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The diatom Didymosphenia geminata (Bacillariophyceae) has garnered increased attention as a nuisance and invasive species in freshwater systems. Historically described as rare yet cosmopolitan, a suspected new variant of D. geminata has the capacity to inundate kilometres of river bottom during a bloom. Unlike most other bloom-forming algae, D. geminata proliferates under high water quality (i.e., low turbidity and low nutrient) conditions. To inform management strategies, the environmental factors and conditions that promote bloom events must be ascertained. Our study of the Bow and Red Deer rivers in southern Alberta, Canada, provides supporting evidence that the mean flow regime is associated with bloom development, based on a significant negative relationship detected between D. geminata biomass and mean discharge (r2 = 0.30). While flow regulation by dams can create the stable flow environment preferred by D. geminata, our results indicate that flow regime (rather than just proximity to dam outflows) is the likely mechanism, in addition to other environmental factors, such as water clarity, temperature, pH, conductivity, and total phosphorus. We discuss the formidable challenges to D. geminata management, particularly along unregulated river reaches, yet also recognize the unique research opportunities that this organism poses for the growing field of invasion biology. [ABSTRACT FROM AUTHOR]