Tidal Exchange Newsletter of the Elkhorn Slough Foundation
Winter 2011
ChangesTells in Perspective History the Story of Changing Tides
T
he construction of the Parsons Slough sill is complete. The sill, an outcome of the Tidal Wetland Project, is essentially an underwater retaining wall built to slow the ebb tide currents that are responsible for eroding slough marshes—a solution for the vexing problem of scouring currents that have increased erosion in recent years. This effort was led by the Elkhorn Slough National Estuarine Research Reserve and supported by the Elkhorn Slough Foundation. 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. By examining evidence from the slough’s past, the Tidal Wetland Project was able to create a solution to preserve the slough’s future. The lower reaches (nearest the harbor) of the slough have always been more influenced by the bay than the upper reaches. Now the
slough’s lower reaches have become more marine, or ocean influenced, and the extent of this influence is moving inland as the slough bed deepens. The erosion of the slough’s marshes and mudflats that causes this deepening has its roots in the modification of its hydrology over the span of the past 140 years. There are many influences that lead to this change. The building of the Southern Pacific Rail line through the slough’s eastern wetlands in 1872, the diversion of the Salinas River, the diking of marshes for grazing and cultivation, the creation of Moss Landing Harbor, and the subsequent breaching of aging dikes and levees have all changed the way water flows through the slough dramatically. All these modifications contributed to marsh loss. First, the reduction in river-borne sediments, previously supplied by the Salinas River before it was diverted, has resulted in ‘starving’ the marshes of substrate. As sea
level rises, salt marsh plants usually trap suspended sediments and grow upward to stay above the mid-tide line. Without a steady supply of sediment, the plants will be submerged for longer periods of time and eventually ‘drown’. Second, the increase in the speed of ebb tide currents flowing out of the slough into the bay has eroded marshes along the main channel, sweeping marsh sediments into the bay and the Monterey Bay Submarine Canyon. Land use changes have also contributed to an altered hydrology in the slough. In Parsons Slough— the southeast corner of Elkhorn Slough—more than 450 acres of salt marsh were diked, drained and converted to pasture land and farm use. As the marsh soils dried, they were compacted and subsided, or sank, up to five feet below their historic level. In the early 1980s the levees were broken, and the water that rushed in drowned the marsh plants that had