The rolling acres of grassland stretching across the center of the United States are a classic American image. Early European settlers of this eco-region were so impressed by these endless grasslands that they compared them to the ocean, and named their wagons "prairie schooners" after large ships of the time. The prairie grasslands begin with the Great Plains at the eastern edge of the Rocky Mountains and extend all the way to the Appalachian Mountains in the eastern part of the country. The Rocky Mountains prevent moist air from moving over the Great Plains, and this "rain shadow" helps to keep the prairie grasslands extremely dry. However, it is not just the lack of rain that makes the prairie a harsh place to live. Twelve thousand years ago, retreating glaciers left behind a flat landscape open to extreme heat in the summer and extreme cold in the winter. The lack of geographic barriers or cover means that the wind runs rampant across the plains, leading to the "black blizzards" of the 1930s Dust Bowl and continuously endangering agriculture.
Despite these extremes, many plants and animals such as wildflowers, pronghorn antelope, mule deer, prairie dogs, and coyotes make their homes in the prairie grasslands. In addition, small, isolated wetlands dot the dry prairies, providing much-needed water and aquatic habitat for birds. In the Northern Great Plains, these wetlands formed as the glaciers receded and left round, sunken areas behind them. Rain and groundwater fill these depressions during certain times of year, creating scattered wetland habitat known as "prairie potholes." The Prairie Pothole Region in the Northern Great Plains contains 5-8 million small wetlands and some of the most important freshwater resources in North America. Bullrushes, sedges, and cattails grow on the edges of these potholes because they prefer standing water, and these plants in turn provide food and shelter for other species, such as birds. More than half of the migratory waterfowl in North America depend on prairie potholes for their survival.