Restoring Assateague Island's Salt Marsh by Filling 1930s Mosquito Ditches

Salt marsh on Toms Cove, overlooking the Coast Guard station and boathouse. NPS.

Assateague Island National Seashore, on the Maryland and Virginia coast, offers undeveloped barrier island beaches, coastal bays, salt marsh and maritime forest. Its salt marsh does vital work for the bay: it recycles nutrient pollution, stabilizes fragile shorelines, and gives habitat to wildlife, including rare and endangered species.

But more than 87 miles of ditches, dug in the 1930s to control mosquitoes, have damaged the island's marsh. The National Park Service is restoring its natural wetland functions by filling the ditches with sand and replanting native vegetation. Another five hundred thirty acres are targeted for restoration next to the island's only old-growth forest, an area the park has named a priority for a proposed wilderness area.

The problem with mosquito ditches

An estimated 90 percent of salt marshes from Maine to Virginia have similar ditch systems. On Assateague they still scar the landscape, though they have not been maintained or cleared since 1939.

The ditches were meant to lower the marsh water table and drain mosquito breeding pools. Instead, lower water tables and drained pools have degraded the wetlands and the services they provide. Ditching can change native vegetation and harm wildlife habitat, and it reduces a marsh's capacity to store floodwater and recycle nutrients — essential to water quality. And 55 percent of the affected marsh lies within the proposed wilderness area.

A method that works

In a 2008 pilot project, the Park Service found that the most effective, lowest-impact way to restore the marsh was to refill the ditches and replant them with native plants. Early results showed that sheet flow — water moving across the marsh surface — had returned, and that swings in groundwater level had become slower and smaller.

Progress

The Park Service has restored more than 400 acres of marsh at Assateague with this technique, which it pioneered. When this was written, work was expected to continue into 2014, perhaps through 2016, depending on funding. Volunteers, including students from local schools, have given thousands of hours, learning restoration by working beside park staff.

Monitoring follows protocols of the Park Service's Northeast Coastal and Barrier Network Inventory and Monitoring Program, comparing the marsh before and after ditch restoration as part of long-term monitoring of the seashore's ecosystems:

• water levels at tidal stations;
• monthly groundwater levels and water quality;
• yearly surveys of marsh and seagrass vegetation, nekton (estuarine fish) and animal use.

The results guide adaptive management, so managers can adjust and improve their techniques.

Sources

Based on "Crystal Clear: Restoring the Salt Marsh at Assateague Island," part of the series Crystal Clear: A Call to Action, National Park Service; a work of the United States government in the public domain.