Effects of dams on stream connectivity and access to source populations for aquatic species restoration

Background/Question/Methods

Approximately 8% of stream reaches in Wisconsin are dammed. The effects of these dams on stream ecosystems are well documented and dam removal is widely touted as an important stream restoration strategy. However, mitigating factors such as habitat suitability and dispersal ability of source populations, may suppress the anticipated benefits of dam removal. Using a database containing records for >4,000 large and small dams, we modeled the removal of dams from Wisconsin streams using GIS-based network analysis and examined changes in stream network connectivity. Then, using an existing model of stream fish distributions, we evaluated the potential effectiveness of removing all dams from state waterways as a tool for restoring populations of seven stream fish species.

Results/Conclusions

Surprisingly, removing all dams from the stream network resulted in only minor increases in mean connected stream length within the dispersal range of many stream fishes. However, the number of severely isolated stream reaches (connected to less than 25 km of stream length) decreased by a substantial 64% (from 7.6% to 2.7% of all Wisconsin streams). Because other environmental factors more often limit fish distributions, removal of all dams increased the predicted ranges of the studied species by only 0.2% to 1.6%. These results suggest that dam removal alone is not a panacea for restoration of depauperate regional fish populations, and reinforces the need to carefully select dams for removal if a restoration goal is to open suitable habitat to fish populations.