Wood Creek Tidal Marsh Enhancement Project

Project Background and Location: Given the dramatic loss of tidal marsh lands in Humboldt Bay, and recognizing the importance of these habitats to juvenile salmonids, tidal marsh restoration is a high priority for all of Humboldt Bay and especially within the Freshwater ecotone. Freshwater Creek still supports the largest population of coho salmon within Humboldt Bay, and is identified as a priority watershed in the Recovery Strategy for California Coho Salmon. The Freshwater watershed also supports chinook salmon, coastal cutthroat, and steelhead populations.

The Biological Assessment of Wood Creek referenced for this project is available as a PDF document, below near the bottom of this page.

Project Components: The Wood Creek Tidal Marsh Enhancement Project would restore tidal hydrology, expand brackish marsh habitat, and remove the Wood Creek tidegate, the primary barrier to fish migration into Wood Creek, to enhance salmonid and tidewater goby access. The project is anticipated to expand the brackish marsh within the construction area from 1.4 acres of existing marsh to 20.7 acres of enhanced brackish marsh and is expected to remain a tidal wetland in perpetuity. The elevations of marsh surfaces are being designed and constructed "at maturity", meaning no sediment aggradation is required to sustain the targeted hydrologic regime and marsh vegetation.

I. General Construction Activities, Site Preparation, and Access

The proposed actions will utilize the existing ranching road access off Myrtle Avenue to bring an excavator, backhoe, dump truck, and possibly bulldozer onto the 23.2 acre construction area and grazed wetland pasture. A temporary access road will be designated around the perimeter of the project area, across the seasonal wetland pasture. All equipment refueling will take place in the upland area next to Myrtle Avenue. No major vegetation clearing is required for this project, but approximately 20.7 acres of non-native grasslands occupying seasonal wetland surfaces are expected to convert to brackish marsh vegetation. All equipment and material staging will take place within the project construction boundary. Silt fencing and other erosion control measures will be installed as needed, to reduce silt and turbidity runoff from Wood Creek during the construction activities. Fish screens will be installed on Wood Creek during ebb tide, downstream of the tidegate and upstream of the proposed pond excavation site, to prevent salmonids and other estuarine fish species from moving into the site during construction.

 II. Excavate and Recontour Site

The primary construction activity of this project is excavation and placement of approximately 3,709 yds³ of material within the construction boundary. The excavations includes: (1) removing  material along 3,900 ft of tidal creek channel to restore tidal flow access to inundate the proposed 23 acre marsh and to provide aquatic habitat; (2) removing former dredge material excavated from Wood Creek in the past and deposited along the north bank, forming a defacto berm that would impede tidal inundation of the proposed marsh site; (3) removing material from a 50 x 90 ft pond adjacent to Wood Creek and; (4) excavating  fill material from a cattle crossing.

All excavations across the seasonal wetland pasture will be done with excavator and backhoe, and will occur on dry ground, separate from Wood Creek and other existing habitat areas. Wood Creek will not need to be dewatered for these actions as the excavation will occur in areas which are not presently slough or stream channels, but are part of the grazed pasture. Where new channels join Wood Creek, a small plug of dirt will be left in place until all the channels and pond are excavated and other construction elements are completed, then the plug will be carefully removed to connect the new channels to the creek. The new channels will become connected to Wood Creek and will receive tide water after the final project activity, removal of the tide gate, is completed. The slough channel excavations will begin at the downstream end of each channel, where the channel will join Wood Creek, and excavators will traverse the seasonal wetland pasture only within the footprint of the eventual excavated channel, so as to minimize disturbance to the surface of the seasonal wetland.

III. Place Fill Material on the Seasonal Wetland and Construct Habitat Features

The excavated material will be used on-site to recreate topographic diversity across the existing seasonal wetland in the form of high marsh surfaces and tidal hummocks. The "cut" material may either be placed directly onto the seasonal wetland surface within reach of the excavator or loaded into a small dump truck and placed a short distance away. Within each proposed tidal slough channel, several pools and habitat structures will be constructed using logs and root wads to provide aquatic habitat diversity (e.g., velocity breaks, scour holes, cover structure) primarily for juvenile coho salmon rearing and tidewater goby. Typically the log structures are buried in the slough channel and embedded into the banks so they remain in place. A minimum of four habitat features will be constructed, each using one or more logs and occasionally a 1-2 ton boulder as an anchor. If large root wads are available to the project, these are also desirable. Log structures may also be used to provide grade control in strategic locations (e.g., at the entrance to the pond) to prevent downcutting.

IV. Upgrade Access Bridge

The existing cattle crossing will be replaced with a 50 ft long bridge after the excavation work is completed, but before the channel plugs and tidegate are removed. Excavation will include removal of the existing structure and hauling the concrete armoring off-site for disposal. The bridge will be set on concrete abutments and span the 20 ft wide creek. During the bridge installation, an approximately 100 ft section of channel will be dewatered with coffer dams to minimize impacts to aquatic species and water quality (primarily turbidity). Fish screens will be installed upstream and downstream of the bridge site, and salmonids will be removed using minnow traps and seine nets. Electrofishing will be used if salinity and conductivity conditions allow. These methods will not allow rescue of tidewater goby, if they are present. A silt fence will be installed on the upstream side of the fish screen to reduce turbidity generated by the in-channel work. Working only during low tide intervals, the excavator will remove the existing structure and pull all the unwanted concrete debris out of the channel and place it into a dump truck to be hauled off-site for disposal. After the channel is cleared out, bridge abutments will be placed on the banks, the new bridge will be placed on top of the abutments, and gravel ramps will be installed for bridge approaches. The abutments will be placed outside the stream corridor. Once the bridge installation is complete, the fish screens will be removed.

V. Remove Waterman Tidegate

A small Waterman tidegate attached to a 2 ft diameter culvert, located south of the main Wood Creek tidegate in the Freshwater Slough dike, is rusted through on the bottom, has become leaky and unstable, and poses a future risk of levee erosion and instability of the existing tidewater goby habitat. The tidegate and culvert will remain in place and a concrete plug will be inserted into the culvert at its midpoint. Approximately six inches of soil will be removed from above the culvert to access the culvert from above. The culvert will then be cut open using hand tools to expose an approximately 2 ft² opening. The concrete will be mixed on site by hand, and the concrete will be set by hand. Provisions will be made so that no uncured concrete will come in contact with the wetted channel. This task will be performed in dry conditions during one low-tide interval.

 VI. Implement the Revegetation Plan

A revegetation plan was developed in coordination with the proposed tidal creek, floodplain bench, and tidal hummock grading plans to create a tidal marsh with native salinity-tolerant vegetation. The goals of the revegetation plan are to: (1) promote the recovery of desirable plant species and marsh types, and minimize invasive species by planting the preferred species assemblages at appropriate hydrologic and elevation zones; (2) minimize surface erosion in areas disturbed by construction activities; and (3) evaluate different revegetation methods that are intended to achieve recovery of desirable marsh types. Three primary revegetation methods will be applied: planting nursery-grown wetland plants, hydro-seeding the preferred plant species, and passive recolonization. An estimated one-half acre, consisting of a four-foot wide strip on each side of the new slough channels, will be manually planted with plugs installed on a maximum of 18 to 24 in centers. As much as 4.5 acres will be seeded with Lyngbye's sedge and hairgrass seed applied at a rate of 155 lbs/acre. Approximately 6.8 acres of undisturbed area will be allowed to recolonize passively. The revegetation will take place in the first fall after the project construction is complete.

VII. Remove Channel Plugs and Wood Creek Tidegate

The final project construction task will remove the remaining dirt plugs left in place at the confluence of the newly excavated tidal creek channels and pond, and the main Wood Creek channel. This task will be done after all other excavations and recontouring are complete, the bridge installation is complete, and the site is ready for inundation from increased tidal prism. The excavator will remove each dirt plug, working successively downstream until all tidal creek channels are opened. Once this is complete, the Wood Creek tidegate will be removed and hauled away.

VIII. Conduct Site Cleanup, As-built Surveys, and Monitoring

Following completion of all project tasks, heavy equipment will be removed and the project area stabilized through installation of erosion and sediment control measures (straw mulch, waddles and baffles, grass seed). As-built surveys and photo-monitoring will document the site conditions relative to construction plans. The cattle exclusion fence will be re-located to protect the newly constructed tidal marsh wetland. Finally, the monitoring phase of the project will begin.