Tidal Wetland Restoration in Connecticut (Part 2)  (Part 1) (Part 3)

   GREAT HARBOR MARSH/LOST LAKE, GUILFORD DRAINED AND SUBSIDED MARSH

This 93 hectare (230 acre) complex consists of two major, interconnected wetlands - Great Harbor Marsh downstream of the railroad and State Route 146, and the upstream Lost Lake (32 hectares or 78 acres, also called Three Corner Marsh) (Fig. 3). Great Harbor Marsh is separated from Long Island Sound by a narrow coastal barrier. In the late 1880s the coastal trolley bed was constructed on this beach and in 1916 an elevated dike was constructed with a tide gate installed in the outlet channel to control tidal flooding. This situation persisted for nearly 40 years, when a hurricane destroyed the tide gates and restored full tidal flow in the early 1950s.  

Fig. 3 Map of Great Harbor marsh/Lost Lake and Long Cove.(165K)

The gating and draining of this wetland complex caused a lowering of the water table in the marsh peat by nearly a meter. This exposed the upper portions of peat to oxygen for the first time, and it began to decompose very rapidly. The overall result was a loss of peat, which caused the marsh surface to drop, or subside, at least to 60 centimeters (two feet) below its former elevation. If we use a conservative estimate of a 60 centimeter decrease in elevation across the entire 93 hectare (230 acre) marsh surface, then the total peat loss is projected at over 765,000 cubic meters (1 million cubic yards). To imagine this much volume, think of it as equivalent to raising the height of a football field 140 meters (460 feet).

Due to the subsidence of the peat surface, within several years of the breach approximately 75% of the marsh was converted from a salt marsh community to intertidal flat/shallow water habitat with no marsh vegetation at all. Only that section of Great Harbor marsh adjacent to the barrier beach had sufficient elevation to retain emergent wetland plants. From there, Smooth Cord-grass began a gradual but progressive expansion out onto intertidal flat, in an upstream direction. Forty years after the loss of the tide gate, most of the Great Harbor marsh now supports wetland vegetation once again (Figs. 4 and 5). Upstream, the Lost Lake area is still largely devoid of vascular plants to this day. 

Great Harbor Marsh has largely restored itself from the standpoint of the amount of wetland that now supports salt marsh vegetation. However, the plant communities are not what they once were. Before tide gates the vegetation was principally a high marsh community, but it is now dominated by the low marsh species Smooth Cord-grass. Draining caused subsidence, but subsequent restoration of full tidal flow to a significantly lower marsh surface did not lead to the restoration objective of re-establishment of the previous plant and animal communities. The lost peat has been replaced by salt water resulting in significant changes in current velocities and flow volumes. The creek has widened from 3 to 27 meters (10 to 90 feet) since the gates came out, a result of the scour from increased current velocity. Furthermore, the marshland immediately upstream of the inlet has been converted to a 0.5 hectare (1.5 acre) pond. Its current diameter is over 60 meters (200 feet), and enlargement is projected to continue until the tidal exchange volume is reduced through the formation of peat and the subsequent increase in wetland elevation. This process is described in the previous chapter on marsh development.

The nearby Leetes Island marsh in Guilford is another example of a subsided marsh. Studies conducted by the U.S. Army Corps of Engineers have shown that restoration can only be accomplished by building a series of structures that will allow more water to leave the marsh on low tide than enters the marsh on high tide. Without such modifications of the natural tidal cycle, too much water will remain on the marsh at low tide, preventing the establishment of salt marsh plants.

Other examples of degraded marshes caused by tide gates include: Old Field Creek and Cove River, West Haven; West River, Mill River and Morris Creek, New Haven; Sluice Creek, Guilford; and Sybil Creek, Branford. DEP is working with many of these communities in the development of salt marsh restoration plans.  

LONG COVE, GUILFORD - RESTRICTED TIDAL FLOW

Long Cove is aptly named since it occupies a long but narrow valley, approximately one mile in length. With an area of only 17 hectares (43 acres), it is separated from Long Island Sound by a narrow, sandy barrier beach (Fig. 24). The road and bridge on this beach were destroyed in the 1938 hurricane and was reconstructed at a landward position. The bridge was replaced by a 42 inch concrete culvert; a short time later, a second culvert was installed with a tide gate for mosquito control. A linear ditch was excavated in the marsh from the new culvert to the upper reaches of the marsh. Eventually the original culvert filled with sediment and was abandoned.

In an attempt to control mosquito breeding, a two step water management program was developed in the 1940s. The tide gate was closed during the summer months to drain the marsh and reduce insect breeding habitat. After the first hard frost in fall, the gate was opened, and the resulting tidal flows removed sediments that accumulated in the creeks and ditches in the summer months. The alternating cycle of draining and flooding continued for forty years. Phragmites gradually and progressively replaced most of the typical salt marsh vegetation.

By the early 1980s, the tide gate had fallen off, reestablishing year-round tidal flushing. It was observed that most of the marsh area never flooded, even during the highest tides, and it was concluded that the remaining functioning culvert was undersized and restricted tidal flow. On the portions of the marsh which did flood with salt water, Phragmites was replaced by salt marsh plants.

The deterioration of this marsh was brought to attention of the DEP by William Tietjen, a long time resident and member of the Indian Cove Association who had personally witnessed the decline of the marsh since the 1940s. This served as the catalyst for the department's first multi-group partnership in tidal wetland restoration. The partnership included the Town of Guilford, the Guilford Land Trust which owned most of the wetland, the DEP and the Mosquito Control Unit (formally the Mosquito and Vector Control Section) of the Connecticut Department of Health Services. Taking into consideration long term changes to the marsh caused by draining, and recognizing the need to increase water levels only to that extent which would regularly flood the marshland (i.e., several centimeters), the reopening of the abandoned culvert was proposed. DEP's Coastal Area Management Program (now the Office of Long Island Sound Programs) provided Guilford with a small grant to restore the channel across the beach and construct concrete training walls to the protect the channel. In 1986 the Mosquito Control Unit removed sediment upstream of the culvert and cleaned those ditches necessary to interconnect the culvert with the main channel.

Eight years later, nearly all of the Phragmites in the central and upper marsh has been replaced by salt marsh vegetation. Phragmites is stunted and decreasing in abundance in the lower marsh. Approximately two hectares (five acres) of pool habitat have formed in the upstream marsh. It is predicted that much of the open water will be replaced by salt marsh vegetation, but in the interim it functions as significant wildlife habitat for waterfowl, shorebirds and wading birds.

Long Cove illustrates successful restoration of a marsh via partial flow restoration using culverts. Examples of salt marshes that have been restored through the installation of larger culverts to increase tidal flow volumes include: Cat's Island, Milford; Caroline Creek, East Haven; and Palmer Cove, Groton.

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June 21, 1997 / DEP's Tidal Wetland Restoration Program / webmaster@po.state.ct.us