1997 Annual Status Report Status and Trend of Submersed and Floating-leaved Aquatic Vegetation in Thirty-two Backwaters in Pools 4, 8, 13, and 26 and La Grange Pool of the Upper Mississippi River System

Thirty-two backwaters of the Upper Mississippi and Illinois Rivers were monitored for the seventh consecutive year in 1997 to determine the status and trend of changes of submersed and floating-leaved aquatic vegetation. Aquatic vegetation was sampled at regularly spaced Sites along permanent transects established in previous years. Species compositions, frequencies of individual species, and the frequencies of sites that supported aquatic vegetation in 1997 were calculated and compared with results from previous years. The status and trend of aquatic vegetation in 1997 varied among the thirty-two backwaters. In general, the frequencies of sites that supported aquatic vegetation in 1997 in the backwaters of Pools 8 and 13 were at or near their highest levels recorded since 1991. In contrast, the frequencies of sites that supported aquatic vegetation in 1997 in the backwaters of Pools 4 were at or near their lowest levels recorded since 1991, despite four of the eight backwaters being better off compared with 1996. Aquatic vegetation was rare in Pool 26 and La Grange Pool in the l990s. The monitored backwaters of Pool 26 and La Grange Pool represent the few places where sizable aquatic vegetation beds were found in their respective river stretches when the transects were established in 1991 and 1992. In 1997, aquatic vegetation was present in two of the four backwaters of Pool 26. The other two backwaters of Pool 26 have not supported aquatic vegetation since after a long-duration flood in 1993. Aquatic vegetation was present in two of the four backwaters of La Grange Pool, but disappeared completely from the third backwater in 1997 and from the fourth (a channel border area) in 1995.