Approximately 15% of the world’s total run-off is presently retained by more than 45 000 large dams. However, the extent of the downstream ecological impacts of those dams is rarely assessed. The longitudinal effects of a large reservoir on the substrate, water quality and riffle macroinvertebrate communities were examined between 0.5 and 18.3 km downstream of Tallowa Dam. The number of taxa and the Australian River Assessment Scheme observed v. expected score generally increased with increasing distance from the dam, average clast size decreased with increasing distance and water quality showed distinct longitudinal patterns. Classification of the macroinvertebrate assemblages identified two groups, one from riffles ~4 km downstream of the dam and one further downstream, suggesting the main impact occurs close to the dam. The difference between the two groups of riffles resulted mainly from the following macroinvertebrates, Edmundsiops (Baetidae), Hemigomphus (Gomphidae), Illiesoperla (Gripopterygidae), Physa (Physidae), Nannoplebia (Libellulidae) and Austrolimnius larvae (Elmidae), occurring less frequently in the near-dam riffles. Water quality was probably the main cause of the altered macroinvertebrate assemblage structure, not altered hydrology, a result attributable to the small operational capacity of Tallowa Dam relative to the annual inflow volumes.