Changes in backwater plant communities from 1975 to 1995 in Navigation Pool 8, Upper Mississippi River

Water elevation in the Upper Mississippi River (UMR) is highly regulated by an extensive system of locks and darns. Completion of this system in the 1 930s created productive, biologically diverse backwater habitats. The statu s of plant communities in these backwater areas may now be threatened by se veral factors, including sediment accumulation, recreational use, and navig ation traffic. Aerial photography. taken in 1975 and from 1991 to 1995, was used to describe vegetation changes occurring in four UMR backwater areas of Navigation Pool 8. The objectives were to determine (1) if changes occur ring in these areas are consistent with hydrarch succession, (2) if the div ersity of their plant communities has declined since 1975, and (3) how a la rge flood event that occurred in 1993 affected the composition and diversit y of plant communities in these areas. Three general cover classes were rec ognized. representing an aquatic to terrestrial gradient. Coverages of spec ific vegetation types were estimated and evaluated using two indices of com munity diversity (vegetation richness and the Shannon diversity index). Tho ugh some vegetation changes were consistent with expected successional patt erns (e.g. increased terrestrialization). other changes were not (e.g. loss of marsh vegetation). Diversity indices and coverages of most aquatic macr ophytes declined from 1975 to 1991/1992 but then increased following the 19 93 flood. The results suggest that disturbance-diversity concepts, includin g the flood pulse model, are applicable to the vegetation dynamics of these systems. Published in 2001 by John Wiley & Sons, Ltd.