Suspended-Sediment Transport Where Rivers Become an Estuary: Sacramento-San Joaquin River Delta, Water Years 1999-2002

Tidal influence from San Francisco Bay extends landward into the Sacramento-San Joaquin River Delta at the confluence of the Sacramento and San Joaquin Rivers. The interplay between the riverine and estuary forcing has created a complex network of channels and tidal wetlands which has been significantly modified by humans over the past 150 years. Interest in restoring tidal action and sedimentation to some diked and subsided former tidal wetlands lead the U.S. Geological Survey to begin collecting data in July 1998 in order to describe sediment transport in the Delta. The data collection program consisted primarily of a network of optical backscatter sensors supported by standard suspended-sediment sampling techniques and flow measurements in order to develop continuous, 15-minute, records of sediment flux at several sites. These data were combined with other available sediment transport data in order to develop a sediment budget with error estimates. Over the four-year period, water years 1999-2002, 6.6±0.9 million metric tons of sediment entered the Delta and 2.2±0.7 million metric tons exited, resulting in 4.4±1.1 million metric tons of deposition. This mass of deposited sediment corresponds to approximately 2.1±0.5 cm of deposition averaged over the entire open water and wetland area of the Delta over the four-year period (or 0.52±0.13 cm/yr). The deposition rate from our sediment flux calculations is virtually identical to the recent deposition rate from analysis of sediment cores. Over the four-year period, 85% of the suspended-sediment came from the Sacramento River, 13% came from the San Joaquin River and 2% came from other sources. Analysis of tidally averaged sediment flux and sediment transport pathways indicates that the San Joaquin riverine signal attenuates more rapidly in the downstream direction (i.e. tidal effects increase) than for the Sacramento River. At least 82% of the sediment entering the Delta from the Sacramento River watershed either deposits along the Sacramento River or moves past Mallard Island and into San Francisco Bay, leaving no more than 18% to move through the complex network of channels toward the San Joaquin River.