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Nicholas Pinter
Geology Department
Southern Illionis University, Carbondale
Reuben A. Heine
Environmental Resources and Policy Program
Southern Illinois University, Carbondale
The goals of this research
were to (1) document long-term trends in flow
conveyance on the Lower Missouri River, (2)
begin to assess what mechanisms have caused
these trends, and (3) update flood frequencies
using the new "stage indexing" technique.
Five stations along
the Lower Missouri River were examined using
specific-gage analysis (SGA), which is a technique
that holds discharge constant in order to observe
trends in a parameter such as stage over time. This
analysis reveals that for all flood conditions
on the Lower Missouri River, stages have systematically
risen for equal discharge volumes over the period
of record. SGA can also be used for analyzing
other time-series parameters such as cross-sectional
area, flow velocity, or channel width, which
illustrate the actual mechanisms of channel
change. The results show that at three
Lower Missouri River stations, velocity appeared
to be the dominate mechanism driving shifting
stage changes. At the two other stations
studied, constriction in channel cross-sectional
area appear to have driven increases in flood
stages.
Rising flood stage
trends imply that large floods will occur more
frequently than previously estimated. The
stage indexing technique updates flood frequencies
by incorporating long-term stage trends in order
to homogenize historical stage data to the current
year. Using this technique, estimated frequencies
for all large flood events at all five Lower
Missouri River stations analyzed were increased,
sometimes dramatically. For example at
Boonville, the largest flood occuring in the
70-year record was the 1993 flood with a stage
of 35.16 ft. Without accounting for the
long-term shifts, the 1993 flood would have
a recurrence interval of at least 71 years,
and recurrence times of 100-500 years are commonly
cited. After indexing, however, the 1993
flood drops to just the 4th largest flood by
stage, and the adjusted recurrence interval
for a stage of 35.16 ft drops to 15-20 years
under present-day conditions.
Such profound changes
in flood response should be recognized on the
Missouri River and incorporated into current
estimates of flood hazard and into strategies
for river management and flood mitigation
You can download this
complete report as a pdf file here: Hydrologic
History of the Lower Missouri River
Maps
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