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.

Water quality trading has been proposed as a cost-effective approach for reducing nutrient loads through credit generation from agricultural or point source reductions sold to buyers facing costly options. We present a systematic approach to determine attenuation coefficients and their uncertainty. Using a process-based model, we determine attenuation with safety margins at many watersheds for total nitrogen (TN) and total phosphorus (TP) loads as they transport from point of load reduction to the credit buyer. TN and TP in-stream attenuation generally increases with decreasing mean river flow; smaller rivers in the modeled region of the Ohio River Basin had TN attenuation factors per km, including safety margins, of 0.19−1.6%, medium rivers of 0.14−1.2%, large rivers of 0.13−1.1%, and very large rivers of 0.04−0.42%. Attenuation in ditches transporting nutrients from farms to receiving rivers is 0.4%/km for TN, while for TP attenuation in ditches can be up to 2%/km. A 95 percentile safety margin of 30−40% for TN and 6−10% for TP, applied to the attenuation per km factors, was determined from the in-stream sensitivity of load reductions to watershed model parameters. For perspective, over 50 km a 1% per km factor would result in 50% attenuation = 2:1 trading ratio.

Geomorphic and hydraulic processes, which form gravel bars in large lowland rivers, have distinctive characteristics that control the magnitude and spatial patterns of infiltration and exfiltration between rivers and their immediate subsurface environments. We present a bedform‐infiltration relation together with a set of field measurements along two reaches of the San Joaquin River, CA to illustrate the conditions required for infiltration and exfiltration of flow between a stream and its undulating bed, and a numerical model to investigate the factors that affect paths and residence times of flow through barforms at different discharges. It is shown that asymmetry of bar morphology is a first‐order control on the extent and location of infiltration, which would otherwise produce equal areas of infiltration and exfiltration under the assumption of sinusoidal bedforms. Hydraulic conductivity varies by orders of magnitude due to fine sediment accumulation and downstream coarsening related to the process of bar evolution. This systematic variability not only controls the magnitude of infiltration, but also the residence time of flow through the bed. The lowest hydraulic conductivity along the reach occurred where the difference between the topographic gradient and the water‐surface gradient is at a maximum and thus where infiltration would be greatest into a homogeneous bar, indicating the importance of managing sand supply to maintain the ventilation and flow through salmon spawning riffles. Numerical simulations corroborate our interpretation that infiltration patterns and rates are controlled by distinctive features of bar morphology.