Assessment of hydraulic restoration of San Pablo Marsh, California.
Inter-tidal marshes are dynamic diverse ecosystems at the transition zone between terrestrial and ocean environments. Geomorphologically, inter-tidal salt marshes are vegetated land-forms at elevations slightly greater than mean tidal levels that have distributed channels formed under ebb (drainage) tidal flows that widen and deepen in the seaward direction. The drainage channels enable tidal flows to circulate sediments and nutrients through the marsh system during normal tidal events, while depositing sediments during storm or seismic events. This dynamic system encourages considerable biodiversity while simultaneously providing water quality enhancement features that service marsh terrestrial life and marine life in the estuary. Reservoir creation limiting sediment transport, anticipated large increases in sea levels as well as agricultural and urban development have resulted in significant loss of inter-tidal marshes and subsequent adverse impacts on waterfowl, infauna and fisheries. The complex and continuously changing marsh channel hydraulics and sedimentary processes have severely constrained quantitative modeling of these marsh systems such that restoration/creation efforts remain something of an empirical science and further assessments are needed. The purpose of this paper is to outline current understanding of salt marsh hydrodynamics, sediment accretion processes and subsequent response of marsh vegetation to set the stage for assessment of a marsh restoration effort along San Pablo Bay near San Francisco, California. Several kilometers of drainage channels were constructed in a 624 ha disturbed salt marsh to restore tidal circulation and vegetation so as to enhance habitat for threatened species (e.g. clapper rail, harvest mouse, delta smelt and potentially anadromous fish species). Two distinct drainage channel systems ('east' and 'west') were installed having similar channel dimensions common to salt marshes in the region, but having design bankfull tidal prism volumes differing by a factor of two. Following channel excavation, main channel tidal flows and sediment loads as well as marsh sediment accretion rates were monitored to assess the relative success of the excavation in restoring tidal circulation and vegetation (Salicornia spp.) to the marsh. Annual aerial surveys corroborated with ground-truthing indicated that marsh vegetation rapidly expanded, from 40 to 85% coverage several years following excavation. The 'east' channel intake was nearly completely silted in within three years. However, channel surveys and flow measurements indicated that the 'east' channel system tidal prism was only about 1200 m3, more than an order of magnitude less than that of the stable 'west' channel system. Marsh sediment accretion rates were on the order of 7-8 mm yr(-1), a rate common to the Pacific coast region that exceeds estimated sea level rise rates of approximately 2 mm yr(-1). East channel network siltation resulted in storm and spring tidal flood ponding such that marsh vegetation coverage decreased to 51% of the marsh area and related habitat expansion decreased. These results are considered in terms of the primary inter-tidal marsh factors affecting possible restoration/creation strategies.