Small Dams: Ecological Risk Assessment
In ecological risk assessment, ecological effects are characterized by determining the potential effects imposed by a stressor, linking these effects to assessment endpoints, and evaluating how effects change with varying stressor levels (Hart et al. 2002). This basic framework can be used to evaluate the effects of dam removal by considering dams to be stressors, dam size (or a related measure accounting for stream/watershed characteristics) to be a measure of stressor level, and determining how the effects of different dams (varying across a range of stressor levels) affect stream condition. For any negative ecological impacts of a dam, the maximum potential benefit of a dam removal would be a return of the stream to pre-dam conditions.
In the language of ecological risk assessment, the relationship between stream condition (response) and dam size (stressor) is called a stressor-response relationship. When a reference curve is also shown, the maximum potential benefit of a dam removal can be illustrated as the difference between the stream response level and reference condition at any point along the stressor-response relationship. In our study, we are determining the effects of dams of varying sizes on stream condition by comparing a stream reach immediately below a dam with an undammed reference reach (either an upstream reach or an adjacent, undammed stream depending on the characteristic being evaluated). For each dam, the degree to which the dam affects stream condition can be measured as the difference between the dammed reach and the reference reach. This difference is then used to compare dams. Below is the hypothetical form of the stressor response relationship for dams, with larger dams hypothesized to have a greater impact on stream condition.
Application of this framework allows an assessment of the potential benefits of dam removal across a range of dam and stream/watershed conditions in the context of specific watershed management goals (e.g. cold water fisheries, nutrient/sediment reduction). This information would be very useful to help select and prioritize dam removal projects, and thereby maximize the effectiveness of dam removal in river restoration.