Greater scientific knowledge, changing societal values, and legislative mandates have emphasized the importance of implementing large‐scale flow experiments (FEs) downstream of dams. We provide the first global assessment of FEs to evaluate their success in advancing science and informing management decisions. Systematic review of 113 FEs across 20 countries revealed that clear articulation of experimental objectives, while not universally practiced, was crucial for achieving management outcomes and changing dam‐operating policies. Furthermore, changes to dam operations were three times less likely when FEs were conducted primarily for scientific purposes. Despite the recognized importance of riverine flow regimes, four‐fifths of FEs involved only discrete flow events. Over three‐quarters of FEs documented both abiotic and biotic outcomes, but only one‐third examined multiple taxonomic responses, thus limiting how FE results can inform holistic dam management. Future FEs will present new opportunities to advance scientifically credible water policies.
Experimental manipulations of streamflow have been used globally in recent decades to mitigate the impacts of dam operations on river systems. Rivers are challenging subjects for experimentation, because they are open systems that cannot be isolated from their social context. We identify principles to address the challenges of conducting effective large-scale flow experiments. Flow experiments have both scientific and social value when they help to resolve specific questions about the ecological action of flow with a clear nexus to water policies and decisions. Water managers must integrate new information into operating policies for large-scale experiments to be effective. Modeling and monitoring can be integrated with experiments to analyze long-term ecological responses. Experimental design should include spatially extensive observations and well-defined, repeated treatments. Large-scale flow manipulations are only a part of dam operations that affect river systems. Scientists can ensure that experimental manipulations continue to be a valuable approach for the scientifically based management of river systems.biological conditions in these systems may not be attributed solely to the level of streamflow during the experiment. Unlike experiments on land, lakes, and small streams in experimental watersheds, flow manipulations involving whole rivers or estuaries can rarely, if ever, be isolated from their social context. Stakeholders have diverse interests in how water is used, and water managers operate facilities and systems to achieve multiple objectives. The overarching issue for scientists involved in large-scale flow experiments, then, is to design scientifically credible and tractable investigations that simultaneously inform water management about policies to achieve long-term objectives.We review the global practice of flow manipulations in rivers as large-scale experiments to guide future efforts in this burgeoning area of interest using examples from over 40 systems (see the supplementary material, available online at http: //dx.doi.org/10.1525/bio.2011.61.12.5). We focus on flow manipulations intended to achieve ecological objectives because of their direct relevance for informing dam operations but recognize that investigations of natural flow events and manipulations not intended for ecological outcomes provide useful information for managing rivers and advancing river ecology. We identify how flow experiments have elucidated and addressed facets of the complexity in river, floodplain, and estuary ecosystems. These examples lead us to a core set of challenges and principles for conducting effective large-scale flow experiments that have both scientific and social value.
Hourly fluctuations in flow from Glen Canyon Dam were increased in an attempt to limit the population of nonnative rainbow trout Oncorhynchus mykiss in the Colorado River, Arizona, due to concerns about negative effects of nonnative trout on endangered native fishes. Controlled floods have also been conducted to enhance native fish habitat. We estimated that rainbow trout incubation mortality rates resulting from greater fluctuations in flow were 23-49% (2003 and 2004) compared with 5-11% under normal flow fluctuations (2006-2010). Effects of this mortality were apparent in redd excavations but were not seen in hatch date distributions or in the abundance of the age-0 population. Multiple lines of evidence indicated that a controlled flood in March 2008, which was intended to enhance native fish habitat, resulted in a large increase in early survival rates of age-0 rainbow trout. Age-0 abundance in July 2008 was over fourfold higher than expected given the number of viable eggs that produced these fish. A hatch date analysis indicated that early survival rates were much higher for cohorts that hatched about 1 month after the controlled flood (∼April 15) relative to those that hatched before this date. The cohorts that were fertilized after the flood were not exposed to high flows and emerged into better-quality habitat with elevated food availability. Interannual differences in age-0 rainbow trout growth based on otolith microstructure supported this hypothesis. It is likely that strong compensation in survival rates shortly after emergence mitigated the impact of incubation losses caused by increases in flow fluctuations. Control of nonnative fish populations will be most effective when additional mortality is applied to older life stages after the majority of density-dependent mortality has occurred. Our study highlights the need to rigorously assess instream flow decisions through the evaluation of population-level responses. Egg and larval mortality resulting from the operation of nuclear power plants (Barnthouse et al. 1988), hydroelectric dams (McKinney et al. 2001), and natural causes (Methot 1983; Crecco and Savoy 1987; Peterman et al. 1988) can potentially reduce the abundance of adult fish populations. The extent of the impact will depend on the proportion of
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