Pool 13 drawdown: Predicting success rates and affected areas

The likelihood of 1- or 2-foot drawdowns, and the area affected by such alternative
drawdowns, was estimated for Pool 13 on the Upper Mississippi River. Minimum water
surface (elevation) requirements were compared to computed water surface profiles to
determine a critical low flow that would allow a navigation channel 400 feet wide and
10.5 feet deep. An upper limit on flow was established based on the flow at which open
river conditions would exist for a given drawdown. The range in flows that would allow
for a drawdown was used to estimate success rates using historical daily discharge data.
Success rates were determined for a variety of drawdown durations between two time
periods, May 1 August 15 and June 15 August 15. The greatest effect of the drawdown
would be near Lock and Dam 13. Higher discharges, as well as distance from the dam,
lessen the drawdown amount. Areas that would be affected by these two drawdown
scenarios were predicted by overlaying maps of water surfaces with depths using a
geographic information system. Although the drawdown effects on most physical and
biotic components of Pool 13 are unknown, some general drawdown effects likely to
occur because of the changes in water surface elevation are presented.