Frequently Asked Questions

The primary objective was to collect baseline environmental data throughout Mill Brook and its watershed to help inform potential future management decisions and improve the ecological functions and values of the resource. All Committee recommendations for managing and improving the Mill Brook habitat are derived from analysis and reports provided by our consultants: Neal Price of Horsley Witten and entomologist Greg Whitmore. Their report findings are based on data collected during the Committee's recent data collection effort. Both consultants designed and quality-checked this data collection effort.

Before any management action at Mill Pond can commence, the Committee's recommendation is that the Town fund an alternatives analysis and feasibility assessment for potential actions to be conducted by qualified hydrologists, engineers, and wetland specialists. This study will investigate potential positive and negative impacts from said potential actions, as well as evaluate mitigation strategies to address any identified concerns. An assessment of sediment management alternatives would be included in that feasibility study. Permitting for a project affecting Mill Pond will be extensive. Dam removal will require approximately 5-8 different state, local, and federal permits, depending on specific circumstances, including sediment management. MassDEP will require consultation on a sediment sampling plan and hydrologic and hydraulic modeling of anticipated sediment mobilization from a quantity and quality perspective as part of its permitting requirements. These precautions are a requirement before the Town and/or State can permit any such project.

Neal Price: In general, stream areas downstream of a dam are historically "sediment starved" due to the decades and centuries of a dam's retention of sediment behind it. The removal of a dam in general starts the process of a river returning to its more naturalized hydraulic and sediment movement characteristics. This process occurs gradually over time as individual storm and flow events redistribute accumulated sediment from the former impoundment to downstream areas that have been previously deprived of that sediment. The above-mentioned permitting process will evaluate the quality and quantity impacts of the sediment redistribution process on downstream receiving areas.

Greg Whitmore: When removing a dam, there are numerous options to consider regarding sediment. Dam removal is highly regulated to protect downstream ecosystems:

Visit the Mass.gov Website
Visit the Mass.gov Website

Similar to the previous question, a feasibility assessment will be completed to assess alternatives, and detailed permitting will be required for the selected alternative before any project can move forward. Hydrologic and hydraulic modeling of the selected alternative's water level and flow impacts will include an evaluation of the impacts on wetland resources. Potential alteration for loss of valuable habitat or harm to private property upstream and down must be evaluated and considered before an action occurs.

Neal Price: In general, dam removals result in a return to the riparian and wetlands resource areas and habitat communities that existed before dam construction. Relative to the current impounded conditions, dam removal generally results in no to minimal loss in the overall resource area, with the impact being more about a change in resource area type than overall loss. For example, the greatest change is typically a conversion from land under water to bordering vegetated wetlands. In Massachusetts, such conversion of wetland types is allowed in the permitting process because the overall change is generally a net positive regarding habitat and ecological functions and values.

Greg Whitmore: If the dam is removed, the ponded upstream wetland areas will convert to a free-flowing stream ecosystem similar to that found in upstream sections of Mill Brook. The process could be expedited by planting native shrubs/trees to supplement the riparian zone and stabilize sediment.

Neal Price: The only ways to reduce temperature in a stream or impoundment are to increase the amount of cold groundwater inflow, reduce water retention time at the water surface exposed to solar and atmospheric warming, or provide shading. Dams and impoundments reduce groundwater inflow, increase retention time, and broaden the open water surface area to reduce shading from the shoreline. Therefore, the effects of impoundment diminish all traditional or natural means of decreasing temperature.

Greg Whitmore: There are no realistic options.

While all impoundments create the same impacts, the Mill Pond and dam are owned by the Town of West Tisbury and can be managed by its citizens. Other impoundments throughout the brook are privately owned. The committee's recommendations are restricted to infrastructure and areas where the Town has the authority to implement management actions.

Analysis of continuous water temperature data throughout the brook shows that slow-moving water in ponds increases water temperatures to unhealthy levels, whereas free-flowing sections of the brook cool temperatures back down to healthier levels. Removing the impoundment at Mill Pond and restoring the natural flow at this location will eliminate the heating impacts from Mill Pond and provide approximately one mile of healthy stream habitat. While removing all dams would be ideal, removing any single dam will provide similar benefits over the course of its influence on the stream. Removing the impoundment at Mill Pond will also benefit by restoring habitat connectivity with Tisbury Great Pond for anadromous fish species such as River Herring and American eel, returning to spawn in Mill Brook.

Neal Price: For any aquatic system, multiple human stressors tend to affect its Ecological health. Improvements to the overall system health need to begin with one project. It is almost always financially and politically impractical to attempt to fix all of the problems in a system at one time; therefore, changes need to be accomplished incrementally.

Greg Whitmore: Every impoundment compounds cumulative negative effects. Removing any impoundment will lower the overall temperature of downstream sections, increase dissolved oxygen, and allow fish and invertebrates to move freely. In a perfect world, removing dams from lowest to highest would make sense, but any removal is a victory. A gradual drawdown of impounded sections would release a relatively small amount of the overall sediment load in each impoundment as the new channel forms.

The stabilized sediment areas adjacent to the stream will be vegetated with wetland plants such as cattails, sedges, reeds, cardinal flower, and others suited to wet soil. The use and management of this new open space/town part can be determined through a focused Town visioning process to create a conceptual design. Such a design should focus on Ecological health, public access, and aesthetic concerns, and include connectivity with adjacent conservation and Town properties. Monitoring and management for invasive species should also be included.

The two primary concerns associated with climate change are increased storm intensity and floodwaters from high rainfall events, and high water temperatures degrading the cold-water stream habitat. High rainfall events will increase stress on existing spillway and road-crossing infrastructure. Removing the impoundment at Mill Pond will restore the natural floodplains in this area for improved stormwater management and reduce the risk of a catastrophic event. Reestablishing the natural flow will reduce high water temperature impacts and restore stream and habitat connectivity to nearly a mile of Mill Brook.

Greg Whitmore: I agree with this answer.