ECOLOGICAL CONNECTIVITY IN ALLUVIAL RIVER ECOSYSTEMS AND ITS DISRUPTION BY FLOW REGULATION

The dynamic nature of alluvial floodplain rivers is a function of flow and sediment regimes interacting with the physiographic features and vegetation cover of the landscape. During seasonal inundation, the flo od pulse forms a 'moving littoral' that traverses the plain, increasin g productivity and enhancing connectivity. The range of spatio-tempora l connectivity between different biotopes, coupled with variable level s of natural disturbance, determine successional patterns and habitat heterogeneity that are responsible for maintaining the ecological inte grity of floodplain river systems. Flow regulation by dams, often comp ounded by other modifications such as levee construction, normally res ults in reduced connectivity and altered successional trajectories in downstream reaches. Flood peaks are typically reduced by river regulat ion, which reduces the frequency and extent of floodplain inundation. A reduction in channel-forming flows reduces channel migration, an imp ortant phenomenon in maintaining high levels of habitat diversity acro ss floodplains. The seasonal timing of floods may be shifted by flow r egulation, with major ramifications for aquatic and terrestrial biota. Truncation of sediment transport may result in channel degradation fo r many kilometres downstream from a dam. Deepening of the channel lowe rs the water-table, which affects riparian vegetation dynamics and red uces the effective base level of tributaries, which results in rejuven ation and erosion. Ecological integrity in floodplain rivers is based in part on a diversity of water bodies with differing degrees of conne ctivity with the main river channel. Collectively, these water bodies occupy a wide range of successional stages, thereby forming a mosaic o f habitat patches across the floodplain, This diversity is maintained by a balance between the trend toward terrestrialization and flow dist urbances that renew connectivity and reset successional sequences. To counter the influence of river regulation, restoration efforts should focus on reestablishing dynamic connectivity between the channel and f loodplain water bodies.