Current View of Chequessett Neck Dike and Rendering of Final Bridge Design
Following several decades of hydrologic and ecological research, an incremental restoration of tidal exchange is proposed for the Herring River estuary (Wellfleet and Truro, Massachusetts). The project will be adaptively managed through regular monitoring and assessment of system response to stepwise increases in tidal flow through the Chequesset Neck dike and the modification or removal of other man-made restrictions.
Monitoring variables have been selected to address both ecological and social concerns, vetted over a lengthy scientific and public review of ecosystem status, including consideration of the sensitivity of public and private infrastructure (Herring River Technical Committee). Predictions of system response have been greatly aided by hydrodynamic modeling of tide heights and salinity distribution for a full range of restoration scenarios ranging from the status quo to unrestricted tidal exchange.
The Chequessett Neck Road dike at Herring River is causing drainage of the salt marsh and poor tidal flushing. Drainage of the salt marsh leads to prolonged exposure to air causing the salt marsh soils to decompose and release sulfuric acid into surrounding soils. When disturbance by activities like maintaining mosquito ditches, these soils leach toxic acidity and aluminum into the remaining surface water. Before the National Park Service asked the Barnstable County Mosquito Control Project to cease work in mosquito ditches, acidic water was linked to the death of thousands of American eels, river herring, and other aquatic animals. Poor tidal flushing is causing the depletion of dissolved oxygen in the water. Dissolved oxygen enables aquatic animals to breathe. The dissolved oxygen in the Herring River sits at dangerously low levels as it is today. Without tidal flushing, fecal coliform bacteria have been concentrating at the river's mouth. Due to the high bacteria levels, the Massachusetts Division of Marine Fisheries has closed acres of productive and otherwise harvestable shellfish beds in both the river and Wellfleet Harbor.
The water quality of the Herring River and Wellfleet Harbor will be substantially improved with the restoration of tidal flushing and the salt marsh. Infusions of oxygen-rich water from Cape Cod Bay and Wellfleet Harbor will occur twice daily providing more oxygen for the system. Tidal flushing will reduce bacteria loading by dilution and high salinity, which shortens the lifespan of the bacteria. Tidal restoration also will buffer acidity contamination by resaturating wetland soils with salt water. Along with higher high tides, there will be lower low tides that will improve drainage of ponded mosquito breeding sites on the wetland surface. Restoration will lead to improved habitat conditions for many microorganisms, shellfish, finfish, and wildlife. Restoring the natural connection between salt marshes and coastal waters is critical for the nutrients, sediment, and marine life between the ocean and highly productive estuarine habitats.
The Chequessett Neck Road dike at Herring River substantially reduced tidal exchange, reducing the area being flushed with salt water. Over time, the salt marsh turned into a freshwater marsh, completely changing the natural landscape for those creatures that depended on it. Freshwater marsh vegetation started to grow where salt marsh vegetation could no longer survive. Within our lifetimes, large regions behind the dike have progressed rapidly from a marsh to open meadow to an upland forest ecosystem. Invasive non-native plants have taken over along with dying shrubs and trees. One of these species is common reed, a tall invasive reed grass that has negative effects on salt marsh ecosystems. It displaces native species, reduces biodiversity, chokes waterways reducing flood retention, and offers little value for wildlife. If no action is taken, continued forest and shrub growth and expansion of invasive common reed will displace the more open, herbaceous habitats in the upper part of the system that are relied upon by many species. Restored salinity will kill many of the salt-sensitive exotic plants that have invaded the floodplain and create favorable conditions for salt marsh vegetation to return.
Salt marshes at Cape Cod National Seashore are vulnerable to climate change and sea level rise. Restoring a native salt marsh at the Herring River will provide a place for this critical habitat to exist once again. Salt marshes are among the most biologically productive ecosystems on earth and play an important role in filtering out nutrients and protecting from storm surges. Salt marshes serve as critical habitat for a host of important animal species including fishes, shellfish, and birds. Saltmarsh sparrows, for example, are a declining species and state species of special concern that are completely reliant on this habitat for breeding. Diamond-backed terrapins, a state threatened species, will also benefit from the restoration as it will expand their foraging, mating, and nesting habitat by more than 750 acres. Small and large mammals will also be able to utilize foraging on salt marsh vegetation and the habitat it provides. Salt meadow cordgrass is a valuable forage plant for white-tailed deer. Northern harriers, a state threatened species, and other raptors will have a hunting ground for meadow voles that inhabit the salt meadow cordgrass. The Herring River Technical Team, including CCNS staff, has developed a Habitat Management and Monitoring Plan that describes changes to species and habitats currently inhabiting the project area in more detail.
Historically, the Herring River supported a thriving coastal river ecosystem and one of the largest nurseries for recreational and commercial fisheries in the Gulf of Maine. Records from the town of Wellfleet in the 1800’s report that more than 200,000 river herring were netted annually from the river. Seasonal herring counts conducted Friends of Herring River volunteers indicate the current usage of the river is an order of magnitude below pre-1909 levels. The dike’s small opening restricts water flow in and out of the upper parts of the estuary and marsh, decreasing the mean tidal range. This makes it difficult for fish to enter the river and reduces the submerged and intertidal habitat available for shelter and foraging. Over the past century, this, along with poor water quality, has made it harder for fish to successfully reproduce and survive in their original numbers. Some species, such as brook trout have disappeared from the marsh entirely.
Restoring tidal range will ensure that estuarine fish can access the salt marsh surface during the high tides and follow the tide back towards the sea during the ebb. Estuarine fish, especially killifish, consume mosquito larvae. Restoring their habitat would reduce mosquito breeding. Unrestricted tidal exchange also allows for a natural passage of migratory fish, like river herring, eels, and white perch, that must travel between salt and freshwater habitats to spawn and complete their life cycles. High tides also support the movements of many other groups of animals that regularly feed or reproduce in the marsh, from silverside minnows to lobsters, bluefish, striped bass, terrapins, and even seals and dolphins. The restored tidal range yields ebbing tidal waters, high in plant nutrients and organic detritus from the creek sediments and marsh, to enrich near-shore waters and feed resident shellfish and finfish. Nitrogen exports from marshes in the form of ammonium used by phytoplankton and the organic detritus coated with nutritious microscopic life are beneficial to the food webs of shellfish. Reestablishment of tidal range will restore favorable conditions and restore physical access to habitats for finfish and shellfish.
Sediment cores retrieved from the Herring River system indicate that it had been a stable salt marsh for approximately 2000 years. The presence of salt water in the system, inhibiting freshwater plant colonization and the balance between deposited sediment and the slow rise of sea level driven by glacial meltwater maintained this salt marsh ecosystem. Today, the Herring River system is a heavily impaired area. A natural unaltered salt marsh can compensate for gradual rise in sea level through the deposition of organic material layered with sediments washed into the marsh system on flood tides. The Herring River dike blocks the influx of inorganic sediments (sand and silt), handicapping the marsh’s ability to compensate for rising sea level. After dike construction, decaying organic matter became the dominant remaining marsh deposit.
After the salt marsh was ditched and the river channelized for mosquito control the discharge of water to the sea from the marsh increased and organic deposits (peats) began to dry out and shrink. The pore space between grains of sediment and organic debris had been supported by water, but as the peats dried out these pores collapsed under their own weight, and the marsh surface sank still more. In the drained section of the marsh, organic material was exposed to the air and began to decompose more quickly, also contributing to subsidence. Presently, the restricted marsh surface elevation upstream from the dike is about 70 cm (over two feet) lower than the natural marsh surface just downstream. A casual observer can note the differing elevations from the hill above the dike, as well as the large difference in tidal range.
Restored tidal range leads to higher sediment transport and deposition onto the wetland surface, as sediment-carrying flood tides again flood over creek banks and onto the marsh plain. This surface has subsided over the past 100 years of diking; therefore, restored sedimentation can allow the wetland surface to rise, reducing inland erosion, trapping sediment, and increasing storm-surge protection for roads and other structures at the edge of the floodplain. Flood tides, and especially storm tides, over time transport massive amounts of sediment onto the marsh surface, helping it to rise and keep above the rising sea level. This sediment also promotes the growth of salt marsh grasses that convert solar energy and carbon dioxide to organic matter at rates almost unmatched on this planet.
Blue carbon refers to the carbon stored in wetlands and seagrass beds. These natural habitats store massive amounts of carbon and prevent carbon dioxide loading to the atmosphere. Decomposition in waterlogged, oxygen-poor salt marsh peat is slow and turnover time of the stockpiled carbon is long – thousands of years. Wetlands store carbon at a rate many times faster than even tropical forests, well known for their carbon-storing abilities. Carbon storage is one of many reasons that makes coastal wetland protection and restoration vital. The degraded ecosystem has emitted an estimated 730,000 metric tons of carbon dioxide into the atmosphere since the dike was built in 1909. The Herring River Restoration Project presents a unique opportunity to achieve greenhouse gas benefits from a large-scale tidal wetland restoration.
Research led by the United States Geological Survey in Woods Hole, Massachusetts in partnership with Cape Cod National Seashore and other colleagues indicates that the compromised diked and waterlogged wetlands are significant sources for human-caused methane, a greenhouse gas with global warming potential 40 times that of carbon dioxide. Methane emissions from the Herring River are nearly 15 times higher than those in natural marshes. Restoring saltwater inundation is expected to largely eliminate methane production. The USGS used published information on changes in greenhouse gas emissions and on expected changes in acreage of salt marsh due to restoration to estimate the change in greenhouse gas exchange between the ecosystem and the atmosphere. The salt marsh is expected to increase by 677 acres, which will bring about a great variety of environmental benefits. The marsh will reduce greenhouse gas emissions at an astonishing rate. Calculations are that 4,018,572 grams of carbon dioxide equivalents per acre per year will be reduced. Projecting forward, this equates to a decrease of 2,721 metric tonnes of carbon dioxide equivalents per year which translates into a total reduction of 81,633 metric tonnes per every 30 years the project is ongoing. This thirty-year benefit can be illustrated by the following statistics. In terms that equate to current events, this project will equal avoiding burning 90 million pounds of coal. The project benefits can further be likened to avoiding burning 9.3 million gallons of gasoline or avoiding logging 551 acres of forest. On a proactive basis, it would equate to operating 22 average wind turbines for a full year! The magnitude of the benefits of the Herring River Restoration Project is indeed comprehensive in scope and provides environmental benefits for many years to come.
The Herring River runs from Wellfleet Harbor northeast about four miles to Herring Pond in north Wellfleet, and northwest a similar distance to Ryder Beach in south Truro, covering nearly 1,000 acres. Before the Herring River was subjected to ditching, channelization, and diking, it supported a vibrant coastal river ecosystem including estuary and salt marsh habitats. This natural system has been impaired since 1909 but the Herring River Restoration Project seeks to restore it for the benefit of future generations, including all who call it home and recreate there.
Restoration means better boat access throughout the Herring River estuary with returned tidal exchange allowing higher high tides to give access to open marsh instead of the drained shrub thicket that exists today. It also means reducing invasive species like common reed (Phragmites australis) and nuisances like mosquitoes. The Herring River can once again be a place to find extensive, abundant and diverse marine resources for observation, education, and harvest both within the estuary and the nearby coastal waters. The completion of this project will serve as a model for restoring other estuaries in Massachusetts and along America’s coasts. The Herring River is a natural asset and once it is repaired, people can continue to enjoy the scenery and recreation it has to offer for generations to come. To learn more about the Herring River, the restoration project, and opportunities to attend education and recreation programs, check out the Friends of Herring River website.
Project Area and Elements
Cape Cod National Seashore will soon begin removing dead trees and shrubbery in the Duck Harbor area of the Herring River in Wellfleet to promote the recovery of native salt marsh vegetation in the area. Vegetation removal is expected to begin later this winter.
Since January 2021, the 120-acre Duck Harbor floodplain has had periodic over wash of saltwater breaking over the dunes on Cape Cod Bay. Higher high tides occurring for 3-5 days during most months allowed saltwater to flow rapidly inland and slowly drain back out through the Herring River and into Wellfleet Harbor. The saltwater accumulation in Duck Harbor caused a massive die-off of upland and freshwater trees and plants that had colonized to the area following the diking of the Herring River in 1909.
Removing the dead vegetation at Duck Harbor will promote the natural recruitment of salt marsh plants and increase the ecological productivity of the area, while helping to minimize breeding habitat for mosquitoes by facilitating flow and drainage of water. Tree and shrub removal will be accomplished with heavy duty mulching equipment and will be accompanied by intensive scientific monitoring to document ecological changes. Park staff will be on-site regularly to monitor the work area. The dead vegetation will be mulched and spread amongst the area to promote growth and vitality for the native species.
Park scientists are optimistic about the revival of native salt marsh species, as saltwater tolerant plants have already been observed returning to the area. The near-term future of dune over washes is unknown, but Duck Harbor will eventually experience routine saltwater flow from the Herring River after the dike at Chequessett Neck Road is replaced with a bridge.
Project Videos and Photo Galleries
Last updated: March 8, 2023