The Elkhorn Slough
Tidal Wetland Plan
Elkhorn Slough, the largest estuary of the Monterey Bay National Marine Sanctuary, joins the Pacific Ocean at the very center of Monterey Bay. The 2,970 acres of Elkhorn Slough tidal habitats include a main subtidal channel flanked by extensive intertidal salt marshes, mudflats, tidal creeks and salt pannes. These areas provide critical habitat for more than 135 aquatic and migratory
bird, 550 marine invertebrate and 102 fish species. In addition to providing habitat, Elkhorn Slough offers research, educational and recreational opportunities for kayakers, birdwatchers, hikers, boaters, students and scientists.
The 1999 Elkhorn Slough Watershed Conservation Plan identified human alterations to the tidal influence and hydrology as one of the major threats to Elkhorn Slough's coastal wetlands. Since the 1880s, human actions have caused the loss and degradation of hundreds of acres of tidal wetlands in Elkhorn Slough. Examples of past modifications include railroad, highway, dam and levee construction; upland clearing and cultivation; diking and draining of tidal wetlands; river diversion; intense groundwater pumping; and harbor construction. These changes have greatly disrupted the important balance among tidal influence, geomorphology, and sediment and freshwater supply that sustain Elkhorn Slough's estuarine habitats.
Research studies have confirmed dramatic rates of tidal erosion and marsh loss in the slough. Bathymetric studies conducted by Rikk Kvitek, California State University Monterey Bay, indicate that the mean cross-sectional area of Elkhorn Slough increased
by 24 percent in just eight years (1993-2001). Eric Van Dyke and Kerstin Wasson of Elkhorn Slough National Estuarine Research Reserve (ESNERR) analyzed marsh loss, bank erosion and tidal creek changes over time. GIS analysis of Elkhorn Slough tidal habitats demonstrates that the mean percent cover of salt marsh vegetation in undiked marshlands decreased approximately 44
percent between 1931 and 2003. (See Figure 1, opposite.) The mean width of 196 tidal creeks increased from 2.5 meters in 1931 to 12.4 meters in 2003. Bank erosion rates along the main channel of Elkhorn Slough are between 0.3 and 0.6 meters per year, with some areas that approach rates of 2.0 meters per year. In light of the significant rates of tidal erosion and marsh loss in Elkhorn Slough, carefully planned management strategies are needed to conserve and restore these critical habitats.
In September 2004, ESNERR and California Department of
Fish and Game, with funding from the National Oceanic and Atmospheric Administration, initiated a planning process to
develop an Elkhorn Slough Tidal Wetland Plan (TWP) with a
wide variety of partners. The purpose of this collaborative process is to conserve, enhance and restore tidal habitats in the Elkhorn Slough watershed by developing strategies to address hydrological management issues. The TWP Strategic Planning Team, the
primary decision-making body overseeing the planning process, consists of coastal restoration scientists, managers, and planners; directors of key conservation organizations; and representatives
of jurisdictional, regulatory and governmental entities. During
the past year, they developed a consensus statement outlining strategic planning principles, a vision, goals and objectives for
the TWP. Summarized, the goals are to conserve the existing
highest-quality estuarine habitats; restore and enhance degraded estuarine habitats, with a special emphasis on those with the highest loss rates; and restore and enhance natural processes (hydrology and geomorphology) to sustain a more stable estuarine system.
The team has also agreed that the current tidal habitat trends are not acceptable and that new management actions are necessary.
A TWP Science Panel, consisting of more than 40 multidisciplinary (biological, hydrogeomorphic, physiochemical, estuarine restoration) experts has met bimonthly over the past year. This group has been instrumental in characterizing what is known about historical changes, tidal habitats, physical processes and causes of tidal erosion and marsh loss. There is now general agreement that the modification of the Elkhorn Slough mouth for the creation of a harbor in 1947, permanently fixing a deeper opening to Monterey Bay, is the main cause of subtidal erosion and more recent marsh loss. Contributing factors include decreases in sediment supply (diversion of the Salinas River), dike/levee failure and removal,
the presence of the Monterey Canyon, sea-level rise, wave action and other biogeochemical processes. The panel acknowledges that the process of marsh loss is complex, but the increased tidal range and duration of tidal inundation on the marsh plain (due to the mouth modifications and land subsidence) in combination with
the decrease in sediment supply are contributing factors.
|Figure 1. Changes to the extent of acreage (hectares; ha) of tidal habitats in Elkhorn Slough from 1870 to 2000 (Van Dyke and Wasson 2005)
The TWP Science Panel and Strategic Planning Team have also agreed that the Elkhorn Slough system is not currently at equilibrium. Their predictions for tidal habitats over the next 50 years, if
no actions are taken, include the continued deepening and widening of the channel and tidal creeks, increase in salt marsh conversion to mudflat and tidal creeks, and erosion of sediments in soft-bottom areas.
The next major step in the tidal wetland planning process will
be to develop and evaluate potential strategies that achieve the goals to conserve and restore tidal habitats in Elkhorn Slough. Possible strategies to address marsh loss and tidal erosion may include actions to reduce the tidal influence to specific areas or the entire system, to supply sediments to increase the elevations of subsided marsh areas and to restore appropriate levels of tidal exchange to areas behind water-control structures. Key agency and community stakeholders and outside experts will be able to provide input on the draft strategies. The anticipated result of the TWP
will be that the partners will be in place to obtain funding, oversee implementation and conduct research and monitoring of the recommended conservation and restoration strategies.
Strategies in the Elkhorn Slough TWP will aim to meet the shared vision of the Strategic Planning Team: "We envision a mosaic of estuarine communities of historic precedence that are sustained by natural tidal, fluvial, sedimentary and biological processes in the Elkhorn Slough Watershed as a legacy for future generations."
For more information, please visit www.elkhornslough.org/tidalwetlandplan.htm.
Elkhorn Slough National Estuarine Research Reserve