Stream ecosystem response to small dam removal: Lessons from the Heartland
Introduction
Interest is rapidly growing in examining scientific problems that occur at the interface of disciplines. This trend represents not only a growing interest in how one discipline can inform another, but also how new questions are emerging that cannot be addressed within the confines of a single discipline. To this end, the Binghamton symposia have traditionally dealt with interfaces between geomorphology and some other scientific discipline. In 1995, geomorphologists turned their attention to “Biogeomorphology” (the relationships between biota and geomorphic forms and processes) and resultant papers (Hupp et al., 1995) considered how geomorphology affects biological processes, and vice versa. Examining the papers presented at the 1995 conference leads to an important question: what is “biology” to a geomorphologist? Sixteen of the 21 papers focus on vegetation, three on zoological factors (e.g., cows), one on the microbial dynamics of rock surface weathering, and one on food web dynamics on floodplains. This strong focus on vegetation indicates a bias toward examining biological factors already known to affect geomorphic processes and toward studies that are interested in the shifting geomorphic template as a means of describing changing habitat availability for large and conspicuous groups of organisms such as fish and riparian trees. Few studies have examined the role of geomorphic forms on ecological dynamics over a range of trophic levels or on biotic interactions between species (but see Power et al., 1995).
Beyond illustrating a potentially skewed research perspective, what geomorphologists consider to be “biology” also has ramifications for their input to ecosystem management or restoration schemes, like river restoration. If the presence of a particular species of fish is considered to indicate a healthy stream or river, then geomorphic design of a channel for restoration efforts will focus solely on creating forms and processes that facilitate survival and reproduction of that species. This is an inadequate approach, as managing or designing for restoration of the entire ecosystem recovery is a more desirable goal (Ward et al., 2001). Developing a more thorough understanding of how geomorphic forms and processes affect ecological dynamics is important beyond the pervasive habitat perspective.
Our primary goal is to examine how geomorphic forms and processes affect stream ecosystems across a range of trophic levels. To address this goal, we synthesize a number of case studies of dam removal in the state of Wisconsin. Dam removals represent large-scale disturbances and are significant from a variety of perspectives (Heinz Center, 2002); observing how the coupled geo–eco systems recover following a common disturbance is instructive for both the individual and coupled systems. We have limited our review to Wisconsin because a large number of dams have already been removed in the state (Doyle et al., 2000), there are several completed and ongoing studies of dam removal within the state (reviewed below), and examining a single region limits variation because of physiographic factors.
We begin by briefly reviewing small dam removal in the U.S., summarizing our previous observations of how rivers geomorphically respond to dam removal, and describing the dominant geomorphic changes that can be expected at sites similar to those in Wisconsin. We then cover five attributes of stream ecosystems affected by dam removal: fish, vegetation, macroinvertebrates, unionid mussels, and nutrient dynamics. We have placed preference on using studies that have already been published or will be published, although we have also used general observations and modeling where empirical data are scarce. In each case, we briefly describe the site, study methods, and results, and then discuss the ecological response to dam removal and how geomorphic forms or processes were significant factors in controlling these responses.
Section snippets
Small dam removal
As is the case with most states in the U.S., dams dominate Wisconsin's riverways (Graf, 1999). The vast majority of these structures are characteristically small and abundant in small and mid-order channels. Pervasive decline in the structural integrity of dams provided the inspiration for dam removal in Wisconsin as early as the 1960s. The initial wave of dam removal as a safety and management action drew little attention beyond the borders of the state. However, a second wave of removals in
Riparian vegetation
Dam removal exposes previously inundated reservoir sediment and forms new sediment surfaces downstream by sediment transport and deposition. Shafroth et al. (2002) suggested several scenarios of vegetation changes likely to occur upstream and downstream following dam removal. Initial vegetation in impoundments will tend to be dominated by weedy plants that grow quickly, have high seed production, and have effective propagule dispersal mechanisms. Eventually, colonizing species should give way
Synthesis
In this review of case studies from Wisconsin, we show that dam removal can affect stream ecosystems in multiple trophic levels; and in each case, ecological changes could be related to geomorphic changes. By developing the ability to predict the mechanisms, rates, and magnitudes of geomorphic responses to dam removal, we will also begin to be able to predict ecological responses. At this point in time, however, we have only a cursory and qualitative understanding of the physical and ecological
Acknowledgements
This work would not have been possible without the extended cooperation by the Wisconsin Department of Natural Resources, particularly Sue Josheff and Meg Galloway, who aided us in identifying many of the dam removal sites for research opportunities. Many of the ideas presented here resulted from research supported by funding from several sources, including the Bradley Fund for the Environment, a Horton Grant from the American Geophysical Union, NSF grant DEB-0108619, the Showalter Fund, Fish
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2022, Journal of Environmental ManagementCitation Excerpt :This bias is carried forward into basin-scale stream connectivity assessments that rely on such databases, leading to underestimation of river fragmentation (e.g. Magilligan et al., 2016; Couto and Olden, 2018; Jones et al., 2019; Belletti et al., 2020; Sun et al., 2020). Nevertheless, the negative impact of low head dams on aquatic communities, as well as the ecological benefits of their removal, are well documented (Watters, 1996; Scruton et al., 1998; Porto et al., 1999; Bednarek, 2001; Doyle et al., 2005; Kil and Bae, 2011; Nislow et al., 2011; Hitt et al., 2012; Smith et al., 2017). Indeed, a 1.5 m dam may present an equally impassable barrier to aquatic organisms as a 20 m dam.
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