Morphohydraulic of a dam-impacted large river: The São Francisco River, Brazil

Abstract

Many studies have highlighted dam impacts in the reduction of the transmission of nutrient loads and suspended sediments downstream, increasing coastal erosion (sediment budget) and water residence time, beyond modifying the channel planform on a reach scale. However, the morphohydraulic changes, which are induced by flow regulation, have been little explored. This study has sought to analyze temporal changes in the at-a-station hydraulic geometry from 9 cross-sections along the São Francisco River, comparing the pre and post-dam periods. In addition to that, it was also sought to compare our results with other tropical river systems and assess the relationships of morphohydraulic parameters with channel patterns and functional trends. We have used field data collected by the National Water Agency and the Geological Survey of Brazil covering the middle, lower-middle, and lower courses of the São Francisco River. Potential functions were extracted using the least squares fit method. We have applied the Mann-Whitney non-parametric statistical test to assess any dataset changes. Finally, we have analyzed the data through the ternary diagram. Our results have demonstrated that the São Francisco River has high bank stability and variations in flow are mainly accommodated by changes both in depth and flow velocity. We found changes in the sensitivity of width, depth, and velocity in the post-dam period that imply high mobility of the cross-sections in the ternary diagram. Changes in land use patterns are potentially associated with these observed changes. Despite the São Francisco banks being stable in the analyzed cross-sections, the sensitivity of the channel width increased in the post-dam period. Differences were observed between the response patterns in the post-dam period in the physiographic provinces of the São Francisco River. However, the increase in width sensitivity predominates, with few exceptions. Hydraulic geometry analyses have proved to be essential for the characterization of the functional dynamics of river systems and allowing inferences about sediment transport conditions, flow stage, and channel stability.