Extended Water-Level Drawdowns in Dammed Rivers Enhance Fish Habitat: Environmental Pool Management in the Upper Mississippi River.

Environmental Pool Management (EPM) can improve ecosystem function in rivers by restoring aspects of the natural flow regime lost to dam construction. EPM recreates summer baseflow conditions and promotes the growth of terrestrial vegetation which is inundated in the fall, thereby improving habitat heterogeneity for many aquatic taxa. A three-year experiment was conducted wherein terrestrial floodplain areas were dewatered through EPM water-level reductions and the resulting terrestrial vegetation was (1) allowed to remain or (2) removed in paired plots in Mississippi River pool 25. Fish assemblage and abundance were quantified in paired plots after inundation. Abundances of many fish species were greater in vegetated plots, especially for species that utilize vegetation during portions of their life history. Fish assemblages varied more between plot types when the magnitude of EPM water-level drawdowns was greater, which produced greater vegetation growth. Young-of-year individuals, especially from small, early maturing species and/or species reliant on vegetation for refuge, feeding, or life history, utilized vegetated plots more than devegetated plots. Vegetation growth produced under EPM was heavily used by river fishes, including young-of-year individuals, which may ultimately positively influence recruitment. Increased habitat heterogeneity may mitigate some of the negative impacts of dam construction and water-level regulation on river fishes. Annual variability in vegetation responses that occurs under EPM enhances natural environmental variability which could ultimately contribute to increased fish diversity. Low-cost programs like EPM can be implemented as a part of adaptive management plans to help maintain biodiversity and ecosystem health in anthropogenically altered rivers.