The reproductive behavior of adult Calopteryx splendens males and females inhabiting the Nida River, south Poland, was studied and compared during a pre-flood and a post-flood year. The flood disturbance in 2010 caused a decrease in aquatic macrophytes, thus reducing availability of potential territories and consequently, significantly influencing male behavior towards a frequent non-territorial strategy. Many males in the post-flood population had damaged wings due to extremely aggressive contests. Male–male tandems were commonly observed; this is an uncommon behavior in C. splendens. Although the sex ratio was male-biased throughout the whole study, we observed more males in the post-flood year. We also observed less-frequent copulations and ovipositions during the post-flood year. The only unchanged characteristic was population density, which did not differ before and after the flood disturbance. Floods have significant impact on damselfly reproductive sites and this, due to changes in behavior and sex ratio, may result in further consequences on population dynamics.