Studies concentrating upon the epilithon were conducted in five tributary rivers flowing into the Athabasca River: the Muskeg, Steepbank, Hangingstone, MacKay, and Ells rivers. The species composition of the epilithic algae was determined during June to November 1978. Diatoms and blue-green algae dominated numerically except in the Hangingstone River where chlorophycean species replaced the latter group. Seasonal fluctuations in algal species and numbers were followed together with seasonal measurements of standing crop and primary productivity. These latter results probably underestimate true productivity because non-circulating chambers had to be used until circulating ones were constructed. To examine the chief determinants causing species, standing crop, and productivity fluctuations, various chemical and physical factors were measured, their fluctuations described, and relationships examined. This preliminary analysis showed no single nutrient or physical factor to be responsible. Instead, a complex interaction of factors is involved. Current velocity appears to be the most important. Comparisons of the mean standing crops and mean discharge rates produced a highly significant correlation among these rivers. Other factors, including nitrate-nitrogen, dissolved silica, irradiance, and water temperature, were important. However, due to the small data base, these results should be viewed as tentative. Largest mean standing crops for the June to November period occurred in the Steepbank, Ells, and Hangingstone Rivers, while largest mean production rates occurred in the Ells and Muskeg rivers. The MacKay River possessed the smallest standing crop and was the least productive.