The Changing Shape of the Cape & Islands: Herring River, Wellfleet, Chapoquoit Beach & Black Beach, November/December 2017 Cape Cod LIFE | capecodlife.com

The map in the top right shows the extent of the historic salt marsh (in tan) compared with the extent of the salt marsh today (black). Tidal restriction has reduced the marsh from approximately 1,000 acres before installation of the dike to about 10 acres today. The aerial photograph in the background shows the Herring River and marsh, with Wellfleet Harbor in the foreground.

“By the late 19th century, the resources of Cape Cod Bay had been severely over-harvested, reducing the tidal wetlands’ economic and social values,” Portnoy says. “In addition, deforestation and sheep grazing caused soil run-off into the Herring River and its tributary creeks, making them no longer navigable.” These lost values, plus a chronic mosquito nuisance, made it easy to convince town residents to dike the river, Portnoy explains.

The dike reduced the opening of the river from 450 feet to approximately 18 feet, severely cutting down on the oxygen-rich water flowing into the salt marsh. It also blocked the sediment carried upstream, which had allowed the salt marsh to grow and thrive. Subsequently the marshland has receded, with only 10 acres remaining today.   

Tim Smith, restoration ecologist at the Cape Cod National Seashore and a member of the Herring River Restoration Committee, says the negative effects of the dike were not known at the time it was built.

“People back then thought they were doing the right thing,” Smith says, “but we’ve since learned how important tidal wetlands and estuaries are regarding coastal fisheries, water quality, wildlife, nutrient and carbon cycling, recreation, and the character of small seaside towns.”

By 1971 the original tide gates of the dike had deteriorated. Larger amounts of seawater had begun seeping north of the dike, which had become a freshwater ecosystem, laying stagnant and occasionally flooding areas of the Chequessett Golf Course. The idea of a potential restoration of the river was debated at that time. Instead the state approved and partially funded rebuilding of the dike in 1974. The rebuilt dike is now more than four decades old and deteriorating again.

The extensive damage to the salt marsh caused by the dike has also had a more subtle influence on the surrounding area, according to Portnoy.

“When water drains from normally water-logged salt marsh peat, it sinks like a dried-out sponge,” he explains. “Portions of the Herring River flood plain have sunk two to three feet since the dike’s creation. This greatly compromises the coastal wetland’s ability to protect nearby upland properties during storm surges.”