III – Inventory and Assessment of Tidal Restrictions

The first phase of the assessment was an inventory of tidal restrictions within the Scarborough Marsh estuary. Because tidal restrictions caused by roads, railroads, bridges, culverts, and tide gates may significantly affect water levels, salinity, and sediment transport, the number and severity of tidal restrictions is a key indicator of the ecological integrity of a tidal marsh system.

The tidal restrictions inventory and assessment was based on the Phase 1 tidal crossing assessment developed by the Parker River Watershed Association (Purinton and Mountain 1997). National Wetland Inventory (NWI) data on Maine Audubon’s geographic information system were used to identify tidal wetlands within the study area. A second data layer showing road and railroad crossings of the marsh was used to identify potential tidal restrictions. The potential restrictions were then marked on a base map for use in the field (Figure 3).

Volunteer Tidal Marsh Stewards gathered data on the type (bridge, culvert, etc.), height and width of the restriction. Because restrictions usually have higher current velocities than natural tidal creeks, water passing through a restriction often erodes the creek bank at either end of the culvert or bridge. Restrictions may also result in changes in the plant community. Evidence of the severity of the restriction, as indicated by scour pools and differences in vegetation above and below the restriction were also noted. The field protocol and a sample data form are included in Appendix A.

The restrictions were then rated according to the restriction classification scheme on the Parker River Watershed Association Phase 1 Data Sheet (Appendix A). Each restriction was given a score of 1 (best) to 5 (worst) in each of three areas: a) evidence of flow restriction, as indicated by the amount of erosion and pooling; b) ratio of culvert or bridge width to river width; and c) comparison of vegetation above and below the restriction. The score for each of the three areas was then summed to provide an overall rating for the restriction. Thus, the best possible overall rating based on the three scores would be a total of 3; the worst overall rating possible would be a score of 15.

The results of the tidal restriction inventory and assessment are summarized in Table 1. Of the seventeen restrictions identified in Figure 3, three (# 6, 9, and 10) were not rated because further field checks indicated that they were located at the natural transition point from tidal to freshwater wetland. The overall rating ranged from a score of 5 (best) to 12 (worst). Six received relatively low scores (overall score of 6.5 or less), six received medium scores (7 to 10), and two received relatively high scores (greater than 10). A discussion of the most severe restrictions is included below.

1. Classification based on Purinton and Mountain, 1997. 1 = Best Condition (nearest to natural); 5 = Worst Condition. See Appendix B for details.
2. Classification for culvert/channel ratio based on low steel dam located just upstream of Blue Point Road.

Restriction # 1 – Jones Creek, Pine Point Road. The Jones Creek restriction at Pine Point was rated one of the most severe. At this point a narrow (7 ft.) concrete box culvert constricts a creek that has a natural channel width of about 34 ft. Significant pooling was observed, and the vegetation is much different on either side of the restriction. Undisturbed low and high salt marsh dominated by Smooth Cordgrass and Salt Meadow Hay is located seaward of the restriction. Inland of the restriction the marsh is poorly drained and dominated by a short form of Smooth Cordgrass. For many years tidal flow to this section of the marsh was blocked by a tide gate. The tide gate was removed approximately 10 to 15 years ago to allow alewives access to spawning areas. After the gate was removed, salt water killed some of the freshwater vegetation that had invaded the marsh west of Pine Point Road during the time the tide gate was in place. While the short form of Smooth Cordgrass occurs naturally in poorly drained areas of high marsh, its dominance in the Jones Creek marsh indicates that years of freshwater influence likely caused the original salt marsh peat to decay. Although salt water has returned, Salt Meadow Hay, the typical high marsh grass, has been unable to re-occupy the site.

It is likely that all of the large seasonally-tidal freshwater wetland south of the Scarborough town line in Old Orchard Beach was salt marsh at one time with a tidal inlet through the beach somewhere near Surfside. This inlet has been blocked and all freshwater draining the northern half of Old Orchard Beach now runs through a ditch connecting the Old Orchard marsh complex to Jones Creek and hence into the Scarborough River. The additional freshwater input to this area likely reduces the salinity of the Jones Creek marsh below the level that might be expected due to this relatively severe restriction. In 1988 a dam was installed on Jones Creek by the Maine Department of Inland Fisheries and Wildlife approximately ½ mile south of Pine Point Road to create a freshwater impoundment. This area is now dominated by uniform growth of Broad-leaf Cattail (Typha latifolia).

Restriction # 15 - Dunstan River, Eastern Road. The Eastern Road crosses the marsh about one mile seaward of Route 1. Where the river cuts through the Eastern Road is spanned by a steel footbridge approximately 100 ft. in length. There is some evidence of scouring above and below the bridge. The river channel is about 125 ft. wide at this point. The section of the marsh inland of the Eastern Road is dominated by the short form of Smooth Cordgrass, whereas typical high marsh plants such as Salt Meadow Hay and Black Grass (Juncus gerardii) dominate below the restriction. The differences in vegetation seem surprising given the small amount of apparent restriction created by the bridge. Other clues, including dead birch trees located within what is now salt marsh on the inland side of the Eastern Road, indicate that a more severe restriction was historically located at this point.

The Eastern Road was the site of the Portland, Saco and Portsmouth Line, a railroad built across the marsh in the early 1840’s². Tidal flows to the section of the marsh lying inland of the railroad were cut off in the 1870’s or early 1880’s. The tide gates and an extensive network of ditches were used to drain the marsh and encourage the growth of salt hay. The tide gates and ditches were so effective that the 1916 edition of the USGS topographic map of the area shows no marsh inland of the Eastern Road, except for a small area along Cascade Brook. The tide gates were maintained until the 1940’s or early 1950’s. After the marsh was purchased by the Maine Department of Inland Fisheries and Wildlife (MDIFW) in the 1950’s the tide gates were used to flood the area with fresh water to encourage the growth of wild rice, a freshwater marsh plant favored by waterfowl. The gates were eventually removed by MDIFW. During a large storm in the 1950’s (sometime after June, 1953 as indicated by aerial photography) the river cut a new channel and broke through the road at the site of the steel bridge that exists today. The old stream channel is now filling in with silt.

Tidal flow to the marsh inland of the Eastern Road has been greatly improved by the elimination of the tide gate and the storm that created the new, wider opening through the Eastern Road. In order to further evaluate the magnitude of the present restriction and the historical impact of the tide gate, a detailed study of marsh and tide elevations was undertaken along the Eastern Road (see Section V).

Restriction #3 – Cascade Brook, Blue Point Road. This restriction is located near the upper limit of tidal range. While the bridge spans 91% of the tidal creek width, a low steel dam located just upstream of the bridge blocks tidal flow on all but the highest tide. This bridge was installed by the Maine Department of Inland Fisheries and Wildlife to create freshwater habitat for waterfowl and other waterbirds while allowing fish passage. Downstream the vegetation is a mix of typical salt marsh plants with stands of Phragmites located along the upland edge. Upstream of the dam salt marsh plants (primarily Smooth Cordgrass) are limited to the fringe of the tidal creek. Narrow-leaf cattail and wild rice are found in the remainder of the marsh.

Restriction #12 – Libby River, Black Point Road. The Libby River, which is 30 ft. wide in this area, passes through a 5.5 ft. diameter culvert. This culvert was installed in 1993 when Black Point Road was rebuilt. Prior to that date a 5 ft. diameter culvert was located at this point (Linnell 1994). Distinct erosion pools are located above and below the restriction. Linnell’s study also indicated that the marsh inland of the road was drier, less saline, and that Phragmites was located inland but not seaward of the road. His study also indicated that the marsh on the inland side of the road averaged 6.6 inches lower in elevation than the marsh on the seaward side. This indicates that the peat probably has decayed and subsided due to the restriction of tidal flows. The Black Point Road was re-built in 1996. Unfortunately, a larger culvert that would allow free tidal flow was not installed at the time the road was rebuilt.

Restriction #12 – Libby River, Black Point Road. The Libby River, which is 30 ft. wide in this area, passes through a 5.5 ft. diameter culvert. This culvert was installed in 1993 when Black Point Road was rebuilt. Prior to that date a 5 ft. diameter culvert was located at this point (Linnell 1994). Distinct erosion pools are located above and below the restriction. Linnell’s study also indicated that the marsh inland of the road was drier, less saline, and that Phragmites was located inland but not seaward of the road. His study also indicated that the marsh on the inland side of the road averaged 6.6 inches lower in elevation than the marsh on the seaward side. This indicates that the peat probably has decayed and subsided due to the restriction of tidal flows. The Black Point Road was re-built in 1996. Unfortunately, a larger culvert that would allow free tidal flow was not installed at the time the road was rebuilt.

² Becky Delaware, Past President of the Scarborough Historical Society, provided much of the historical information in this report.