Aquatic vegetation responses to structural and nonstructural restoration methods in the upper Mississippi River.
Yin, Yao*,, Langrher, Heidi,
ABSTRACT- The establishment of a nine-foot navigation system within the Upper Mississippi River (UMR) during the early twentieth century created expansive backwater areas that were quickly colonized by aquatic vegetation. After approximately four decades, the effects of permanent impoundment of the river began to reduce the distribution of aquatic plants. Biologists and environmental engineers have used both structural and nonstructural methods to promote aquatic vegetation growth to restore/enhance fish and wildlife habitats in Navigation Pool No. 8, UMR, near La Crosse, Wisconsin. The structural method consisted of construction of sand and rock islands. The nonstructural method consisted of a water level draw-down in two consecutive summers. We analyzed the aquatic vegetation data collected from 1998 to 2003 through the Long Term Resource Monitoring Program, an element of the Environmental Management Program of the Upper Mississippi River System administered by the U.S. Army Corps of Engineers, to differentiate the effects of each method. Our analyses reveal that both methods triggered immediate positive responses by submersed, emergent, and rooted floating-leaf plant species. The strength of the structural method is that it is very effective in establishing abundant growth of many plant species. One potential weakness is the permanency of the constructed structures. In our case, the structure triggered an explosion of Nelumbo lutea (Willd.) Pers., a highly invasive species that is difficult to control and that could potentially exclude other desirable wildlife food species from the project area. In comparison, the water-level draw-down method does not establish any permanent structures and could be applied repeatedly. However, it triggered moderate responses that may disappear very quickly.
Key words: draw-down, aquatic vegetation, habitat restoration, artificial island
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