Water-level management is widespread and illustrates how contemporary climate can interact directly and indirectly with numerous biological and abiotic factors to influence reproductive success of wildlife species. We studied common loons, an iconic waterbird sensitive to timing and magnitude of water-level changes during the breeding season, using a before-after-control-impact design on large lakes in Voyageurs National Park (Minnesota, USA), to assess the effect of anthropogenic changes in hydroregime on their nesting success and productivity. We examined multiple competing a priori hypotheses in an information-theoretic framework, and predicted that magnitude of changes in loon productivity would be greater in the Namakan Reservoir, where water-level management was altered to mimic a more natural hydroregime, than in Rainy Lake, where management remained relatively unchanged. We determined outcomes from 278 nests during 2004–2006 by performing boat-based visits every 3–5 days, and measuring hydrologic, vegetative, and microtopographic covariates. Relative to comparably collected data for 260 total loon pairs during 1983–1986, productivity (chicks hatched/territorial pair) increased 95% in the Namakan Reservoir between the 2 time periods. Nest success declined in both lakes over the 2 study periods but less so in the Namakan Reservoir than in Rainy Lake. Flooding was a primary cause of nest failures (though second nests were less likely to flood). Nest predation appears to have increased considerably between the 2 study periods. Top-ranked models suggested that timing of nest initiation, probability of nest flooding, probability of nest stranding, and probability of nest success were each related to 2–4 factors, including date of initiation, timing of initiation relative to peak water levels, changes in the elevation of the nest edge, maximum water-level change between initiation and peak water levels, and maximum water-level change between initiation and nest outcome. The top model for all variables except stranding each garnered ≥82% of total model weight. Results demonstrate that water-level management can be altered to benefit productivity of common loons. However, nuanced interactions between land-use change, invasive species, human development, recreation, climate change, and recovery of top predators may often complicate both management decisions and interpretation of water-level impacts on wildlife. © 2013 The Wildlife Society.