Processing of alder (Alnus tenuifolia) was investigated during autumn and winter to determine the influence of stream regulation on leaf litter processing. Study sites were the Colorado River below Granby Dam and the Fraser River, an unregulated tributary. Alder leaves (5-g units) were attached to bricks and placed in riffles. Leaching controls were retrieved after 48 h; thereafter five replicates were collected from each site after 17, 38, 52, and 66 d. There were significant differences (P < 0.05) in processing rates between sites. The loss rate coefficient (k) was much higher (k = 0.0462) for leaves incubated in the regulated section of the Colorado River than for those in the unregulated tributary (k = 0.0235). The hypothesis that reductions in macroinvertebrate shredders would decrease processing rates of leaf litter in regulated streams was not supported by the results. The "winter warm" thermal regime below the reservoir seems to compensate for the virtual absence of shredder species apparently by enhanced microbial processing.Key words: Colorado River, leaf litter, macroinvertebrates, Rocky Mountains, stream regulation, temperature