Effects of Variable Annual Hydrology on a River Regulated for Navigation: Pool 26, Upper Mississippi River System

Abstract

Abstract We investigated impacts of water level regulation for navigation on water quality, plant distribution and fish communities in Pool 26 of the upper Mississippi River during an extreme drought and following a return to typical flows. We also compared biological responses in three hydrologically distinct pool reaches. The third worst drought in 53 years occurred during 1989 and resulted in dam operations that created stable water surface elevations and low current velocities through the year. These conditions resulted in high water clarity that allowed development of submersed and floating-leaved aquatic plants in channel borders and backwaters; fish communities were dominated by centrarchids (lentic species). The drought was followed by two years of moderately high flow. Higher discharge resulted in lower water clarity and higher current velocity. When combined with drawdowns, such conditions limited submersed aquatic plant production and promoted mesic plant species. Centrarchid abundance declined, with the lower reach showing severe declines (from 95% of the catch in 1989 to 15% in 1991).