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-2- I I I I I I I I I I I I I I I I I I I punctuated with many high. valleys, but peaks and mountain spurs dominate the physical features. Extending eastward from the Rocky Mountains are the interior plains, which in general have a flat to gently rolling topography. The plains cover about 2~2 million acres and generally slope from west to east. Two prominent land forms within the interior plains are the Black Hills of western South Dakota and northeastern Wyoming and the Sand Hills area of north-central Nebraska. In the southeastern part of the basin is a 7-million-acre area of hilly to mountainous land called the interior or Ozark Highlands. Climate--primarily because of its midcontinent location, the basin experiences weather that is known for fluctuations and extremes. The weather tends to fluctuate widely around the annual averages with, unpredictable occurrence and degree of fluctuations. Normal annual precipitation varies from west to east in the basin, averaging over 32 inches in the Rocky Mountains on the western boundary, about 17 inches on the interior plains, and over 36 inches in the Ozark Highlands. About 70 percent of the precipitation occurs as rainfall during the growing season. Human Population--The population of the basin in 1~75 was estimated at ~ million or about 4.3 percent of the national total. Although the basin's population has slowly increased since 1~40, its proportion of the U.S. population has declined. All metropolitan areas in the basin have experienced large population increases since the 1930's. An important ethnic group in the basin is the native American Indian. The estimated total population of the basin's 23 reservations in 1975 was 65,400. Econo.y--Since 1862, when the Federal Homestead Act opened land in the Missouri River Basin to agriculture, the basin has been an extremely important