Davis' most influential scientific contribution was the "geographical cycle", first defined in his 1889 article, The Rivers and Valleys of Pennsylvania, which was a model of how rivers erode uplifted land to base level. His cycle of erosion suggests that (larger) rivers have three main stages of development, generally divided into youthful, mature and old-age stages. Each stage has distinct landforms and other properties associated with them, which can occur along the length of a river's upper, middle, and lower course.