Rivers are easily identified in dryer areas by trees and scrub that grow along their banks, especially here on the Great Plains. The Great Plains are pretty much defined by not having trees, because the almost constant wind and regular brush fires kept trees from growing even before this land was converted to farms. But the trees would grow near rivers, where adequate water made holding on easier.
While corn had become the favorite grain of the early colonists, newcomers spreading across the prairie in the mid 1800s wanted wheat. Wheat was what rich people ate. Wheat needed good soil to grow well. In Europe, the poor lived on marginal lands, where rye, oats, and millet would survive, but not wheat. So wheat became part of the mix on the Great Plains. It never came close to competing with corn, as far as acreage and importance, but still it has a significant presence. The gold of the ripening wheat is beautiful, especially in the wind, which makes it ripple like the sea.
Other crops in the area include milo (grain sorghum), hay, and alfalfa. But still, corn is king. (And if you’d like to know more about sorghum, I did a post on it on my World’s Fare site: https://worldsfare.wordpress.com/2014/12/01/sorghum/.)