Significant Topographic Changes in the United States
Hydrologic interference is a primary direct anthropogenic process, and dam construction is one type of interference that usually has obvious effects. Certainly, earth materials are moved during dam construction, but the larger impact is the inundation by the reservoir behind the dam.
The figure below shows an example of a large reservoir expansion that was detected by the elevation differencing and thresholding approach. This reservoir is Lake Pleasant, located northwest of Phoenix, Arizona, and is part of the Central Arizona Project, a large water supply system.
A dam constructed on the Agua Fria River in the 1920s created Lake Pleasant. In the early 1990s the U.S. Bureau of Reclamation constructed a much larger dam and the size of the reservoir nearly tripled. The volume calculated from the difference grid associated with the change polygon can be used to characterize the additional water supply resulting from the new dam construction and reservoir expansion. Given the calculated volume of 7.57 x 108 cubic meters, a current population of the Phoenix-Mesa-Scottsdale metropolitan area of 3.86 million people, and a per capita daily water use of 218 gallons, the additional capacity stored in Lake Pleasant equates to about an 8-month supply of water for the entire Phoenix metropolitan area.