OCR Text |
Show and economic development differ greatly within the basin. Yet extensive areas are monotonously similar. Most of the Missouri Basin is plains country, a large part originally grassland. Gently undulat- ing in places, elsewhere tablelike over great dis- tances, tbie plains stretch nearly 800 miles from the Canadian line to the southern margin of the basin, and from the Rockies to the Mississippi River. (See figure 3.) Here and there the plains are interrupted by ribbons of hill lands which have been dissected along the borders of the major stream courses. Several areas within the Missouri Basin are znotably different, among them the Rocky Mountains, Big Horn Mountains, the Sand Hills region of northwestern Nebraska, the Black Hills of southwestern South Dakota, and the Ozarks of southern Nfissouri. The Black Hills and Sand Hills may be consid- ered major interruptions of the plains surface. The Continental Divide, however, terminates the plains everywhere on the western side of the basin, where the Rockies rise sharply as a major moun- tain range. Many peaks and ridges approach 14,- 000 feet afcove sea level, but even these mountains are dwarfed by the area of the vast plains to the east. In its natural or unmodified state the Missouri River Basin belonged in large measure to America's grasslands. Tall grasses prevailed in the eastern part of the basin and short grasses were common in the drier western portions. The nature of the grass reflected in a general way the amount of precipitation, which declines from east to west and to some ezxtent from south to north. The vital and pervading characteristic of these dominating grasslands is their water problem. There a:re humid lands in the basin, particularly in Nebraska, Iowa, and Missouri, where 30 to 45 inches of rainfall can be expected in a year. But precipitation everywhere decreases westward, drop- ping to a Jow of 5 inches in the Big Horn Basin of Wyomixig near the Rockies, which again are humid with from 20 to 45 inches of rainfall annually. Most of the basin is semiarid. (See figure 1.) To the once migratory buffalo and the Indians who depended on them these local variations caused little hardsliip because large areas could be searched for grass. The white man has settled down to develop an intensive agriculture within a given area. In this pur-suit, he has found the limited moisture supply insufficient to meet his regular and con- 162 tinuing needs. Consequently, conservation of the available water supply is essential. The most significant feature of the physical en- vironment for land use is the great degree of varia- tion in precipitation from year to year. In some years, or for a series of years, the plains rainfall will be far above normal. Inevitably such periods have been followed by years when rainfall is far below normal. The period from 1930 to 1950 has shown the full range of these cyclical variations. In almost every year from 1930 to 1940 rainfall was well below average. During this period, dry farm crops failed year after year for lack of moisture. Some hay lands and range lands failed to produce enough forage to sustain even a small fraction of the herds which had been carried by these lands during the previous "wet" period. The 1940's saw a complete reversal of these conditions. Every year during the 1940's was above normal in pre- cipitation with attendant high agricultural pro- duction. The geographical variation in yearly precipita- tion also affects land use. Some parts of the west- ern plains experience droughts almost every year while nearby areas have adequate rainfall. Average rainfall figures may be misleading. Over much of the western section of the plains area the annual rainfall is so low-10 or 12 inches-that production of unirrigated crops seems impossible. But well over half of all the rainfall received comes during the three early summer months, at a time when days are long and when the rain is most use- ful to plants. Consequently, grain can be raised with dry farming methods in areas of extraordi- narily low annual rainfall. But even considering this peculiarity, much of the basin can be stricken with crop failure in drought years. Some Features of Settlement Many of the cultural features of the basin cor- respond remarkably to climatic characteristics. For instance, well over half the population of the basin is concentrated along its humid eastern border. Here are the most dependable farm lands. Here also are the principal manufacturing industries. Population density declines consistently toward the West. The dry western part contains only 4 per- cent of the basin's 7,000,000 people. Here, how- ever, are such well-known cities as Denver, Lara- mie, Cheyenne, Rapid City, Great Falls, and Helena. |