The dynamics of the litter stratum of a streamside forest ecosystem in Illinois, USA, were described. Changes in litter fall, leaf decomposition and soil organic matter content were determined in relation to time and to the coenocline of canopy species induced by the gradient of flood variables of the Sangamon River. Gradient variation in litter fall was determined during 2 yr (Sept. 1973-Aus. 1974 and Sept. 1974-Aug. 1975) by litter-fall traps. Leaf litter decomposition and soil organic matter were measured during the 1974-1975 sampling year. Total litter fall ranged from a low measured in 1973-1974 in the flood plain of 520.8 g m-2 yr-1 to a high of 977.1 g m-2 yr-1 measured in the upland during 1974-1975. Slower decomposition rates in the upland resulted in turnover rates exceeding 1 yr. All leaf litter falling in the flood-plain section of the gradient was either decomposed or washed into the stream during flood during each year. Soil organic matter in the upland averaged 4.17% and showed an increase between autumn levels and those recorded after decomposition of leaf matter was nearly completed.