The first phytoplankton studies on Daugava River were undertaken in 1924, when the Riga sewage collection system was expanded. Dammed reservoirs were constructed in 1940 (Keguma HPP); 1962 (Plavinu HPP); 1975 (Riga HPP). In total 480 species of algae (65 blue-greens) were determined. First reports of cyanobacteria bloom in the cascade of HPP were stated in 1977. It is established that Microcystis settled to the bottom of reservoirs in October, after having dominated about three months. Enhanced temperature above 27^oC in July and August 1999 was the main reason for extensive cyanobacterial bloom by mixed composition dominated by Microcystis species (mainly M. aeruginosa, M. wessenbergii, M.viridis ). Mean July ñ August temperature for Daugava reservoirs is 21-22.2^oC. The maximum of phytoplankton biomass more than 85 mg L^-1 was recorded in August 1999 in the lower Reservoir of Cascade. Former observed summer maximum biomass was 24.7mg L^-1 in July 1991. In September 1999 the Microcystis species were partly replaced by Anabaena spp. and Oscillatoria spp. Conspicuous large phytoplankton biomass constituted more than 92% of M. aeruginosa and dense surface scums of cyanobacteria have deteriorated water quality in Riga reservoir ñ one of the sources of the drinking water for Riga the Capital of Latvia, the Daugava River estuary and Riga Gulf. The results from September 1999 indicate that colonies of Microcystis associated with Anabaena spp., Oscillatoria spp., Spirulina sp. and unicellular diatoms are forming 1.5-3.0 cm thick green layer on top of the sediment. The data suggested that these high amounts of Microcystis in sediments would cause microbial activity ultimately leading to phosphorus leakage from sediments and potentially increasing hazard for Riga drinking water.