This study examines the distribution and ecology of the morphologically complex genus Calothrix, which contains species particularly hard to identify due to morphological variability with changing water conditions. Up to a dozen species have been reported previously in US freshwater environments. Using literature review to establish previously reported occurrences and the extensive dataset from the Environmental Protection Agency National River and Streams Assessment, this study mapped the current confirmed distribution of 7 species in United States rivers and streams. Results from data analysis showed Calothrix spp. occurring in 42 States and establish its occurrence in both flowing and standing, typically alkaline or neutral waters. Ecological traits exhibited by individual species included C. fusca (Kützing) Bornet & Flahault indicating pH may be a limiting factor, with this species preferring more alkaline waters. In addition, conductivity levels influenced filament structure. Data indicates that Calothrix epiphytica West & G.S. West responds to higher light availability and is also influences by turbidity levels. This study expands our ecological knowledge of species belonging to the genus Calothrix, begins to document the current distribution of these species and the morphological data presented represent the first metapopulation data summary for US Calothrix distribution. The study also confirms the need for a polyphasic approach when studying this group and suggests that molecular data be incorporated into continent-wide studies in the future.