The effect of a thermal effluent on the attached algae of the Provo River, Utah, USA, was studied from 1975 to 1977. Data for macroscopic and microscopic algae were collected and analyzed. Diatoms, Cladophora glomerata, and Hydrurus foetidus dominated the flora. The thermal effluent significantly affected the algal flora in a section of river 100 to 135 meters long immediately below the discharge point. Cladophora growth was increased and Hydrurus was absent in this area. In addition, diatom production was often higher and diversity lower than in the rest of the river. Community structure was unique from all other adjacent areas. Small temperature increases which occurred as effluent and river waters mixed farther downstream were apparently not as important to the algal flora as other environmental factors.