How did a large part of the Central Lowlands turn into a human-made desert?
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Answer:
The Central Lowland is a flat-lying region located between the Appalachian Mountains to the east and the Great Plains to the west (Figure 4.5). It extends from the Canadian Shield in the north to the Atlantic Coastal Plain in the south and is part of the North American craton (the older, stable part of the continent).
The Minnesota River lies entirely within the Central Lowland (CL) physiographic province and almost entirely within the Central Tall Grasslands terrestrial ecoregion. A small portion near the confluence with the Upper Mississippi lies within the Upper Midwest Forest/Savanna Transition zone (Ricketts et al. 1999). The basin has been greatly influenced by glaciation, as demonstrated by the identification of 27 different geomorphic sections based on glacially formed features. Soils in the basin are primarily Wisconsin Drift overlying igneous and metamorphic rock or poorly consolidated shales and sandstones. The poor drainage of soils in some regions, especially the upper end of the river, historically created broad areas of wetlands and lakes. Characteristic of the mesic conditions of the Central Tall Grasslands, dominant vegetation in much of the basin prior to settlement were tallgrasses, primarily big bluestem, switchgrass, and Indian grass. Vegetation in the eastern one-third of the basin consisted of oak–prairie mix and forests of oak, maple, basswood, ash, and elm. Only remnants of presettlement vegetation can be found today, with much of this limited to the river valley.
Average annual precipitation varies from 60 cm/yr in the west to 76 cm/yr near the mouth. Precipitation is primarily rain, occurring in the greatest amount during the summer (Fig. 8.14). Precipitation is lowest in the winter, falling as snow. The major climatic event in the basin is in the spring, when rain falls over the melting snow and thawing ground. Average snowfall in the basin is 117 cm/yr (as snow). Mean annual temperature is about 7.5°C, with an average monthly range of −11°C to 23°C
About 95% of the basin is used for agriculture, with corn and soybeans as the major crops. Wheat production is also important in the basin, as is hog and cattle production. Less than 1% of the basin is urban, with Mankato and part of the Minneapolis–St. Paul metropolitan area as the major urban areas on the river. The remainder of the basin is an agriculture/forest mix, although a few areas of wetland, prairie, oak forest, and prairie-woodland persist, largely in the river valley and riparian zone.