One of the widespread sources of river pollution is wastewater coming from both wastewater treatment plants and stormwatater system. Wastewater can vary significantly in composition and concentration of substances introduced into water bodies. Municipal effluents may contain significant amounts of organic matter and ammonia [Strom et al., 1976]. Storm drains are diverse in composition and depend on the nature of the surface from which the water collects, but carry more suspended solids and less nutrients. The research was aimed at the assessment of the effect of surface runoff collected by stormwater system from the territory of the city of Lublin on the Bystrica River using popular environmental indices, calculated on the basis of periphytonic algae species abundances: species number, Shannon's H, rarefied species number, Pielou's evenness, trophic diatom index (TDI). We observed, that the correspondence between a species diversity and the quality of the environment is not always straightforward. So, periphytonic algae diversity increases under the influence of runoff, as evidenced by the Shannon index. Nevertheless, pronounced changes are noted in the structure of the algal community, as shown by the Pielou index and NMDS. However, these changes in the structure are invisible if we rely on the trophic diatom index (TDI).