Freshwater resources are at risk globally from increased eutrophication and toxic algal blooms. These cyanobacterial and algal events typically occur in the summer when the reservoir is well-stratified and rich in nutrients from erosion and runoff. Prediction of bloom occurrence requires an understanding of the factors which trigger bloom formation, and on which spatiotemporal scales. The biochemical and physical contributors have been studied in depth, but there is still uncertainty in the temporal and spatial scales at which climate or hydrologic contributors are important. We identified the primary hydrologic drivers of harmful algal bloom biomass within Lake Allatoona and Lake Lanier, Georgia, using a combined in-situ and satellite remote sensing dataset. Linear and multiple regression suggest that in shallower and more energetic reaches of the reservoir, drought severity is the primary contributor to biomass, while storm duration drives growth of harmful algal blooms in deeper, more lacustrine environments.