EFFECTS OF VARIABLE ANNUAL HYDROLOGY ON A RIVER REGULATED FOR NAVIGATION - POOL 26, UPPER MISSISSIPPI RIVER SYSTEM

We investigated impacts of water level regulation for navigation on wa ter quality, plant distribution and fish communities in Pool 26 of the upper Mississippi River during an extreme drought and following a ret urn to typical flows. We also compared biological responses in three h ydrologically distinct pool reaches. The third worst drought in 53 yea rs occurred during 1989 and resulted in dam operations that created st able water surface elevations and low current velocities through the y ear. These conditions resulted in high water clarity that allowed deve lopment of submersed and floating-leaved aquatic plants in channel bor ders and backwaters; fish communities were dominated by centrarchids ( lentic species). The drought was followed by two years of moderately h igh flow. Higher discharge resulted in lower water clarity and higher current velocity. When combined with drawdowns, such conditions limite d submersed aquatic plant production and promoted mesic plant species. Centrarchid abundance declined, with the lower reach showing severe d eclines (from 95 % of the catch in 1989 to 15 % in 1991).