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Restoration of Tidally Restricted Esturaries

Cape Cod NS
Mark Adams
Restoration of Historically Restricted Estuaries, Cape Cod National Seashore, Massachusetts.

John Portnoy, Ecologist, Mark Adams, GIS Specialist

The Atlantic coast’s estuaries and salt marshes have been historically under-appreciated and misunderstood which has led to their widespread diking, draining and development. Cape Cod National Seashore was created in 1961 to protect some of these remaining coastal resources for recreation, conservation and scientific study. Even with National Park status, managers encounter many obstacles to the restoration of natural salt marsh functions in historically altered areas. Fragmented ownership and jurisdiction, privately developed inholdings and incomplete knowledge of the consequences of altered hydrology comprise a few of the obstacles to tidal restoration. But if we cannot restore and maintain natural ecological functions on public lands then chances are slim that coastal wetlands can be preserved elsewhere, lacking legal protections and public interest mandates.

The benefits of naturally functioning tidal salt marshes can be enjoyed by present generations of National Seashore users through carefully phased restoration programs that include assessment, modeling, restoration and monitoring. Estuaries on Cape Cod in various stages of restoration include Hatches Harbor (Provincetown), East Harbor (Truro) and Herring River (Wellfleet). Steps have included topographic mapping with survey-grade GPS, vegetation and water quality inventories, predictive hydrologic modeling, community consultation and scientific oversight, reconstruction of tidal culverts and staged re-introduction of tidal influence. Post-restoration monitoring is underway at Hatches Harbor and East Harbor and pre-restoration assessment at Herring River.

The Role of GIS.

Geographic Information Systems have become integral to natural resource monitoring at Cape Cod National Seashore. GPS units are standard field equipment for all science staff and spatial data management for vegetation, wildlife and hydrologic monitoring is a cooperative function across all of our programs. The GIS staff participates in sample design, data collection and analysis, particularly where special expertise is needed. Detailed tidal floodplain elevation data collected by survey-grade GPS and total station is required as input to hydrodynamic models that predict tidal influence under different culvert openings.

Hatches Harbor Salt Marsh Restoration Project.
In cooperation with the Town of Provincetown, Federal Aviation Administration and Massachusetts Aeronautics Commission, the National Park Service (NPS) has been incrementally restoring tidal exchange to the diked portions of Hatches Harbor since March of 1999. The overall objective of this project is to restore native salt marsh functions and values to the tide-restricted wetland to the extent possible without compromising safety at the Provincetown Municipal Airport.
After an hydrodynamic assessment, large culverts were installed through the Hatches Harbor Dike by the NPS in the winter of 1998-99 to accommodate increased tidal flow. These gated culverts have been opened in small increments each year to ensure Airport safety from flooding and to control and adaptively manage ecosystem response. Cape Cod National Seashore (CCNS) staff and cooperators have monitored system response intensively since 1999, with base line data collected in 1997 before new culvert construction.
Monitoring has included tide heights, sedimentation, sediment-water quality, wetland vegetation, nekton (fin-fish and decapod crustaceans) and mosquitoes within both natural (unrestricted) and diked portions of the Hatches Harbor flood plain.
A critical variable affecting wetland plant vigor is the depth of dewatering and consequent root-zone aeration during the growing season. Prior to tidal restoration, the restricted wetland surface often remained flooded even through the low tide period; with enlarged culverts, low-tide porewater levels have decreased substantially. This, together with increased salinity and low sulfide should encourage the re-establishment and increase the vigor of salt marsh plants.
The mosquito species composition and abundance of floodwater-breeding in coastal marshes is affected by salinity, flooding duration and areal extent, and access to breeding sites by (primarily fish) predators. The objective of mosquito monitoring at Hatches Harbor is to assess whether changes in hydrography caused by the tidal restoration affect mosquito species composition and abundance. Brackish and salt marsh mosquito production has increased as higher high tides have increased flooding extent and salinity on the wetland surface. Increased breeding appears to be focused on swales that flood with tidal water just southwest of the Airport taxiway. Because these swales may be an artifact of diking, creek construction to improve tidal exchange is an adaptive management option that may expedite habitat restoration and reduce floodwater mosquito production.
Restoration began incrementally in 1999 and is still in the initial stages of a process that will span decades; so far, tide-restored vegetation is changing in a direction similar to Spartina-dominated marsh.
Herring River Proposed Salt Marsh Restoration

The Herring River and associated salt marshes in Wellfleet (MA) represent the largest riverine estuarine system within the Cape Cod National Seashore. Core analysis has shown that salt marshes occupied the flood plain for 2000 years before European settlement.

With seventeenth century settlement, two millennia of natural marsh development were interrupted by the placement of dikes blocking tidal flow to salt water marshes. By 1910, the entire system to the river mouth had been diked, for the purpose of reducing salt marsh mosquitoes. The final 1908 diking was undertaken amid great controversy and concern for the effects of the structure on salt hay production and the lucrative herring fishery. Salt hay farms disappeared and the herring run, whose annual auction at the turn of the century produced enough income to pay all elected officials in the Town of Wellfleet, shortly declined. The mosquito nuisance persisted, however, causing the town and state to dig drainage ditches in the marshes behind the dikes and eventually channelize parts of the river. Later, two houses were constructed within the 10-ft MSL contour (i.e. within the flood plain) behind the dike about the time the Seashore was created by Congress around 1960.

By the 1960s, under the diking and drainage regime, most of the original emergent marshlands had given way to freshwater wetland plants and even upland shrubs and trees. Saltwater wetland values, rather than being replaced by those associated with freshwater wetlands, were largely lost.

Beginning is the 1980s, NPS scientists documented major water quality problems associated with the long history of diking and drainage, including surface water acidification, the release of oxic metals and oxygen depletion, each causing repeated and massive fish kills. Despite the long history of marsh alterations aimed at their control, the dominant species of nuisance mosquito for the region was found to breed in abundance throughout the acidified waters of the diked floodplain. Apparently, water quality imposed a chemical block to important mosquito predators, particularly fish. Ecological assessments of the system from 1983 to 1985 documented depauperate fish and shellfish populations upstream of the dike, and minimal use of the system by shorebirds (sandpipers, plovers etc.).

With tidal restoration, typical marsh-estuarine vegetation would increase in extent; water quality problems, including stream anoxia and acidity, would diminish; estuarine nursery habitat for fish would be improved; and benthic invertebrate populations (including hard clams, soft-shelled clams, oysters and non-commercial species) would increase. Mosquito populations are expected to decrease due to enhanced tidal circulation and natural predation by fish.

GIS is a component of the monitoring tools needed to resolve outstanding obstacles to restoration, including:

-- the potential for salt water intrusion into domestic wells adjacent to the flood plain;
-- options to protect the Chequesset Neck golf course from regular flooding by tidal input and freshwater runoff; and
-- options to protect the residential dwellings currently within the flood plain

With NPS funding and refined bathymetric and tide-height data, an improved hydrodynamic model of the system was completed in 2001. Presentation of this study’s results has prompted additional local discussion and raised further questions. NPS and cooperating scientists have responded to these concerns with a research and monitoring plan that should resolve outstanding environmental questions.

GIS will continue to support these and other Cape Cod salt marsh restoration projects through detailed topographic and bathymetric surveys, spatial records of monitoring sites and maps that synthesize data in compelling ways. Much of the scientific data is subtle and complex. GIS staff will continue to develop analysis and simulations that aid managers and neighboring jurisdictions to visualize the results of monitoring and build support for sound management decisions.

Portnoy , J.W., S.M. Smith, C.T. Roman & E. Gwilliam. , 2002. Hatches Harbor Salt Marsh Restoration: 2002 Annual Report. U.S. National Park Service, Wellfleet MA.

Portnoy, J.W., 2003. Briefing Statement on past and proposed management of the Herring River Estuary, April 2003. Cape Cod National Seashore, U.S. National Park Service, Wellfleet MA.

Cape Cod National Seashore, 99 Marconi Sire Road, Wellfleet MA 02667
Cooperative Ecosystems Studies Unit, National Park Service, University of Rhode Island, Narragansett RI 02882

April 08, 2004