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and the Black Hills of western South Dakota. Slopes and stream gradients generally are steep, and natural erosion, while slow in most places with heavy vegetative cover and boulder-armored channels, is rapid where soft rocks are exposed. However, the sediment contributed to streams is not excessive. ]n the north-central part of the Great Plains and in the large inter-mountain valleys of Wyoming are found the basin's "rough lands and badlands." These are vast and rather high areas where the bedrocks consist largely of silstone, soft shale, clay, and soft and hard sand- stones. Natural vegetation is relatively thin, offering little protection from erosion, which is extremely active. Soils generally are shallow, but with some local exceptions. The "hilly lands" appear intermittently from the bluffs of the Missouri River in eastern Missouri and Iowa to the foothills of the Rocky Mountains. Here, the soils range from shallow to deep, but most are capable of supporting a good natural growth of grasses or trees. Erosion has become very rapid where such lands are cultivated without proper conservation management. Much of the hilly land is grazed, but substantial areas in the eastern part of the basin are used for clean-tilled crops. ]n the wooded Ozark Plateau in Missouri south of the Missouri River and in the grassy Flint Hills of Kansas are found the "hilly to mountainous lands." Their parent material is largely hard sedimentary bedrock and the soils range from very shallow to quite deep. "Moderately sloping lands" are found mostly in areas of soft rocks, glacial till, and loess interspersed with a few reaches of hard rocks. Soils range from moderately deep to very deep, with some shallow soils and rock outcrop. Most of these llrellS originally supported tall grasses in the eastern parts and short grasses in the dryer western parts, and natural erosion was moderate. Today, a large share of the land is used for cultivated crops, especially in the central and eastern parts of the basin. "]ntermountain valleys" are the areas of the Rocky Mountains in which significant farming occurs. Silty, clayey, or sandy sediments accumulated more rapidly than the streams were able to remove them, and these sediments have formed moderately deep to deep soils which generally show only moderate erosion. T he "hummocky (dune) lands" include the 24,OOO-square-mile Sandhills area of north-central Nebraska and similar smaller areas in other states. These lands consist of hummocky reaches of loose dune sand currently stabilized by grasses. There are thousands of small basins, some of them semi-marshy and others dry, with many small lakes generally representing exposures of the ground-water table. Soils are generally thin, though the loose-sand deposits are rather thick. An Aeo]ian topography together with the sandy soils produce very little direct runoff to streamflow, but IO streams in the area accumulate unusually steady flows, mostly from ground-water accretions. The streams carry much .>and sediment derived from their channels and limited tributary drainage areas. "Level and undulating glacial drift lands" are under- lain by alluvium, loess, and glacial drift of greatly varying character. The soils are generally deep to very deep and natural erosion is quite slow. Water erosion is active at higher levels, but the soils washed from the knolls usually collect in intervening basins. CLIMATE The basin's climate has a great influence on how its people live and on their socio-economic structure. Obviously, it has a strong influence on the basic agri- cultural industry, due primarily to the seasonal and variable regimen of precipitation, temperature, and wind. The climate within the basin is determined largely by the interactions of three great air masses which have their origins over the Gulf of Mexico, the northern Pacific Ocean, and the northern polar regions. They regularly invade and pass over the basin throughout the year with the gulf air tending to dominate the weather in summer and the polar air dominating in winter. It is this seasonal domination of the air masses and the frontal activity caused by their colliding with each other which produces the general weather regimens found within the basin. Primarily because of its mid-continental location, the basin experiences weather that is known for its fluctu- ations and extremes. There are also variations between areas within the basin. Winters are relatively long and cold over much of the hasin, while summers are fair and hot. Spring is cool, humid, and windy; autumn is cool, dry, and fair. Averages are misleading for seldom does "average" weather actually occur. ]nstead, weather tends to fluctuate widely around the annual averages, with the occurrence and the degree of the fluctuations being unpredictable; climatic averages have to be thought of as generalizations of the more common occurrences over a period of time. The one-year minimum precipitation for the entire basin is 70 percent of the long-term average, but one-year minima for the individual subbasins range from 46 to 66 percent of average. Table 1 shows both the minimum annual and 5-year averages of precipitation over the basin and within the subbasins in com parison to the long-term averages. Normal average annual precipitation ranges from about 40 inches in parts of the Rocky Mountains and southeastern parts of the basin to as low as 6 to J 2 inches immediately east of the Rocky Mountains. Variation of the average annual precipitation pattern over the basin is shown on figure 5.