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I I I I I I I I I I I I I I I I I I I ,QOO,~M CHAPTER 8 - MIDDLE MISSOURI SUBBASIN SUBBASIN DESCRIPTION The Middle Missouri Subbasin covers an area of 14.8 million acres in the east-central portion of the Missouri River Basin. As shown in figure 8-1, it includes lands drained by all of the direct tributaries of the Missouri River from Sioux City, Iowa, to Kansas City, Missouri, except Nebraska's Platte River. Principal among these are the Little Sioux River draining the northern portion southwestward, the Nishnabotna River flowing southward from the center of the subbasin, and the Nemaha River flowing southward along the eastern boundary. The topography in general is a rolling, irregular plain. Along the main stem of the Missouri River and its principal tributaries are flat bottomlands with bluffs often having several hundred feet of local relief. The elevation of the Missouri River varies from 1,220 feet (mean sea level) at Sioux City, Iowa, to 750 feet at Kansas City, Missouri. The subbasin drains generally north to south and extends about 325 miles north-south, and 175 miles east-west. Average annual precipitation varies from 26 inches in the northwest to 36 inches in the south but annual precipitation often varies greatly from those averages. July is normally the subbasin's warmest month and January the coldest. Mean daily maximum temperatures in January range from 26 degrees in the north to 40 degrees in the south, and in July, range from 86 to 92 degrees. The average frost-free period varies from approximately 150 days in the north to over 180 days in the extreme south. Although most lands in this subbasin have been either agriculturally or industrially developed, native ecosystems such as eastern deciduous forests or grasslands persist in areas where dominant natural features have inhibited 8-1