Twenty-nine years of geomorphic change at Elkhorn Slough, California
Thesis (M.S.) Division of Science and Environmental Policy
This study utilized high-precision surveys to estimate 29 years of elevation change on the Elkhorn Slough marsh plain. There were 3 objectives to this study: 1) characterize the spatial variation in rates of net erosion/deposition and net vertical change with respect to the benchmark, 2) compare net vertical change rates to estimates of projected rate of sea-level rise in the region, and 3) determine linkages between land cover type and rate of net vertical change. We resurveyed 11 of the 13 original cross sections using the same methodology to collect new surface elevations for comparison with the original 1980 dataset. Overall, survey points on the marsh plain averaged 0.5 cm/yr of accretion (SD = 0.4 cm/yr), but an estimated rate of overall subsidence of 0.4 cm/yr across the slough reduced vertical movement to an average of 0.1 cm/yr. When compared to a low sea level rise scenario of 0.25 cm/yr, rapid marsh deterioration will result if no management actions mitigate a rising sea. Only 26 of the 149 survey points (17%) contain vertical change rates that will outcompete a 0.25 cm/yr sea level rise scenario. Additionally, mudflat and tidal creek categories had erosion rates relative to the benchmarks of 0.7 cm/yr and 1.6 cm/yr, respectively. Respective net vertical loss becomes 1.1 cm/yr and 2.0 cm/yr, when the estimated 0.4 cm/yr background subsidence rate is considered. Further study is needed to identify and quantify individual components of benchmark movement to be able to quantify observed subsidence at each cross section, as opposed to applying a best estimate given available data. Resource managers at Elkhorn Slough National Research Reserve have been weighing four management alternatives to reduce the rate of marsh plain loss: 1) no action, 2) a new mouth, 3) sill at the current mouth, and 4) sill at Parsons Slough to reduce tidalvolume. It is recommended that resource managers focus attention to restoration alternatives that directly mitigate erosion, increase deposition, and/or mute sea level rise effects, Restoration of Parsons Slough (Alternative 4) appears to be the most cost effective way to reduce tidal volumes below the junction and mitigate erosional forces. Cross sections closer to the mouth of the Slough show some of the highest accretion rates, so a tidal sill recommended in Alternative 3 might ultimately decrease these rates by limiting tidal inundation onto the marsh plain. With the restoration of Parsons Slough, the tidal volumes will be reduced below the Parsons Slough junction that will inherently reduce tidal forces and scour, while maintaining the healthy marsh plain accretion rates closer to the mouth of the Slough. Increased biologically productive area will be a further benefit of selecting Alternative 4.