New methods for estimating annual and long-term suspended sediment loads from small tributaries to San Francisco Bay

Estimates of suspended sediment loads entering San Francisco Bay (California, USA) have been generated in the past by a number of researchers. Recently, it has been confirmed that the loads entering the Bay from the Central Valley are decreasing over time; as such, the loads from small local tributaries may constitute an increasing component of the overall Bay sediment budget. Previous estimates of suspended sediment loads entering San Francisco are outdated because of the wealth of new data, evolving land uses, and the availability of new methods of analysis. Our report presents updated estimates of suspended sediment loads entering San Francisco Bay from local tributaries. This information is essential for developing management strategies for many surface-reactive pollutants (e.g. certain trace metals and hydrophobic organic pollutants). In our study, we explore and evaluate hydrological, physical, and land-use characteristics of the San Francisco Bay watersheds to predict relationships between watershed sediment loads and geomorphic processes, and ultimately, to provide an updated estimate of regional suspended sediment loads from small tributaries. Based on this analysis, peak flow explained most of the variability in the sediment loads. Measured annual suspended sediment loads in Bay Area watersheds vary inter-annually by two to four orders of magnitude. Regionally, the new discharge- and land-use-based estimate of contemporary average annual suspended sediment loads entering the Bay is 1 300 000 t. This is equivalent to an average of 155 t km(-2). It is estimated that 35% of this load is associated with mostly urbanized watersheds.