UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS
Location: Water Quality and Ecology Research
Title: Floodplain Backwater Restoration: A Case Study
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 15, 2010
Publication Date: June 15, 2010
Citation: Shields Jr, F.D., Knight, S.S., Lizotte Jr, R.E., Bryant, C.T., Wren, D.G. 2010. Floodplain backwater restoration: A case study. Proceedings, 9th Federal Interagency Sedimentation Conference, Joint Federal Interagency Conference Organizing Committee, Washington, D.C., CD-ROM.
Interpretive Summary: Ponds, lakes, and wetlands on floodplains next to rivers are an important component of riverine ecosystems, but are becoming increasingly rare in agricultural landscapes due to sedimentation and flood control activities. Two floodplain backwaters (severed meander bends) along the Coldwater River in northern Mississippi, USA were studied for four years. One backwater was modified by diverting agricultural runoff away from the backwater and by building low water control structures to increase dry season water depth. Positive responses in treated backwater water quality and fishery were measured, but similar changes did not occur in the untreated site. These findings are of interest to land managers interested in riverine aquatic ecosystem restoration and management.
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.