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<br />River Flow <br /> <br />The deluge across the upper Mississippi River Basin <br />produced record setting peak flowrates and water levels <br />in many tributaries and in the main stem rivers, <br />including a large reach of the upper Mississippi, over <br />the full reach of the middle Mississippi, and over much <br />of the length of the lower Missouri River. Flooding <br />began in the northern portion of the upper Mississippi <br />River Basin in June and then moved southward with the <br />shifting of the storm-producing weather pattern and the <br />travel of the flood flows downstream as summer <br />progressed. <br /> <br />Rainfall was particularly heavy between June 17 and 20 <br />in southwest Minnesota and northwest Iowa, causing <br />record flooding on the Minnesota River. The next <br />major pulse of precipitation occurred from June 23-25. <br />Runoff from these rains combined with flood flows <br />from the Minnesota River to initiate the first flood crest <br />that moved down the upper Mississippi River. <br /> <br />Following a short, dry period, a prolonged siege of <br />heavy precipitation occurred from June 30 to July II. <br />This included extreme amounts of rainfall on July 9 in <br />Iowa, which produced record flooding on the Raccoon <br />and Des Moines rivers. Just as the crests from these <br />two rivers reached Des Moines, a relatively small, <br />convective pocket dumped several inches of rain on the <br />crests rapidly boosting the river levels and flooding the <br />city's water treatment plant. The intense rainfall during <br />this period also led to record flooding on pottions of the <br />lower Missouri River and combined with the crest <br />already rolling down the Mississippi to establish record <br />river stages from the Quad Cities area on the upper <br />Mississippi River downstream to Thebes, Illinois, on <br />the middle Mississippi River. <br /> <br />Another major precipitation event occurred from July <br />21-25. The heaviest rains were focused farther south <br />than the earlier events, with especially heavy rain <br />falling over eastern Nebraska and Kansas, leading to the <br />second major crests on both the Missouri and <br />Mississippi rivers. Hydrographs of river stages <br />(elevations) over time for the Missouri River at Kansas <br />City and the upper Mississippi River at the Quad Cities <br />are shown in Figure 1.5. <br /> <br />12 <br /> <br />J, <br /> <br />THE FLOOD OF 1993 <br /> <br /> <br />The Kansas City graph shows two flood peaks, one <br />caused by the June 30 to July II rainfall and the other <br />by rain falling from July 21-25. The Quad Cities graph <br />shows only the single peak from the earlier period. <br />This comparison demonstrates the generally southern <br />focus of this second event. Both peaks are evident on <br />the hydro graph for the Mississippi River at St. Louis <br />(Figure l. 5). While flooding from the latter rainfall <br />period did not extend as far upstream on the Mississippi <br />River, new record river levels occurred at many <br />locations downstream and on much of that portion of <br />the Missouri River that flows through Missouri. Figure <br />1. 6 shows those reaches of main stem and tributary <br />rivers where peak stages exceeded previous record <br />levels and where they reached unusually high but not <br />rcecord levels. <br /> <br />'.~ ' <br />'.~.. <br />.'~ <br />1 <br /> <br />Above normal rains continued to occur over parts of the <br />flood-affected region during August, especially over <br />Iowa where accumulations were twice the normal <br />monthly amount over much of the state. By mid- <br />September, however, rainfall began to diminish and <br />rivers began to recede. Then, at the end of September, <br />a strong system of thunderstonns deposited I to 3 <br />inches of rain over the State of Missouri and 7 inches <br />or more from the central part of the state eastward. <br />The consequence was major flash flooding on many <br />tributaries and new flood crests on the lower Missouri <br />and middle Mississippi rivers, Farmlands behii1(1 <br />previously breached levees were reflooded and two <br />people drowned in separate incidents. Many roads <br />were washed out and there was much damage to <br />property in Missouri. <br /> <br />~ <br /> <br />i <br /><of <br />~ <br />4- <br />.~ <br /> <br />Conclusion: Wet antecedent soil and <br />swollen river conditions, record rainfall, and <br />significant upland runoff resulted in 1993 <br />flood flows that ranged from below the 100- <br />year up to the SOO-year recurrence interval <br />magnitude at many locations. <br />