Suspended-sediment dynamics in the tidal reach of a San Francisco Bay tributary
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To better understand suspended-sediment transport in a tidal slough adjacent to a large wetland restoration project, we deployed continuously measuring temperature, salinity, depth, turbidity, and velocity sensors in 2010 at a near-bottom location in Alviso Slough (Alviso, California, USA). Alviso Slough is the downstream reach of the Guadalupe River and flows into the far southern end of San Francisco Bay. River flow is influenced by the Mediterranean climate, with high flows (∼90 m3 s−1) correlated to episodic winter storms and low base flow (∼0.85 m3 s−1) during the summer. Storms and associated runoff have a large influence on sediment flux for brief periods, but the annual peak sediment concentrations in the slough, which occur in April and May, are similar to the rest of this part of the bay and are not directly related to peak discharge events. Strong spring tides promote a large upstream sediment flux as a front associated with the passage of a salt wedge during flood tide. Neap tides do not have flood-directed fronts, but a front seen sometimes during ebb tide appears to be associated with the breakdown of stratification in the slough. During neap tides, stratification likely suppresses sediment transport during weaker flood and ebb tides. The slough is flood dominant during spring tides, and ebb dominant during neap tides. Extreme events in landward (salt wedge) and bayward (rainfall events) suspended-sediment flux account for 5.0 % of the total sediment flux in the slough and only 0.55 % of the samples. The remaining 95 % of the total sediment flux is due to tidal transport, with an imbalance in the daily tidal transport producing net landward flux. Overall, net sediment transport during this study was landward indicating that sediment in the sloughs may not be flushed to the bay and are available for sedimentation in the adjacent marshes and ponds.
KeywordsEstuaries Tides Tidal slough Sediment flux Stratification
The authors gratefully thank Paul Buchanan, Robert Castagna, Amber Powell, Chris Silva, Kurt Weidich, Brooks Weisser, and Rob Wilson for the field work and data management on this project. The manuscript has been improved though discussions with Scott Wright. Funding for this work has come from the US Army Corps of Engineers, California State Coastal Conservancy, the Regional Monitoring Program for Water Quality in San Francisco Bay, and the US Geological Survey Priority Ecosystems Science Program. The Santa Clara Valley Water District supported the sediment measurements at the Guadalupe River station. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
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