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Parsons Slough Sill Completed!
The construction of the Parsons Slough sill is complete. An outcome of the Tidal Wetland Project (TWP), the sill is essentially an underwater retaining wall built to slow ebb tide currents that are responsible for eroding slough marshes and the soft mud habitat on the slough bed. More than 100 scientists, managers and conservationists met over a six-year span to study, understand and design a solution for the habitat changes caused by increased erosion.
The construction of the underwater sill has been a major undertaking. TWP's Strategic Planning Team and Science Panel focused on the Parsons Slough sill as the most cost effective way to reduce the erosive force of the currents. The work was well reviewed - requiring 14 permits from state, federal and local agencies. "This has been an extraordinary undertaking," said TWP Director Bryan Largay, "with enthusiastic cooperation from many partners, agencies, scientists and conservationists. We are very pleased that the construction has gone so smoothly and we're excited about the benefits of this project to the long-term health of this remarkable place."
The Parsons Slough Project was made possible by multiple grants of planning funds, including support from the David & Lucile Packard Foundation, the Resources Legacy Foundation Fund, the California Coastal Conservancy and the US Environmental Protection Agency. Construction funding came through the American Recovery and Reinvestment Act. In addition to conserving exceptional natural resources, this project has saved or created 107 jobs - a key goal of the Recovery and Reinvestment Act. The Parsons Slough Project will also be leaving Kirby Park, which served as the project staging area, in tip top shape.
Although construction is completed, the research continues. Throughout the next several years the wetlands and wildlife will be monitored to study the impact of the sill and change in tidal flow. This research will provide vast data for Elkhorn Slough and estuaries everywhere. The Parsons Slough project is truly a community effort. Local scientists, landowners and regulators have worked hand in hand to make Elkhorn Slough an enduring place of inspiration, and the project itself directly improves community access to this shared space.
A review of the Project history and intent: The increased erosion of the slough's marshes and mudflats has its roots in hydrology modifications over the past 140 years. The building of the Southern Pacific Rail line through the slough in 1872, the diversion of the Salinas River (1900s), the diking of marshes, the creation of Moss Landing Harbor (1940s), and the subsequent breaching of aging dikes and levees have all dramatically changed the way water flows through the slough. All these modifications led to changes in two processes that contributed to marsh loss. First, the reduction in river borne sediments, supplied by the Salinas River before it was diverted, resulted in 'starving' the marshes of substrate. Without a steady supply of sediment to keep the marsh above high-tide line, plants are submerged for longer periods of time and eventually 'drown' like an over-watered house plant. Second, the increased speed of ebb tide currents flowing out of the slough erodes the marsh banks and exports precious mud from the slough.
Land use changes also contributed to an altered slough hydrology. In Parsons Slough, more than 450 acres of salt marsh were diked, drained, and converted to pastures and farms. As the marsh soils dried, they were compacted and subsided, or sank, up to five feet below their historic level. In the early 1980's levees were broken; water rushed into these dry areas and once again changed the system.
The Parsons Slough sill will return tidal flows to a more natural rate and reduce the loss of marsh and fine sediments. This will conserve these threatened habitats that play such a major role in Elkhorn Slough.
 | | The completed sill is visible adjacent to the railroad bridge. The submerged center section is barely visible. |
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