Revisiting the homogenization of dammed rivers in the southeastern US
For some time, ecologists have attempted to make generalizations concerning how disturbances influence natural ecosystems, especially river systems. The existing literature suggests that dams homogenize the hydrologic variability of rivers. However, this might insinuate that dams affect river systems similarly despite a large gradient in natural hydrologic character. In order to evaluate patterns in dam-regulated hydrology and associated ecological relationships, a broad framework is needed. Flow classes, or groups of streams that share similar hydrology, may provide a framework to evaluate the relative effects of dam regulation on natural flow dynamics. The purpose of this study was to use a regional flow classification as the foundation for evaluating patterns of hydrologic alteration due to dams and to determine if the response of rivers to regulation was specific to different flow classes. We used the US Geological Survey (USGS) database to access discharge information for 284 unregulated and 117 regulated gage records. For each record, we calculated 44 hydrologic statistics, including the Indicators of Hydrologic Alteration. We used a sub-regional flow classification for eight states as a way to stratify unregulated and regulated streams into comparable units. In general, our results showed that dam regulation generally had stronger effects on hydrologic indices than other disturbances when models were stratified by flow class; however, the effects of urbanization, withdrawals, and fragmentation, at times, were comparable or exceeded the effects of dam regulation. In agreement with the existing literature, maximum flows, flow variability, and rise rates were lower whereas minimum flows and reversals were higher in dam regulated streams. However, the response of monthly and seasonal flows, flow predictability, and baseflows were variable depending on flow class membership. Principal components analysis showed that regulated streams occupied a larger multivariate space than unregulated streams, which suggests that dams may not homogenize all river systems, but may move them outside the bounds of normal river function. Ultimately, our results suggest that flow classes provide a suitable framework to generalize patterns in hydrologic alterations due to dam regulation.