Population dynamics, diversity of phytoplankton and their combination with different environmental factors in Sultansuyu (SsDL) and Surgu Dam Lake (SrgDL) were investigated. While water samples were taken from water surface and Secchi depth for pigment analysis (Chl-a, Chl-b, carotene, pheophytin), they were taken vertically from surface to bottom of lakes for physico-chemical analysis. Diversity, evenness, richness, and dominancy indices were performed for the structure of phytoplankton population due to water quality of the dam lakes. A total of 114 taxa were identified in SsDL and 96 taxa were identified in SrgDL. While Cyanobacteria population changes and abundance were strongly correlated with SiO2 concentration (r= 0.915, p< 0.01), phytoplankton were not significantly correlated with NO3-N and SRP concentrations in SsDL. Total phytoplankton abundance was correlated with pH (r= 0.809); Euglenozoa was strongly correlated with temperature (r= 0.995) in SrgDL. In the assessment of Pearson's correlation analysis and RDA analysis, the interaction among community structure, temporal dynamics, and environmental variables was clearly seen. The present study provides a valuable baseline data on ecosystem quality of two dam lakes towards semi-arid condition.