In our study we investigated two polluted rivers of Eurasia from silicate and carbonate regions. We revealed an algal diversity consisting of 184 algal species and cyanoprokaryotes in the Rudnaya River, and 175 in the Qishon River. The distributions of species over the 7 higher taxa were very similar for both rivers with diatom prevealing. Bioindicational analysis in respect salinity, acidification, oxygenation, and organic pollution show that the water is cleaner and the diversity is higher in the Rudnaya River than in the Qishon River. The indices of saprobity S ranged similar. The impact of pollution on Rudnaya River increases downstream. In tha case of Qishon River, the impact of pollution decreases downstream.As a result of CCA, we revealed biosensors group sensitive to borates and fluorides in the case of Rudnaya River and these were: Ankistrodesmus acicularis, angustus, Scenedesmus acuminatus, Lyngbya kuetzingii, Neidium ampliatum, and Sellaphora rectangularis. In the case of Qishon River, Audouinella pygmaea, Characium ornithocephalum, and Chamaesiphon amethystinus were found as biosensor species. We found that algal biodiversity is more sensitive to technogenic pollution in the silicate province being more tolerant to the same organic pollutants in the carbonate province. Therefore, the combination of bioindicational methods and statistics are effective for determination of the main factors influencing algal diversity, indicators or biosensing species for the most important environmental variables.