Vast Dakota Territory, at the geographic center of the continent, was divided into North and South Dakota in 1889. Rich loess soils favor agriculture, and farms cover 90 percent of the state's land area. But the practice of draining glacier-formed prairie potholes to increase cropland destroys critical habitat for migrating birds. Extremes in weather and in world markets subject wheat farmers to cycles of boom and bust, but petroleum and lignite production assists the economy. Garrison Dam, on the Missouri River, produces 400,000 kilowatts of electricity, providing extensive irrigation for the surrounding area.