Controls on the delivery of fluvial sediment to the coastal ocean: The Salinas River, California

Understanding the controls on the magnitude and timing of sediment delivery to the coastal ocean is important when interpreting the role of rivers in the biological, chemical, physical and geological coastal zone systems. A global database (1500 rivers) was assembled to estimate global fluvial delivery of water and sediment to the coastal ocean. I estimate an annual delivery of 35,000 km3 of freshwater, 4 * 106 tonnes of dissolved solids and 18.6 * 106 tonnes of suspended sediment. The global delivery of fluvial water and sediment, both suspended and dissolved, is dominated by southeast Asia, due to the unique climatic, geologic and geomorphic character of the rivers in this region. Over 30% of the global freshwater discharge to the oceans is estimated to originate in southeast Asia and Oceania. This same region of the world is responsible for over 30% of the dissolved solid input and an astounding 70% of the suspended sediment. The Salinas River, central California, was the focus of an in-depth study on the controls on sediment delivery from a small (11,000 km2), semi-arid watershed. The Salinas River was chosen because there is a significant historical dataset from monitoring efforts throughout the basin, and it was thought to react to El Nino conditions in a similar manner to the rivers of southern California. This river discharges into the Monterey Bay (Central California) an average of 0.4 km3 of water and 3.3 tonnes of sediment per year. Basin scale control on the discharge of the river is dominated by the underlying geology as well as the anthropogenic changes to the watershed (e.g., damming of 2 large tributaries and high concentration of agriculture). Despite the highly altered nature of the Salinas River, the fluvial discharge is dominated by short-lived, intense meteorological events. The frequency of these events is determined by the oceanic and atmospheric conditions present offshore. Historically, large flood events on the Salinas River almost entirely correlate with El Nino events. However, not all El Nino years produce large flood events. It was determined that the probability of a large flood on the Salinas River is determined not only by the presence of El Nino conditions, but also by the interaction between the Pacific Decadal Oscillation with the ENSO. The coinciding of warm phases of both of these large-scale phenomena produces significantly higher annual discharges than any other combination of the climatic phenomena. Estimating the sediment load of the river from historical gauging records allows us to determine the role the Salinas River plays in the sediment budget of the Monterey Bay. The Monterey Submarine Canyon bisects the bay, restricting the shelf space and creating 2 separate sedimentary environments. The southern shelf bypasses most modem fluvial sediment to the canyon, whereas the northern shelf stores most of the sediment delivered. The estimate of Holocene sediment discharge from the rivers and creeks of the bay indicate that > 60% of fluvially delivered sediment is lost to the canyon and deep ocean. The transport pathways of this removal are currently unknown, but hyperpycnal flow from the mouth of the Salinas River is hypothesized.