FLOODPLAIN RIVER BACKWATER RESTORATION: A CASE STUDY

Abstract: Current thinking in stream ecology emphasizes the importance of floodplain backwaters within lowland riverine ecosystems. However, these types of habitat are becoming increasingly rare as development is transforming floodplain landscapes in fundamental ways. Two floodplain backwaters (severed meander bends) along the Coldwater River in northern Mississippi, USA were studied for four years. The study sites contained up to 2.5 m of sediments deposited since cutoff, and connections to the river were brief and sporadic. During drier months, water quality reflected combined effects of nonpoint source pollution and very shallow water depths. One backwater was used as an untreated reference, and the other was modified by adding a weir that included an operable culvert that was opened during winter to maintain river connectivity but closed during drier months to increase water depth. Furthermore, placement of the weir diverted agricultural runoff away from the modified backwater. Responses to treatment included reduced river connectivity, increased summer water depth, moderation of severe diurnal water quality fluctuations, and reductions in concentrations of solids (~70%), nutrients (~30%-60%), and chlorophyll a (~50%). Similar changes were not observed in the untreated backwater. Fish species richness, numbers and biomass were unchanged following rehabilitation, but trophic structure shifted away from omnivorous species and toward predators, making the treated site less similar to the degraded reference. Ecological services provided by floodplain riverine backwaters may be enhanced by modest management measures, but regaining and maintaining connectivity with adjacent ecological functional patches is difficult