Downeast Salmon Federation (DSF), through a contract with Inter-Fluve, have published a study on the existing conditions of the West Branch Pleasant River marsh.
The study investigated all aspects of the system as it currently exists, including road crossing features, vegetation, soils, salinity, water levels, and legacy agricultural features. The study also included estimates for future tidal inundation patterns based on sea level rise predictions, and comparisons to the adjacent tidal marsh areas just downstream from the tide gates. The present condition of the West Branch Pleasant River (WBPR) marsh complex above the tide gates is very different than adjacent marshes found along the tidal reaches of the Pleasant River. Currently, the West Branch marsh would best be described as “brackish”, or somewhere between fresh and salt. Evaluating the existing features of the tidally restricted marsh was an important first step to understand the viability of its restoration. The full 200 page report can be found here.
Road crossings
Six road-stream crossing structures are present throughout the marsh complex. These crossings include the tide gates on the Ridge Road Dike Bridge and five upstream road crossings that are not currently sized to accommodate freshwater flooding or tidal flow. There are known flooding issues at two out of five road crossings upstream from the tide gates (Water St. and Point St.), and the Addison Rd. crossing is in critical condition and in need of replacement. Read more about each of the crossings here.
Soils and vegetation
All areas of the West Branch marsh show impacts of past agricultural use, including embankments, ditches, and sluices. These features changed the properties and movement of water within the marsh, which over time led to changes in the soil and they types of plants that can grow. Upstream from the tide gates, mostly freshwater and partially salt-water tolerant plants grow in the marsh. Although highly salt-tolerant plants are not currently present upstream of the tide gates, the mix of brackish and salt-tolerant species throughout the marsh suggests that the shock to plant communities from saltwater reintroduction may not be as extreme as in other gated marshes.
The vegetation of the WBPR marsh was surveyed in 2006 as part of an earlier study, and again in 2023 as part of this report. Overall, minor changes were seen in the types of vegetation found. Some of these changes appeared to be related to beaver activity in the upstream reaches of the marsh. Closer to the tide gates, the changes were likely due to the salt water that currently leaks through the tide gates. One site just north of Ridge Road saw a shift from an invasive species of cattail to native saltwater cordgrass and brackish-tolerant white meadowsweet and goldenrod.
Salinity and water levels
Throughout the West Branch, water levels and salinity are controlled by tides and precipitation runoff. Years of wear and tear and damage from storms have reduced how well the clapper gates work, allowing some tidal flow to move upstream. Additionally, the West Branch receives freshwater inputs from an 11 square mile watershed.
Above the tide gates at Ridge Road, the tide range is about a quarter of that in the Pleasant River, and the tide is detectable all of the way to Addison Road. Runoff from rainfall and snowmelt becomes trapped behind the gates during high tides, which increases the chances of flooding.
No tidal signal was observed at Point Street, due to the large beaver dam that sits above the Water Street crossing. However, this beaver activity between Point Street and Water Street traps water from heavy rain and snowmelt, further contributing to the complicated hydrology of the West Branch.
At current sea levels, the Addison Road, Water Street, and Point Street road elevations are lower than the height of the highest tide, if and when Ridge Road overtops. Point Street is especially low. The potential for flooding with future overtopping will increase in the coming years.
Just upstream of the tide gates, the average salinity is similar to the tidal water outside the gates in the Pleasant River. At Water Street and Addison Road, it is about a half to a third as salty as the tidal water.
Predicted extent and depth of tidal water in each of the West Branch marsh subsections during a Mean Higher High Water (daily high tide) event