Over the past 200 years, changes to the Nation's urban areas have been dramatic. Changes that have occurred relate both to the location of urban centers, as well as to the spatial extent of land dedicated to urban uses. Urban areas at the beginning of the 19th century were located primarily along major rivers or bodies of water, as waterways provided the most efficient means for transporting goods and people. As railroads became prominent, urban areas were able to expand or develop away from the water's edge.

Geographic features such as steep slopes, wetlands, and lack of freshwater impeded settlement. In 1902, the National Reclamation Act was passed and with it came funding for the construction of water storage and transportation systems. This encouraged urban expansion in the arid west. After World War II, the Nation's urban areas continued to expand outward away from the city center as populations migrated to the margins of urban areas, where land was less expensive and the environment was less polluted. In 1956, the Federal Highway Act and the building of Interstate highways further facilitated urban expansion across the Unite States. Rural towns, small industrial centers, and farmland were engulfed by expanding urban centers.

Over the past 200 years, numerous social, cultural, economic, and political incentives have encouraged urban expansion. In the 1800s, the industrial revolution influenced where people lived and worked. Many people shifted from agricultural production in rural areas to factory work in urban centers. Advances in transportation systems, such as rail transport in the 19th and early 20th centuries, followed by the mass production of the automobile and convenient air travel, facilitated a mobile society and a national economy. Economic growth and a population boom after World War II spurred increased suburbanization-the shifting of residential areas to the outlying section of a city or to a separate municipality-on the fringe of urban areas. Other economic and political incentives that shaped the urban environment included Federally backed home loans, credit and tax mechanisms that encouraged new development, and less restrictive municipal ordinances regarding building codes, environmental laws, and zoning regulations.

Throughout the past two centuries land use changes associated with increasing urbanization have had impacts that resonate at local, regional, and even national scales. Landscape changes resulting from urbanization can be mapped and studied over time. Understanding these changes requires a study of the causes of change as related to social, economic, and political influences. Understanding these changes also requires analysis of how urbanization physically spreads across the landscape. The knowledge gained from studying urban land-use change can be helpful when it flows into local, regional, and national decisionmaking that relates to land-use decisions that impact the people, the economy, and the environment. Deriving a correlation between physical change and the explanations of the causes of change can help anticipate and mitigate the impacts of future change.

Throughout the past two centuries changes to the Nation's urban areas are inextricably linked to population changes. The Nation's population started growing slowly along the eastern seaboard during the 17th and 18th centuries, accelerated in the second half of the 19th century, and then continued steadily spreading westward throughout the next hundred years. Currently, nearly 80 percent of the U.S. population resides in urban areas. Land area dedicated to urban use continues to expand, although differently than it has in the past. Most newly urbanized areas are much less densely populated and less intensively developed than they were 50 to 100 years ago.