The influence of a hypolimnetic reservoir discharge on breakdown of aspen (Populus tremuloides) leaves was investigated during the late summer and autumn at sites above and below a high mountain reservoir. During the two-year study, leaf packs (5-g envelopes) were secured to stream substrate in riffle/run areas. Leaves protected from invertebrates with fine mesh did not demonstrate differences in leaf loss between sites suggesting that microbial processing was equivalent. There were. however, significant differences in leaf loss in coarse mesh envelopes between sites, with losses higher at the site above the reservoir. Aquatic invertebrate communities colonizing the leaf packs also differed from above and below the reservoir suggesting that invertebrates played a role in the differing leaf loss. We hypothesize that during late summer and fall, leaf breakdown is slower below the reservoir because of the altered invertebrate community.