Water is the fundamental requirement for humans and for society. Humans have dammed, dyked, channeled, diverted, and pumped water for millennia to control freshwater for society, often changing the flow of streams and rivers to meet human needs. Dams provide substantial and undeniable benefits to society, including drinking water, irrigation, flood control, and – particularly in the 20th and 21st centuries – hydropower. But these benefits come at environmental and economic costs. Dams often result in the loss of important ecological attributes and ecosystem services provided by rivers and streams. By turning a river into a string of artificial lakes punctuated by impassable barriers, dams can wreak havoc on populations of commercially valuable migratory fish like salmon. Expensive mitigation practices like construction of fish ladders and the transport of juvenile salmon downstream via barges or trucks are often implemented to counteract the effects of dams with mixed results. In recent years, removal of aging dams to restore vitality to rivers is a realized possibility, including high profile cases like the 2011-2012 removal of the Elwha Dam on the Olympic Peninsula in Washington State. But even as dam removal gains traction, one certainty of the coming decades is that dams new and old are an inevitable part of a world with growing water and electricity needs. Two questions then arise: how do dams affect freshwater organisms and ecosystems, and can we manage dams in a way that minimizes their negative impacts on the freshwater ecosystems we value?

Understanding how flow alteration affects freshwater organisms is a complex challenge of interest to scientists and managers alike, and flow:ecology relationships are a research focus of the Olden Lab. One important step in establishing flow:ecology relationships is first understanding the relationship between freshwater organisms and the historical “flow regimes” (timing, magnitude, duration and predictability of flow events like floods and low flows) to which they have adapted. What are the major reproductive, movement, and/or behavioral cues provided by streamflow events? We recently asked this question for native freshwater fishes across the United States, and we found that predictable relationships exist between natural (undammed) flow regimes and freshwater fish “life history strategies” – a combination of reproductive tradeoffs including number of offspring, age at sexual maturity, and life span (read more about our study). Since all species can be characterized by life history strategies, we were able to compare fish communities that may have very different species.

We then took those established flow:fish relationships and asked if the life history strategies of fishes below dams are different from undammed similar streams nearby and whether differences are predicted by the new flow regime created by the dam. We again looked at study sites across the United States, and our study included dams used for different purposes (hydropower, flood control, or locks). We found that fish life histories below dams tended to be more “equilibrium” – fishes well-adapted to less disturbance and more consistent flow conditions – and less “opportunistic” – fishes well-adapted to rapidly-changing flow and high levels of disturbance. Dams typically reduce flow variability by dampening extreme flow events (floods or low flows) and can change the timing of major flow events. Our study demonstrates the ability of dams to filter for fish traits (established over millennia) in just a few decades. Furthermore, the evidence of directional filtering (favoring equilibrium strategists and selecting against opportunistic strategists) illustrates the potential of dams to systematically disadvantage some species. To read more about this study, check out this month’s issue of Freshwater Biology.

These studies represent a valuable step and highlight broad patterns across time and space, including many regions and many types of dams. Studies like these are a critical part of the recent call for “environmental flows” – releases of water below dams that mimics the natural (pre-dam) flow regime (more on environmental flows). But such a broad approach as ours results in the loss of finer-scale detail, such as the effect of particular flow attributes (for example, floods or low flow events) or particular dam types (for example, hydropower versus flood control). Understanding mechanisms of these observed patterns of biotic response to dams, such as filtering for life histories, is of critical importance for dam managers. One possible way to improve understanding of these mechanisms is by use of experimental floods below dams to examine specific responses to flow types (more on experimental floods). Improving flow classification of streams and rivers (such as the lab’s recent paper on classification of rivers in Washington State) will also help improve flow:ecology relationships where missing empirical flow data hinders such efforts. Ultimately, region-specific studies or those focusing on a specific type of dam or flow regime may provide the most useful information for dam managers. With increasing demands for freshwater and energy an inevitable consequence of 21st century human growth, a scientific platform for sound dam management will need to remain a priority for managers and scientists alike.

-Meryl Mims