pleistocene sedimentary bedrock produced extremely high (>100 g L−1) suspended sediment concentrations. Average suspended sediment yields ranged between 400 and 6000 t km−2 yr−1 and are significantly correlated with bedrock geology (first order) and land use (second order). Water and sediment budgets of the watersheds suggest that although the Santa Clara River incorporates the majority of drainage area (∼75%), it only contributes approximately half of the water and sediment loads. These reduced yields are primarily a function of tributaries with low yields, rather than natural storage processes (groundwater recharge and sedimentation). The coastal margin wetlands of the channel reduce water and sediment loads by insignificant amounts.; River discharge into the Santa Barbara Channel is very energetic and dominated by hyperpycnal (suspended sediment > 40 g L−1) events. Due to high discharge velocities the horizontal dispersal of river plumes initially is dominated by momentum. Once the momentum dissipates due to friction, the plume is forced by ocean currents and winds. Within approximately 1-km of the river mouth, the river discharge quickly stratifies into a freshened, turbid surface plume and a bottom nephloid layer. Oceanographic measurements reveal that little of the discharged sediment is observed in either the surface or bottom plumes, which suggests that sediment settles rapidly on to the inner continental shelf.