Connectivity and variability: metrics for riverine floodplain backwater rehabilitation

Authors: Shields Jr, Fletcher; Knight, Scott; Lizotte, Richard; Wren, Daniel

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/15/2010
Publication Date: 9/15/2011
Citation: Shields Jr, F.D., Knight, S.S., Lizotte Jr, R.E., Wren, D.G. 2011. Connectivity and variability: metrics for riverine floodplain backwater rehabilitation. In: Simon, A., Bennett, S., Castro, J., and Thorne, C. R. (eds.), The Scientific Basis for Stream Restoration in Dynamic Fluvial Systems: Deterministic Approaches, Analyses and Tools. American Geophysical Union. 2010, Geophysical Monograph Series 194, pp. 233-246, DOI:10.1029/2010GM000985.

Interpretive Summary: Seasonally flooded lakes and wetlands adjacent to rivers (backwaters) in the agricultural landscape are increasingly degraded by sedimentation and nonpoint source pollution, and cost-effective restoration tools are needed. Three backwaters along the Coldwater River in northern Mississippi, USA were studied for four years. One backwater was modified by by building low water control structures, and ecological effects were assessed using pre- and post rehabilitation hydrologic indices. The restoration project resulted in increased summer water depth, moderation of severe diurnal water quality fluctuations, and general water quality improvement in the treated backwater, but not at other sites. The types of fish in the treated backwater shifted in favor of those found in healthier ecosystems. These findings are useful to land managers and conservation agencies charged with aquatic ecosystem management and protection.

Technical Abstract: The importance of floodplain backwaters within lowland riverine ecosystems is well established. However, these types of habitat are becoming increasingly rare as development is transforming floodplain landscapes in fundamental ways. Therefore, rehabilitation, protection, and management of riverine backwaters (floodplain aquatic habitats that are seasonally or periodically connnected to the main channel) are becoming increasingly common, with annual national expenditures now in the millions of dollars. Even with the increasing number of projects, general criteria for selecting restoration goals and evaluating project outcomes are lacking. To address this need, Kondolf et al. (2006) proposed an approach for evaluating river restorations is based on assigning a position to the system in a four-dimensional space that represents hydrologic temporal variability on one axis and connectivity in the three spatial dimensions on the remaining three axes. Use of the Kondolf approach for evaluating restoration of a backwater adjacent to a medium-sized river in northern Mississippi is presented as a case study, in which nearby degraded and less-impacted backwaters were used as references. The restoration project resulted in a reduction in main-channel connectivity and lower levels of variability for the treated backwater. Additional responses to treatment included increased summer water depth, moderation of severe diurnal water quality fluctations, and reductions in concentrations of solids, nutrients, and chlorophyll a. Fish species richness, numbers and biomass were unchanged following rehabilitation, but tropic structure shifted away from omnivorous species and toward predators. Ecological services provided by floodplain riverine backwaters may be enhanced by modest management measures, but regaining and maintaining connectivity with adjacent ecological functional patches remains difficult.