Joint Federal Interagency Sedimentation Conferenet and Federal Interagency Hydrologic Modeling Conference 2015

SEDHYD 2015 Proceedings »

Sediment Budgets, Transport, and Depositional Trends in a Large Tidal Delta

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The Sacramento-San Joaquin Delta was once a large tidal freshwater marsh formed by the accumulation of sediments and organic material following the last ice age. The geomorphology as of 2,000-3,000 years ago included a puzzle of numerous shallow dendritic channels and sloughs surrounding tidal wetlands that were formed at the confluence of California’s two largest rivers. Water and sediments backed up into the Central Valley from the combination of slowing currents, tidal action, and lush wetland foliage. In the late 1800s, the largest wetland on the west coast of the United States was transformed into one of California’s richest agricultural areas of both local and global economic value. Low natural levees around the wetlands and floodplains were periodically flooded and built up in elevation, forming “islands” that were cut off from tidal activity and farmed. Today 50% of California’s freshwater runoff flows through the Delta in a network of mostly clear channelized canals surrounding subsided islands and is then pumped from the South Delta to Central and Southern California to support agricultural and urban demands. More recently, species population declines have been associated with decreased natural habitat. One species of concern is the endangered Delta smelt which requires a turbid environment for feeding, migratory cues, and to avoid predation. Efforts to restore significant aquatic habitat throughout the Delta are in progress. The development of landscape-scale restoration requires understanding of the hydrology and sediment quantity and distribution. The USGS has an extensive network of flow stations throughout the Delta where we also monitor turbidity and sediment. In 2011, we expanded our sediment monitoring network to a total of 17 sites that provide 15-minute continuous data. We are able to quantify sediment budgets on event, seasonal, and annual scales, determine seasonal transport patterns, and long-term depositional patterns. We calculated annual sediment budgets for the Delta, and spatial depositional patterns for three regions and compared these to velocity patterns. The ability to quantify the volume of sediment within each northern, central, and southern region is essential to restoration planning and determination of accretion in the face of sea level rise associated with climate change. Sediment loads were computed from the product of cross-sectional averaged suspended-sediment concentrations and flow. Results from 2011, which was a wet year, show that the largest quantity of sediment was supplied to the north Delta, a smaller volume was supplied to the central Delta, and the smallest volume is supplied to the south Delta. Though the largest quantity of sediment moves into the north Delta, the largest amount of deposition in 2011 occurred in the south Delta, due to differences in hydrodynamic conditions among the regions. Methods for the collection and calculation of suspended-sediment and load data using turbidity as a surrogate in a tidal Delta will be discussed.

Author(s):

Tara Morgan-King    
California Water Science Center
USGS

Scott Wright    
California Water Science Center
USGS

 

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