Response of Vegetation and Fish During an Experimental Drawdown in Three Pools, Upper Mississippi River

摘要

An experiment to improve ecological conditions while maintaining a 9- foot (2.74-m) navigation channel was continued in 1995 and 1996 on the three pools of the Upper Mississippi River managed by the St. Louis District, U.S. Army Corps of Engineers. Water levels were held from 1 to 3 feet (0.3 to 0.9 m) lower than maximum regulated elevations at the dam from about mid-June through July in Pools 24 and 25 and Melvin Price Pool. Water levels were then gradually raised as discharge allowed. Vegetation was surveyed along an elevational gradient in eight areas in 1995 and six areas in 1996. Seven plant genera were identified in 1995 and five genera in 1996. Amaranthus spp. (pigweed), Cyperus spp. (chufa), Echinochloa spp. (wild millet), and Polygonum spp. (smartweed) were found in 30-90% of the sites. Plants grew 7-10 inches (17.8- 25.4 cm) in about 30 days and then grew more quickly as water levels were slowly raised. On July 27, 1995, aerial infrared photographs (1:15,000) were taken of the lower stretches of the three pools. Photographs also were taken of Pool 22, where no drawdown occurred. These photographs were interpreted, specifically searching areas near the waterline with signatures representing the above plants. In Pools 24 and 25 and Melvin Price Pool, between 255 and 880 acres (103 and 356 ha) of plants were measured near the waterline, compared to just 51 acres (20.6 ha) in Pool 22 where water levels near the dam were held at the project pool elevation. A geographic information system model was also used to predict areas that would be dewatered under venous water-level management options. Minnow seining data collected on the three pools from 1986 to 1996 were also analyzed. The total numbers of fish per haul and Simpson's Diversity Index showed no significant detrimental effects during years when water levels were held on the low side of the operating band. We believe that this management experiment was successful and that continuation would be beneficialc.