When glaciers scraped over the continent at the end of the last ice age, they dented the Great Plains with millions of shallow divots that still pock modern-day Montana, Minnesota, Wisconsin, Iowa, and the Dakotas. Few are larger than an acre, but they hold repositories of rainwater and snowmelt that are imperative in an otherwise grassy landscape. The natural potholes are critical breeding and nesting habitat for more than half of our nation's migratory waterfowl—especially ducks. When they dry up, or landowners fill them in, nesting hens compete for resources. A lack of food or space means fewer ducks are born. Like other wetlands, prairie potholes also filter nitrogen runoff from the region's corn, soy, and wheat farms, and hold water so it has time to slowly trickle into groundwater and recharge aquifers that farmers rely on for irrigation water. When there aren't wetlands to retain heavy downpours and surface runoff from paved cities, excess liquid floods crops.