Shallow and deep water aquatic vegetation potential for a midlatitude pool of the Upper Mississippi River System with drawdown

Prior to navigation dam and levee placement, the Upper Mississippi River (UMR) flowed through a wide floodplain supporting a diverse ecosystem. Diversity was created by variable flood frequencies and water flow, but presently high and static water levels supporting river navigation have caused low diversity of aquatic vegetation in locations within the UMR. A pool-scale water level drawdown was proposed as a wetland management tool to mimic historic low water flow for UMR Navigation Pool 18, between Oquawka and Keithsburg, IL. The objectives of this research are to determine plant species, density, and diversity expected for a drawdown in Pool 18. A seedbank and propagule assay was used to evaluate drawdown plant species response. Emergence was tested using river bottom substrate samples collected in 2009 from the proposed drawdown area. Samples were treated at two hydrologic levels: shallow (3-cm depth) and deep (16-cm depth). Dominant species in the shallow flooded treatment were Gratiola neglecta, Leersia oryzoides, Eleocharis palustris, Sagittaria latifolia, and Ammania coccinea. Deep flooded dominant taxa included G.neglecta, S.latifolia, Vallisneria americana, and A.coccinea. Each treatment indicated a seedbank of moderate diversity with a shallow treatment diversity of D=0.56 and deep treatment diversity of D=0.44. Plant density for the shallow flooded treatment was 213 stems/m(2) (+/- 112; 95% CI), and deep flooded hydrologic treatment, 206 stems/m(2) (+/- 82; 95% CI). It is expected that this drawdown will provide an intermediate ecological disturbance resulting in greater species diversity and density currently lacking in this portion of the Upper Mississippi River System.