Hydrologic Studies to Inform Future Adaptive Restoration for the Giacomini Wetlands

FlowWest Hydrologic Consultants

Fish Hatchery Creek (FHC) flows off the Inverness Ridge, through Inverness Park, and then into Giacomini Wetland Restoration Project area. Immediately prior to the 2008 restoration, the former ranchers excavated a cross channel between FHC and another smaller slough, the West Pasture Old Slough. FHC started to fill in with sediment, likely due to a reduction in flow volume, velocity, and scour due to the cross-channel diversion. With the loss of hydraulic capacity in downstream portions of FHC, flood and non-flood flows started to sheetflow across the new marshplain rather than flow through the restored marsh in a defined channel. This change dramatically reduced the quality of habitat for Central California coastal steelhead (Oncorhynchus mykiss) in FHC. Concerns about the adverse impacts to this federally listed salmonid and to natural hydrologic and ecological processes and functions prompted the Seashore to consider an adaptive restoration component to improve performance of the 2008 restoration. Using a small NPS grant to assist with information needs related to this future adaptive restoration project, the Seashore engaged FlowWest to conduct hydrologic studies in support of this future restoration design for a new upper and middle FHC channel alignment that would maximize hydrologic conveyance to improve hydrologic and ecological processes and functions and habitat quality conditions for salmonids and other wildlife while minimizing the potential for flooding of adjacent private property. The information collected for this study built upon previous data collected during the original marsh restoration. Study components included updated post-restoration topography; assessment of base and peak water flows, groundwater input, and tidal influence; and fine and coarse sediment load inputs.