Using historic aerial photography and paleohydrologic techniques to assess long-term ecological response to two Montana dam removals

The restorative potential of dam removal on ecosystem function depends on the reversibility of dam effects and its operations. While dam removal is an established engineering practice, the need for an understanding of the ecological response remains. We used paleoflood hydrology, hydrologic modeling, and aerial photo interpretation to investigate the long-term ecologic responses to dam failure and breach. We investigated downstream geomorphic and vegetation responses to a dam failure (Pattengail Dam in 1927) and a controlled dam breach, which used natural sediment removal (Mystic Lake Dam in 1985). Our data showed vegetation responses indicative of channel and floodplain evolution at Pattengail. The size of the flood following the Pattengail dam failure initiated a series of channel adjustments and reworked over 19 ha of floodplain downstream of the dam. In Mystic, we observed few flood stage indicators and a slight response in floodplain vegetation. We made several findings. (1) Dam removal effects on channel evolution and floodplain development depend on reach types and their responsiveness to flow regime change. (2) Ecologic response to dam removal depends on the sizes and timing of high flow events during and following removal. (3) Paleohydrology can be used to assess historic floods (>20 years). We see the utility of assessing the ecological responsiveness of a system to previous fluvial events or changes in flow regime. Informed about the character of a system based on its history, dam removal scientists can use these tools to set realistic restoration goals for removing a dam.