River impoundment is known to affect the physical, chemical, and biological characteristics of the downstream river, but little is known of the way in which changes of these characteristics interact or the time-scale required to attain a new quasi-equilibrium state. The data presented herein show that channel sedimentation below a tributary confluence, resulting from reduced flow competence and capacity within the regulated mainstream, has produced morphological changes associated with substrate particle-size alteration. Four physical compartments are identified, each representing a different stage of readjustment to the hydrological conditions imposed by the dam. Each stage is shown to have a characteristic fauna. Moreover, the species diversity, composition, and biomass were found to be most different from the natural data during the early stages of readjustment and to approach the natural data more closely as the channel attained a quasi-equilibrium form.