Dam Removal Effects, CT River Watershed

This dataset is a component of a complete package of products from the Connect the Connecticut project. Connect the Connecticut is a collaborative effort to identify shared priorities for conserving the Connecticut River Watershed for future generations, considering the value of fish and wildlife species and the natural ecosystems they inhabit. Click here to download the full data package, including all documentation.

This GIS product represents potential opportunities to restore aquatic connectivity by removing dams. Specifically, this product tabulates the results of a model in which each dam is systematically removed (virtually), one at a time, and the predicted improvement in local aquatic connectedness from the removal is recorded. The delta, or difference, in the aquatic connectedness score, before and after the dam removal for each cell within the affected neighborhood, is computed and multiplied by the average index of ecological integrity of the affected neighborhood. Therefore, improvements are scored higher where conditions are not highly degraded and dam removal may have greater ecological benefits.

Considerations for Using Data Layer

This tool is not designed to quantify benefits to anadromous fish from dam removal; users interested in tools that address anadromous fish may wish to investigate other products such as the Northeast Aquatic Connectivity Project led by The Nature Conservancy along with Northeast state fish and wildlife agencies.

The dam removal effect score (effect) is an index of the potential improvement in local aquatic connectedness to be achieved in places where it matters most -- where the current ecological integrity is not already severely degraded. Based on these scores and the corresponding ranks, dams can be prioritized for restoration. Note, these scores do not take into account other socio-economic considerations, such as whether the impoundment is a public drinking water supply, which ultimately will determine the cost-benefit tradeoffs of any particular dam removal. Given the large number of dams, it may be useful to bin the dams into categories representing high, medium and low impact, or simply threshold the score or its rank at some level to highlight the highest priority dams. This layer may best be used to direct field surveys of dams of interest, during which complete and accurate assessments can be made. It can also be used in combination with the lotic (rivers and streams) and lentic (lakes and ponds) cores to identify places where dam removal may have the added benefit of improving the integrity of the designated aquatic cores.

Use of this layer should be done considering the scope and limitations of this dataset:

• The actual restoration potential of a dam may be quite different than the modeled estimate. For example, unmapped dams certainly exist and affect the real-world aquatic connectivity not reflected in our scores. Incomplete and/or inaccurate data on dam height and other attributes (such as the partial breach of the dam) result in incorrect estimates of aquatic passability. Also, for many dams with incomplete data, especially the smaller dams, we are forced to make an assumption about dam height and also to assume that the dam has not been breached. In addition, unreliability of data on fish passage structures forced us to omit this factor from consideration in the model. Because of these known data gaps and errors inherent in the source data, the data layer should be used cautiously.

• The dam removal effect score represents the potential gain in local aquatic connectivity from removing each dam without considering other natural or anthropogenic barriers (e.g., waterfalls, culverts) or potential nearby restoration actions to improve connectivity. Of course, dams often do not exist as isolated barriers. The score of a dam is dependent to some extent on the degree to which natural barriers and road-stream crossings nearby on the same waterway are also acting as barriers to movement. For example, removal of a dam will result in less improvement in connectivity if there is an undersized culvert a short distance from the dam than if no movement barriers are nearby. The undersized culvert will continue to depress aquatic connectedness even after the dam is removed. Unfortunately, evaluating the combined (and possibly synergistic) effect of multiple restoration activities, such as removing the dam and upgrading the nearby undersized culverts, is computationally beyond the scope of this project, but should be taken into account when prioritizing dam restoration opportunities in practice.