Understanding Tidal Marsh Sedimentation in the Sacramento-San Joaquin Delta, California

Over 90% of the once vast tidal-freshwater wetlands of the Sacramento-San Joaquin Delta have been leveed and removed from tidal and floodwater inundation. Contemporary restoration efforts breach and/or remove the levees surrounding delta islands with the goal of regaining wetland habitat. However, experience so far with levee-breaches, both planned and unplanned, has shown that the transition from shallow open water to Tule marshes occurs slowly, if at all. Sedimentation rates in tidal marshes are an important control on this transition. This two-year study examined potential controlling factors on vertical accumulation including sediment type, and inorganic and organic soil components. Measured rates of surface accretion are in excess of 10 mm/yr. Gravimetric determinations show the main control on variations in soil accretion appears to be mineral sediment accumulation, with highest rates being close to inputs from the Sacramento River and lowest rates in interior marshes in the south-central delta. Rates of organic accumulation are remarkably similar among areas. However, measurements of marsh surface elevation change reflect the volumetric contributions of soil components and appear to be influenced by interannual changes in vegetation associated with minor variations in salinity. Marshes in breached-levee sites showed higher annual rates of marsh surface elevation change than reference marshes. This study demonstrates the importance of examining volumetric rather than gravimetric contributions to marsh soils in evaluating the factors controlling the vertical development of tidal marshes.