THE IMPACT OF TRIBUTARIES ON A REGULATED AND UNREGULATED RIVER

The complex fluvial environment at tributary junctions provides habitats critical for biological diversity. The dynamic equilibrium between mainstem channel morphology and tributary discharge and sediment load may be particularly sensitive to flow regulation. While there has been extensive research on the impact of flow regulation on channel morphology caused by the reduction in the magnitude and frequency of large floods and the trapping of sediment behind dams, much less is known about the direct impact of regulation at and near tributary junctions. We compare the impact of tributaries on similarly-sized regulated and unregulated rivers. The impact was assessed with respect to the following parameters: (1) historical changes in bar extent and channel plan-form morphology pre- and post-regulation; (2) downstream extent of channel bars immediately below tributary junctions as a function of tributary and mainstem drainage areas; (3) rate of sediment movement as quantified using ⁷Be activity; (4) magnitude and extent of tributary induced changes in sediment grain size; and (5) the pre- and post-dam changes in channel slope at tributary junctions. Preliminary results indicate that tributaries have a greater impact on channel morphology on a regulated river than an unregulated river due to the reduced discharge and sediment load post-impoundment.