Preliminary Responses to Removal
Biological response, however, will likely take more time. O’conner et al. (2015) found that the initial response of animals to dam removal has been fairly fast (at least for mobile taxa), pointing to recent dam removals in Virginia, Maine, and the removal of two dams on the Elwha River in Washington. In those cases, migratory eel, lamprey, and Chinook salmon populations responded quickly to improved river connectivity, often colonizing upstream habitat within days or weeks of removal. Less understood, however, is the long-term trajectory of those populations. If spawning habitat was a limiting factor on production, we can assume that access to historical spawning habitat will improve population abundance (though this will take time). In some cases, fishes will return to a range of habitat types that haven not been accessible for decades or longer. These previously inaccessible habitat types promote different environmental selection pressures and, in turn, will promote life history diversity within populations. This is important because improved life history diversity will strengthen population resilience to random processes such as drought, fire, or many of the other disturbances that are anticipated to increase in frequency with the onset of climate change. Downstream habitat will also greatly be affected, particularly by the influx of sediment. In the case of the Elwha River, coarse sand has since blanketed the low gradient reaches of the Elwha River Delta, creating vital habitat for threatened fish, foraging birds, and marine predators.
Dam removal may also have unintended consequences. O’connor et al. (2015) points to a study conducted by Kornis et al. (2015) in which the authors document the removal of a small dam in Wisconsin. The findings suggest that, at least in the short term, there was greater overlap in the fish assemblage patterns above and below the dam after the removal. In particular, reservoir species historically precluded from upstream passage were now able to colonize upstream habitat. In some cases, those reservoir species reduced the abundance of pre-existing fishes above the dam, such as sculpin and stickleback. The authors also found, however, that downstream habitat changes elicited an important response: a reduction in warm water species and improvement in coldwater habitat. Additional consequences of dam removal include the potential for downstream transport of excessive nutrients, pollutants, or even disease. And as discussed earlier, there is also the potential for significant changes to channel morphology , including channel incision upstream into the reservoir or sediment aggradation downstream (both of which may be tempered by the rate of dam removal) with a host of potential biological consequences.
Photo: Ed Homich
Figure 1. Data source: American Rivers.
Modified from O’connor et al. 2015.