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Old Baldy, Canada | photo by Cameron Schaus
Conservation Issues of the Ventana Chapter | monterey county
Parsons Slough project could restore tidal marsh
October 2009
Parsons Slough
Click here for other aerial photographs of Elkhorn Slough. Photo by Tom Neuman

The Elkhorn Slough salt marsh habitat is unraveling. The deterioration has many causes including the construction of Moss Landing harbor in 1947 (which allowed tides full access to the Slough scouring away soft mud), the yearly breaching of the sandbar at the mouth of the Salinas River to protect farmland from flooding, and the diking and draining of large areas of the slough for farming in the early 20th century.

Pickleweed grows around the edges of the salt marsh. It is able to thrive in environments too salty for many other plants. Ventana staff photo.

Elkhorn Slough consists of 800 acres of salt marshes with tidal creeks and 1600 acres of mudflats. In the 1800s there was twice as much salt marsh as mud flats. Today, the salt marsh is dying off because it is wet too much of the time. Additionally the depth of the water channel has changed. In the 1800s the depth of the channel ranged from 5-10 feet; today it is 20 feet deep. The tidal scouring has resulted in a hardened bed which makes it more difficult for clams and other invertebrates to burrow.

Working over the last five years, a group of 100 scientists and conservationists have developed plans to slow down the deterioration and preserve the tidal marshes and other estuarine habitats in Elkhorn Slough. They are planning a test project in Parsons Slough, a branch of Elkhorn Slough.

This project, called the Tidal Wetlands Project, has three goals:

• Conserve estuarine habitat,
• Restore lost estuarine habitat, and
• Restore the processes that sustain the system.

The solution proposed is an adjustable tidal barrier at the entrance to Parsons Slough with detailed monitoring so appropriate adjustments could be made. Almost $4 million has been granted from Federal Stimulus Funds for an analysis of alternatives and partial design of the project which is not expected to inhibit movement of marine mammals or alter the Leopard shark nursery created since the building of the harbor. The beauty of the project’s conceptual design is its reversibility should any impacts be adverse.

Restoration is a delicate balancing act. The Slough needs tidal action to maintain high levels of dissolved oxygen to sustain a healthy invertebrate community. Yet, too much tidal action can scour the Slough, cause erosion of the banks, and keep the salt marsh submerged too long. Restoration managers must also balance the existing needs of the harbor, agriculture, and other users.

Parsons Slough historically supported 400 acres of tidal march, but now only 35 acres remain. In the first half of the 20th century the area was diked off and drained for farming. When the dikes broke in 1982, the tides returned but the salt marsh did not because the land surface had dropped making the area too wet for salt marsh plants.

If the tidal exchange could be reduced, the salt marsh plants would be exposed to salt water for shorter periods of time, increasing the area where salt marsh plants could survive. Reducing the tidal exchange at Parsons Slough slightly would slow currents in many other parts of Elkhorn Slough, increasing the viability of salt marsh and soft mud habitats throughout the estuary.

Ninety-seven percent of salt marshes in California have been lost. If nothing is done to restore natural processes, we are may lose the remainder.

Elkhorn Slough is largely owned by the California Department of Fish and Game.

Parsons Mudflats
CMud flats are increasing at Elkhorn Slough. In the 1800s there was twice as much salt marsh as mud flats. Today the ratio of salt marsh to mud flats is reversed. The salt marsh is dying because it is wet too much of the time. Photo by Barb Peichel

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