Great Plains and Prairies - Environment
The topography and vegetation of the grasslands is among the least varied to be found anywhere in the United States. Early settlers following the Oregon Trail could reach the Pacific coast in one season of travel, in part because the grasslands were so easy to cross. The region lies entirely within the interior lowlands physiographic province. The underlying sedimentary beds dip gently. Elevation increases gradually, almost imperceptibly, from east to west. Along the eastern margin, the elevation is only 500 meters, whereas in the west, Denver, Colorado, claims an altitude of more than 1,500 meters.
Physiographically, the largest portion of the Great Plains is the High Plains stretching along the western margin of the region from south Texas northward to southern Nebraska. Covered by a thick mantle of sediments that are often quite sandy and extremely porous, this section is generally flat. Only along streams such as at Scottsbluff on the Platte River in western Nebraska or at Palo Duro canyon on the Red River in northwest Texas has erosion resulted in substantial local relief. The Lake Agassiz Basin, formerly occupied by the largest of the Pleistocene lakes, is another exceptionally flat area and includes the valley of the Red River of the North in North Dakota and Minnesota.
Not all portions of the region are so unvarying topographically. The most obvious exception are the Black Hills of South Dakota and Wyoming. A large, dome-shaped area of eroded igneous rock, the Black Hills are associated both geologically and topographically with the Rocky Mountains to the west. In southern Texas, the Edwards Plateau is heavily eroded into a canyonlands landscape along its southeastern margin where it is adjacent to the coastal plain.
In central and northwestern Nebraska, the Sand Hills offer a dense, intricate pattern of grass-covered sand dunes, many of which are well over 30 meters high. The dunes were created by sand blowing along the southern margins of continental glaciers during the Pleistocene. Badlands topography--extremely irregular features resulting from wind and water erosion of sedimentary rock--is widespread on the unglaciated Missouri Plateau from northern Nebraska northward to the Missouri River. North of the Missouri River and west of the Lake Agassiz Basin, the glaciated Missouri Plateau, although sometimes flat, is covered with ponds, moraines, and other glacial features.
Although agriculture has destroyed much of the original grasslands vegetation, the moister eastern portions (areas with more than 60 centimeters of annual precipitation in the north or more than 90 centimeters in the south) were originally a continuous tall-grass prairie, where grasses grew between 30 centimeters and 1 meter in height. Along the western margins of the Plains, prairie grasses gave way to bunch grasses--shorter, more separated grasses could succeed in the semiarid conditions of the western Plains.
The prairie grasses have developed deep, intricate root systems that commonly extend much deeper into the soil than the grass blades reach above, allowing them to utilize available water. The tangled root system made the prairies exceptionally difficult to plow. The first settlers often had to employ heavy plows pulled by as many as 20 animals to break the sod. The prairie sod could also be "cut" into large bricks used in the construction of sod houses during the early period of Plains' settlement by Europeans.
The warm, moist tropical maritime air flowing in from the Gulf of Mexico, the prime contributor of moisture to the Plains, commonly curves up the Mississippi Valley and then moves northeast, missing much of the western Great Plains entirely. One result of this pattern is the marked westward decline in average precipitation amounts. In Kansas, for example, average annual precipitation varies from a moist 105 centimeters in the southeast to a semiarid 40 centimeters in the southwest.
Periods of higher than normal precipitation on the Great Plains result when tropical air masses move northwestward from the Gulf of Mexico, which brings these air masses over portions of the Plains. This provident current is far from dependable, however. Fortunately for the Plains farmer, about three-quarters of the precipitation falls during the period of more rapid crop growth, from April to August.
Some of the region's spring and summer precipitation comes in the form of violent thunderstorms. Hail is occasionally a product of these storms. These frozen pellets, sometimes measuring more than 5 centimeters in diameter, have the power to devastate a crop of mature, top-heavy wheat. Much of the southern and west-central Plains experiences frequent hail storms, with parts of western Nebraska and southeastern Wyoming leading the continent in average annual hail frequency.
Tornadoes, which can have funnel wind speeds in excess of 350 kilometers per hour, are another violent result of these storm systems of the Great Plains. Although the area affected by any one funnel is small, the frequent occurrence on the central Plains makes tornadoes a significant regional hazard.
Wind has been a mixed blessing on the Great Plains. Late spring and summer wind velocities on the central and southern Plains are among the highest in interior America. In the past, this served to maximize the efficiency of windmills in the region. However, the persistent winds also mean that the amount of moisture evaporated and transpired by plants is high across much of the region.
The chinook, a winter wind, occurs when dry, relatively warm air from the Pacific Coast pushes over the Rocky Mountains. As it descends onto the Great Plains, it warms still further and is much warmer than the cold, continental air mass commonly found over the region in winter. The Pacific air temporarily pushes the cold air from the western Plains, and a rapid, dramatic temperature rise results. Partly because of this interesting phenomenon, winter temperatures along the higher western area are slightly warmer than along the eastern edge of the Plains. In addition, the length of the frost-free season varies widely around the average from year to year. As with annual temperature range, the variation increases as one moves northward.
Snow, wind, and cold are all part of one of the most devastating weather elements on the Plains: the blizzard. A blizzard occurs in winter when a very cold polar air mass pushes southward along the Rocky Mountains and onto the Plains, breaking the usual west to east storm pattern. High winds, intense cold, and considerable amounts of snow are associated with these storms. A blizzard can last for several days and bring half of the average winter snowfall. Because Plains ranchers usually leave their livestock outdoors during the winter, a severe blizzard may block an animal's access to food and result in high animal mortality.
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