Studies concentrating upon the epilithic algal community were conducted in five tributary rivers flowing into the Athabasca river: the Muskeg, Steepbank, Hangingstone, MacKay, and Ells rivers. Numerically, cyanophycean algae (Lyngbya aerugineo-caerulea, Phormidium sp., Calothris braunii, Nostoc spp., and Anabaena affinis) dominated, followed by diatoms (Synedra ulna, Synedra rumpens, Gomphonema olivaceum, Gomphonema acuminatum, Gomphonema longiceps v. subclavata, Nitzschia fonticola, Nitzschia palea, Achanthes lanceolata, Epithemia sorex, Epithemia turgida, Cocconeis placentula and Cocconeis pediculus). One exception was the Hangingstone River where chlorophycean algae (Stigeoclonium pachydermum and Cladophora glomerata) were next in importance to the Cyanophyta. Seasonal fluctuations in algal species and numbers were influenced by a myriad of interacting factors as were standing crop fluctuations. However, physically disruptive forces, current velocity and discharge, appeared more important than dissolved nutrients. They also affected the chemical composition of the water itself. The mean algal standing crops ranged from 7.94 to 43.23 mg m chlorophyll a in the MacKay and Ells rivers, respectively, with mean values of 30.46, 22.9, and 22.35 mg·m-2 chlorophyll a occurring in the Muskeg, Steepbank, and Hangingstone rivers, respectively. Epilithic algal primary productivity was more closely related to standing crop size than irradiance. The annual production averaged 36.2, 54.4, 71.4, 101.6, and 110.0 gm C m-2 in the MacKay, Hangingstone, Steepbank, Ells, and Muskeg rivers, respectively.