New Zealand rivers were surveyed for filamentous algal proliferations following prolonged periods of low flows to define their extent, standing crop, and composition. Significant growths were found at 167 of the 423 sites surveyed. The geometric mean river bed cover of filamentous algae was 26% in summer and 22% in winter. Proliferations of >40% cover occurred at 16% of the sites. The geometric mean standing crop was 15.3 g/m2(ash‐free dry weight) in summer and 1.87 g/m2in winter. Proliferations of >50 g/m2(AFDW) occurred at 8% of the sites. A taxonomic classification analysis of the summer data identified nine main clusters of sites which appeared to correspond to different catchment land uses. One, or a combination, of the following taxa dominated these clusters:Ulothrix zonata, Phormidiumspp.,Sti‐geocloniumsp.,Spirogyraspp.,Compsopogon coe‐ruleus, Oedogoniumspp.,Cladophorasp.,Cladophora glomerata, Rhizocloniumsp., andMelosira varians. Ulothrix zonataandDiatoma hiemalevar.mesodondominated the sites sampled in winter. There was a close positive relationship between the average standing crop and the average water conductivity of the nine site clusters.