Delaware Center for the Inland Bays

by Marianne Walch, Ph.D.

The Center for the Inland Bays sometimes receives questions about the straight, parallel ditches that are a prominent feature of many of our local saltmarshes.

Who made them and why?

Are they good or bad for the marsh?

Salt Marshes Matter

Salt marshes are important and highly productive coastal ecosystems that support an amazing number and variety of plants and animals. They provide shelter and spawning areas for fish, crabs and many other creatures. The tidal waters that regularly flood and drain the marshes bring nutrients that stimulate plant growth and wash out the decomposing plant material and other organic matter that becomes food for fish and other aquatic life. They protect shorelines from erosion, protect against flooding, and filter pollutants from runoff.

The value of healthy salt marshes for supporting our fisheries and protecting coastlines has not always been appreciated, however. These wetlands are also breeding areas for the Common Saltmarsh Mosquito, a prolific and aggressive biter that is also capable of spreading some types of diseases.

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Method of Mosquito Control?

Prior to the widespread use of chemical insecticides, mosquitos were a serious health problem in coastal areas near tidal wetlands. In the 1930’s, the government began ditching salt marshes as a method of mosquito control. The majority of this work was done by the Civilian Conservation Corps (CCC) under President Franklin D. Roosevelt’s “New Deal” initiative.

The CCC hand-dug thousands of miles of gridded narrow ditches through salt marshes, spaced 100-150 feet apart, with the intention of draining pools of water where mosquitoes might breed. By 1940, 90% of salt marshes on the U.S. Atlantic coast had been grid-ditched to control mosquitoes.

 

Houston, we have a problem.

Draining of marshes continued until the 1960’s, but this practice was found to be only moderately effective in controlling mosquitoes. Furthermore, scientists began to realize that ditching has many long-term negative impacts on salt marsh ecosystems.

Unfortunately, the ditches altered the natural processes and water movement that maintain marsh elevation, hastening degradation and die-off of the wetlands. Many of the non-tidal ponds that occur naturally in salt marshes, and provide habitat and food for wildlife, disappeared. Altered water levels also caused large-scale changes in marsh plant communities, affecting populations of birds and other wildlife.

 

For these reasons, grid-ditching of salt marshes was largely ceased, and other methods are employed to control mosquitoes in tidal wetlands, including surveillance programs and limited use of pesticides. In addition, the State of Delaware now uses a practice known as “Open Marsh Water Management” to control mosquitoes. In this method, linear ditches are filled or plugged, and small, shallow ponds are selectively installed, connected by networks of tidal channels that look and function more like those of a natural, unaltered salt marsh.

This eliminates many of the alternately wet and dry potholes where saltmarsh mosquitoes prefer to lay eggs. At the same time, the permanent water pools provide habitat for foraging birds and small fish that feast upon mosquito larvae. Since the open-marsh management approach was adopted in Delaware in 1979, approximately 750 acres of these practices have been installed in the Inland Bays.