Removing tributary low-head dams can compensate for fish habitat losses in dammed rivers

Abstract

Dam removal has been widely used as a river restoration approach; however, the effectiveness of removing low-head dams in tributaries for fish habitat compensation in a dammed river is still poorly understood. One major impediment to examining the ecological effectiveness of low-head dams is the short length of post-dam hydrologic, geomorphic and ecological observations. To circumvent this challenge, we used a one-dimensional hydro-morphodynamic model to simulate the changes in hydrodynamic features and riverbed elevation. The modeled outputs were then incorporated into a fish habitat model to assess the changes of habitat suitability after dam removal. One open question in dam-removal is the effect that extreme hydrological events have on the trajectory of the post-removal hydraulic, geomorphic and habitat change. Here we studied the changes of habitat suitability to a fish species Jinshaia Sinensis (J. Sinensis) in a tributary, where a low-head dam was recently removed, of the Jinsha River in China. Three scenarios of interannual hydrological variations based on observed flow records were simulated. The post-removal simulations showed that the flow sequence influenced the erosion and deposition rate of sediments but had little effect on net changes in bed elevation. After dam removal, upstream incision eroded the deposition sediments and reconnected the riverbed at the dam location within the first post-removal year. Stream hydraulics became more diverse with an increasing of riffles following the morphological response to the dam removal. The continuous changes in stream hydraulics improved the habitat suitability for J. Sinensis, as the percentage of suitable spawning habitat increased from 17% to approximately 68% before and 10 years after dam removal. The study demonstrated that removing a low-head dam can not only reconnect the passage for a migratory fish to migrate upstream tributaries for spawning, but also improve habitat suitability due to the morphological and stream hydrological responses following dam removal.