Characterization and Causes Analysis for Algae Blooms in Large River System

•The characteristics and causes of algal blooms in a lager river system were analyzed comprehensively.•Algal blooms have significant seasonality variations.•The key influencing factors were identified by Cross wavelet analysis method.•The contributions of three key factors (WL2, TP and WT) for algal blooms were 44.5%, 39.6% and 11.4%, respectively.•Hydrological conditions play a very important role in river algal blooms.