From Encyclopedia of the Great Plains:
This leaves the eastern boundary of the Great Plains, which is not a sharply defined line but an almost imperceptible transition zone from the more humid South and Midwest. The difficulty in identifying the eastern entry onto the Plains was described by Robert Pirsig as he rode his motorcycle west from Minnesota into North Dakota. "There is no one place or sharp line where the Central Plains [i.e., the Midwest] end and the Great Plains begin," observed Pirsig. "It's a gradual change like this that catches you unawares, as if you were sailing out from a choppy coastal harbor, noticed that the waves had taken on a deep swell, and turned back to see that you were out of sight of land." The key landscape evidence for Pirsig was that there were fewer trees on the Great Plains and those that were there had been introduced. The "greenness" encountered farther east had also paled, the streets of the towns were wider, the buildings more run-down. Pirsig concluded that there was less concern with "tidily conserving space" on the wide-open Great Plains.
To compensate for this geographical nebulousness, Plains scholars have sought to define the eastern margin by an arbitrary line, generally the 98th meridian, less frequently the 100th meridian. Perhaps a better definition of the eastern boundary would use a combination of physical, historical, and geopolitical factors. Our boundary follows the eastern border of the states of North Dakota, South Dakota, Nebraska, and Kansas, including these entire units in the region. These states were organized and settled later than the adjoining states to the east, and their institutions and iconographies give them a coherence that should not be divided.
http://plainshumanities.unl.edu/encyclopedia/intro.html
