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What are runnels, the man-made channels that help restore salt marshes?

The salt marshes that are vitally important in protecting shorelines from erosion are increasingly threatened with death by drowning, scientists say.

Sea level rise and human activities such as dredging mean that "ponds" or standing pools of water that submerge and kill salt marsh plants and grass are becoming more common, says Jennifer Bowen, a Northeastern University professor of marine and environmental sciences.

Bowen says that's bad news for coastal environments that depend on salt marshes to slow erosion, act as buffers against storm surges, remove carbon from the atmosphere and take up nitrogen before it can pollute waterways.

Enter the runnel.

Shallow, man-made channels that tend to run no deeper than 12 inches, runnels snake away from the standing pools or ponds of water and connect to natural channels or deep ditches dug decades ago for mosquito control.

"Runnels are designed to connect the pond back to a drainage ditch so that the water that collects in these ponds can run off and allow the vegetation to regrow," she says.

"They're not designed to be a permanent feature of the marsh," Bowen says.

Instead, runnels are a more ephemeral part of the landscape, dug by conservation agencies and other groups when ponds pop up, as they increasingly do, she says.

"You are seeing a greater proportion of ponds over time. There are a number of reasons for that, but sea level rise is certainly one of them. In order for the marsh to be able to keep pace with sea level rise, we need to decrease the amount of ponds and increase the amount of vegetation," Bowen says.

She says the wetland grasses that look like fields of hay—and were used as such by early colonists—serve as nursery grounds for commercially important fish and shellfish and decompose into layers of root-filled mud and peat that "store way more carbon than terrestrial forests."

A by Bowen's lab showed that runnels may also improve the process by which salt marshes remove nitrogen from the environment. The paper is published in the journal Estuaries and Coasts.

There were some questions about whether draining pools of water would introduce levels of oxygen that would impede nitrogen removal, says lead author Hillary Sullivan, who received her doctorate in marine biology from Northeastern this spring.

She studied runnels that had been dug in Massachusetts and Rhode Island and found that the channels did not oxygenate the environment and actually were associated with more nitrogen removal, probably due to tidal flushing.

"The runnels help restore nitrogen removal capacity because they restore the natural tidal hydrology," says Sullivan, who is currently Cape Cod restoration coordinator with Friends of the Herring River.

Some salt marsh restoration projects require large capital expenditures, such as replacing culverts to improve tidal flow. Think of digging runnels as a pop-up solution.

"We do this once the pools have formed," Sullivan says.

In the past, salt marshes could afford to have the ponds drain naturally and slowly, Bowen says. But with rising sea levels, they need some help, in the form of runnels, to keep their plants alive and elevated above the water.