Management of water surface elevations at 27 dams affects much of
the floodplain habitat in a 653-mi stretch of the Upper
Mississippi River (UMR) between St. Louis, Missouri, and
Minneapolis, Minnesota. In 1930, Congress authorized most of
these dams to maintain a 9-foot navigation channel. This report
provides a systemic overview of current operating plans at dams
on the UMR and analyzes historical data to determine how well the
operating plans are being met.
Water level elevations at all 27 dams on the UMR are regulated as
a function of discharge, although plans are specific for each
dam. The U.S. Army Corps of Engineers strives to maintain a
target water level at a specific location in a pool (control
point) over a specific range of discharges. In any particular
pool, the target water level and control point may change as the
discharge changes. Control points can be located anywhere within
a pool, but they are usually near the mid-point of the pool or at
the pool's downriver end. Difficulties in meeting targets are
caused by winds, local rainfall events, ice dams, and rapidly
fluctuating discharges from tributaries having upstream
reservoirs used for peaking hydropower.
Water level measurements and discharge estimates were used to
assess the amount of time water levels were near their targets
during the navigation seasons of 1980 through 1990, inclusive.
Water levels were within their target window an average of 72.5%
of the time for 25 dams analyzed. When target windows were
expanded by �0.5 ft, water levels were in the target window 99.0%
of the time. Yearly water level fluctuations were found to be
caused mostly by discharge and were usually less after dam
construction. Yearly and daily mean water level fluctuations
varied with location within a pool, generally decreasing
downriver.
