ABSTRACT
Watershed urbanization is widespread and increasing throughout the United States. Conversion of rural lands to urban lands is driven by a combination of population growth, net movements of people from rural areas to urban zones, and increased rates of urban land use per capita. The population of the United States reached 273 million in 1999 and is predicted to be 403 million by 2050, a 48% increase (all census data in this article are from www.census.gov/population/estimate/nation). In 1930, about 56% of the population (123 million people) lived in urban areas. By 1990 this percentage increased to 75% (248 million people) and is expected to rise above 80% by 2025. The amount of urban land in the United States has more than tripled from 8,065 sq. mi. in 1950 to 27,838 sq. mi. in 1990. Most of this increase was caused by “urban sprawl” as people moved from higher density downtown zones to newly built lower density suburbs encircling the central city. In 1950, the population density per square mile was 6,121 in urbanized areas, and declined to 3,411 people per sq. mi. by 1990 as cities spread out.
