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<br />to the northwest and are overlain by Tertiary and Quaternary System <br />formations in the upper part of the basin. The latter include river <br />deposits, loess, and sand dunes. Loess covers a wide area and often <br />exceeds 100 feet in depth, River valleys have extensive deposits of <br />alluvium of comparatively recent origin. The lower slopes of most <br />of the adjoining bluffs are covered with talus and slope wash. The <br />soils may be classified according to origin as residual, alluvial <br />and loessial. The loessial soils are very productive within the <br />limitation of available moisture. The alluvial soils of the floo~ <br />plains constitute a relatively small part of the basin area but due <br />to their productive capacity they are of major economic importance <br />in the agricultural development of the basin. <br /> <br />. <br /> <br />S. ECONOMIC DEVELOPMENT <br /> <br />~. Population. The population of the Republican River basin <br />has declined from 238,000 persons in 1930 to 172,000 in 1960. This <br />is sttributed largely to migration from the rural areas to urban <br />centers outside of the basin. Presently, about 40 percent of the <br />basin residents live in towns of over 1,000 population, of which <br />the largest is McCook, Nebraska. The population density varies from <br />less than two persons per square mile in Cheyenne County, Colorado, <br />to 18 persons per square mile in Red Willow County, Nebraska, in <br />which the city of McCook is located. <br /> <br />~. Transportation and industry. Five major railroads have <br />lines traversing the Republican River basin: the Chicago, Rock <br />Island and Pacific; the Atchison, Topeka and Santa Fe; the Missouri <br />Pacific; the Chicago, Burlington and Quincy; and the Union Pacific. <br />The principal cities in the basin are located along one or more of <br />these railroads, which have made liberal use of the easy gradients <br />of the main stream valleys. There are several all~weather Federal, <br />State and county highways serving the cities and counties of the <br />basin. The principal industry is the marketing and processing of <br />agricultural products and livestock and supplying the needs of rural <br />communities. The construction and operation of the large reservoirs <br />in the basin has introduced water-associated recreation as an <br />.important influence in the local economy. <br /> <br />~. Land use and irrigation. Large areas in the Republican <br />River basin are arable, but the rainfall is usually insufficient to <br />insure crops without irrigation. The uncertainty of adequate and <br />timely precipitation greatly limits crop diversification, and large <br />areas are planted only to wheat or are devoted to grazing. The <br />Bureau of Realamation estimates that there are 191,400 acres of <br />irrigable land in the basin, which it has subdivided into five divi- <br />sions, as follows: <br /> <br />. <br /> <br />6 <br />