The Herring Run
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News and Information about the
August 2021
Update on Wellfleet's Duck Harbor Beach Overwash
Above: Over the past year high tides and storm waves have washed over some of the low barrier dunes at Duck Harbor Beach. Sand is being washed into what was the original tidal inlet into Duck Harbor, i.e. before the harbor was completely diked off in 1909. 
See the 1887 map of the area below. (Courtesy of John Portnoy)
Along Cape Cod’s ever-changing shoreline, erosion and movement of sand are part of a natural process that helps to sustain beaches, dunes and marshes. Even unexpected shoreline changes such as the overwash at Duck Harbor Beach demonstrate that the shoreline is functioning to sustain coastal resources.

The Duck Harbor Beach overwash is an observable example of barrier beach dynamics. Barrier dunes maintain their form and function, not by rigidly resisting the onslaughts of rising seas and storm waves, but by strategic retreat, migrating landward or “rolling over” to maintain both seaward-facing beaches and back-barrier wetlands.

 “Barrier” dunes that occur just above the reach of normal tides are occasionally overwashed by storm tides and waves. Most overwashed sand remains within the barrier-beach and marsh system and can be either reworked into new dunes, or can supply much-needed sediment to back-barrier marshes so that they too can keep up with rising sea level.

The Duck Harbor Beach overwash is linked to peak Cape Cod Bay high tides. Flooding seawater inundated back barrier wetlands and, without a pathway to recede, ponded water caused salt-kill of wetland vegetation in the Bound Brook Island area. As sea level rises, future overtopping events could occur. Cape Cod National Seashore will continue to coordinate monitoring of tidal conditions and landside changes associated with the overwash events.

Even if overwash events continue, it is unlikely that a permanent inlet into historic “Duck Harbor” (a sub-basin of the Herring River system) will be re-established. Duck Harbor is no longer an open water body, and is now filled with peaty sediment that impedes water flow. Any breach of the line of barrier dunes will likely be temporary. When the original inlet was diked in 1909, tidal flow into shoaled Duck Harbor was already negligible, and the harbor reportedly un-navigable. This is not to say that a very large storm couldn’t overtop the dune system here, as well as near Ryder Beach (Truro) and even Powers Landing, and temporarily flood the wetlands behind with seawater, a potential recognized by federal flood insurance maps.
Fortunately, any potential flooding effects of the overwash at Duck Harbor will be significantly lessened with the implementation of the Herring River Restoration Project. Re-establishment of historic marsh channels and the new bridge planned at Chequessett Neck Road will enable floodwaters to recede and discharge into Wellfleet Harbor much more quickly than under existing diked conditions. Likewise, measures such as elevation of roads that will be put in place to protect structures from the effects of tidal restoration will also provide greater flood protection compared to current conditions.
Above: The Herring River estuary in 1887, before the 1909 diking of both the main stem at Chequessett Neck and Duck Harbor. Note that by this date there was little navigable open water left in Duck “Harbor”. Barrier dune systems, subject to overwash during storms, persist north of Bound Brook Island (e.g Ryder Beach), Duck Harbor, and Powers Landing. (Courtesy of John Portnoy)
Summer Mosquito Outbreak Linked to
Duck Harbor Overwash
Photo courtesy of Cape Cod Mosquito Control Project
Ponded seawater trapped behind the overwashed Duck Harbor barrier beach has become a breeding ground for nuisance mosquitoes this summer. Cape Cod Mosquito Control Project (CCMCP) has reported as many as 2,000 mosquitoes a day in nearby traps located in Herring River, far in excess of the number of trapped mosquitoes normally observed.
CCMCP and Cape Cod National Seashore (Seashore) are working collaboratively to address the mosquito outbreak. Even though the outbreak did not meet the definition of a public health emergency, out of an abundance of caution the Seashore expedited the issuance of a special permit to allow CCMCP to apply larvicide in the Duck Harbor area. The outbreak should eventually subside as CCMCP’s repeated applications of larvicide take effect, older adult mosquitoes die-off, and wet breeding pools dry out.  

Continued communication and collaboration between CCMCP and Seashore will help determine the most appropriate way to avoid or address future mosquito outbreaks in this area. Longer term, the Herring River Restoration Project will help to ameliorate conditions that are conducive to mosquito breeding. The Herring River Restoration Project will diminish mosquito breeding habitat by restoring system drainage, improving water quality and re-establishing populations of larvae-eating fish species.

A user friendly fact sheet with more information about the Project’s anticipated effects on mosquitoes is available here. A recent article about the mosquito outbreak that appeared in the Provincetown Independent is available here. Questions about the recent mosquito outbreak can be referred to the CCMCP office at 508-775-1510 or directly to the Seashore.
P.O. Box 565
South Wellfleet, MA 02663