Analysis of water levels and discharge

Analysis of water level elevations and discharges was performed
for Pool 25 on the Upper Mississippi River as part of a study to
increase ecological benefits of the operation of Lock and Dam 25. 
This dam was placed in operation in May 1939.  Since that time,
the operating plan for managing water levels has undergone several
changes.  Discharges for Pool 25 were estimated using data from
the closest gaging stations, with corrections accounting for
changes in watershed area.  Discharge estimates since 1939 were
used to compute exceedence frequencies on a weekly, monthly, and
yearly basis.  They were also used to calculate the percentage of
time discharges were within specified ranges for the three time
scales.  Water level measurements at five in-pool stations were
used to calculate the percentage of time water levels were above
specific elevations for the same three time scales.  Only data for
the period between 1952 and 1993 were used for this part of the
analysis because different water level management plans were in
use before 1952.  The results of this work, along with a companion
study, show the expected frequencies when alternative water level
plans may be utilized and the amount of habitat that would be
affected by alternative plans.