The Tidal Wetland Project Team recently made a big change to the restoration project at Parsons Slough. The new approach, called the Low Sill, is expected to reduce tidal scour in Elkhorn Slough from Parsons Slough and the Harbor, but affect habitats in Parsons Slough less, be easier for fish and wildlife to swim across, and will be less apparent to kayakers, reserve visitors and other people enjoying the slough.
Following detailed analysis by the engineering team of Ducks Unlimited and URS, this decision was made by staff of the Elkhorn Slough National Estuarine Research Reserve, in collaboration with our advisory team, which includes Reserve volunteers and staff from the US Fish and Wildlife Service, the California Department of Fish and Game, the California Coastal Conservancy, the Monterey Bay National Marine Sanctuary, and the Restoration Center of the National Oceanic and Atmospheric Administration.
As we developed the design for the adjustable sill, we found that much of the tidal scour occurs during spring tides, when the water rises and falls by about eight feet. As that water pours out of Parsons Slough, it erodes the Elkhorn Slough channel all the way to the Harbor. The new design is more selective: it will affect only these extreme tides, leaving typical tides unchanged. That will minimize any changes to the distribution of habitats in Parsons Slough, thereby limiting water quality changes and impacts to wildlife. Unlike the project considered previously, the Low Sill is not expected to convert large areas of mudflat to subtidal habitat.
Previously, we were considering a structure with a large adjustable section above water. That section would have been managed over several years to gradually reduce tidal exchange to Parsons Slough by as much as one-third. The Low Sill, in contrast, will be largely underwater, will not be adjustable, and will reduce tidal exchange by only a few percent.
Another factor playing a role in this decision was concern about the possibility of ESNERR not being able to adjust it in a timely manner in response to rare tidal events, or of the adjustable structure failing. This new design will be more reliable, as it will have fewer parts and pieces.
The project will still be subject to detailed monitoring. We will be looking at the water quality in Parsons Slough and other important indicators of ecosystem health, such as algal blooms and the responses of a suite of species to the structure.
An adjustable weir is not currently planned. We will conduct detailed hydrologic and ecological monitoring at the project area over the next three to five years. Should that monitoring demonstrate the need for modifications to the structure, such modifications would be considered at that time in a process that includes the opportunity for participation.
