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<br />THE FLOOD OF 1993 <br /> <br />UPPER, LOWER, MIDDLE? <br /> <br />Lending confusion to a discussion of the Mississippi River and its drainage basin is <br />the fact that hydrologists divide the basin, including tributary basins, into two parts: the <br />upper and the lower; and the river into three reaches -- the upper, middle, and lower. <br />Division between the upper basin and lower basin is at Cairo (above the mouth of the Ohio <br />River). For the Mississippi River itself, the reach upstream from St. Louis is called the <br />upper Mississippi River wpper Miss.), the reach between St. Louis and Cairo is the middle <br />Mississippi River (middle Miss.), and the reach downstream from Cairo is called the lower <br />Mississippi River (lower Miss.). <br /> <br />Other major tributaries include the Minnesota, <br />Wisconsiu, Iowa, Des Moines, and Illinois rivers, ail of <br />which drain watersheds greater than 10,000 sq. mi. in <br />area (Figure I.I). <br /> <br />centuries. By 1824 early steamboat travel and <br />commerce created a demand for navigation <br />improvements. Urban and ruraI populations continued <br />to grow, creating an increased demand for forest <br />lumber resources and agricuIruraI products. Most early <br />urban settlements were located on or near rivers to be <br />close to water supplies and transportation arteries. By <br />the late 1800s, settlers had cleared and drained many <br />wetlands for agriculture and planted higher floodplain <br /> <br />The Missouri River, which drains all or part of ten <br />states and 74 percent of the upper Mississippi River <br />Basin, contributes only 42 percent of the long-term <br />average annual flow of the Mississippi River at Sl. <br />Louis. The Missouri River does contribute the most <br />sedimeut in the upper Mississippi River Basin. <br />Hydrologically the Missouri River Basin is divided into <br />upper and lower portions with demarcation at Sioux <br />City, Iowa, The upper and lower basins contain <br />314,600 sq. mi. and 214,700 sq. mi. respectively. <br /> <br />VOLUMES OF WATER <br /> <br />. <br />" <br />When quantifying large volumes <br />of water, a measuring unit as small as a <br />gallon results in numbers in the billions <br />or trillions and makes perception <br />difficult. Water engineers and scientists <br />have adopted a larger unit and, therefore, <br />employ smaller, somewhat more readily <br />envisioned numbers. That unit is the <br />acre-foot and represents the volume of <br />water standing one foot deep over an <br />area of one acre. Thus the mean annual <br />volume of water that runs off the upper <br />Missouri River Basin can be expressed as <br />25 million acre-feet rather than <br />8,145,720,000,000 gallons. <br /> <br />Runoff from the upper basin is coutrolled in great <br />measure by regulation of six large dam and reservoir <br />projects on the main stem Missouri River operated by <br />the U.S, Army Corps of Engineers (USACE). The <br />drainage area (279,400 sq. mi.) above Gavins Point <br />Dam, the dam furthest downstream, eucompasses about <br />90 percent of the upper Missouri River BasIn and over <br />50 percent of the total Missouri River Basin area. The <br />amount of water that runs off the upper basin annually <br />averages 24.6 million acre-feel. I <br /> <br />History of Development <br /> <br />The upper Mississippi River valley was settled by <br />European immigrants during the 18th and 19th <br /> <br />4 <br /> <br /> <br />-f:- <br />, <br /> <br />t <br />,. <br />Jt <br /> <br />'l <br /> <br />~:- <br />'f <br />. <br />.>. <br /> <br />l <br />'J~ <br />, <br />-;~ <br /> <br />'{ <br />