Impact of Climate Change and Land Use on the Southwestern United States

Human impacts on the landscape

Population Growth of the Southwest United States, 1900-1990

Martin Chourre and Stewart Wright
U.S. Geological Survey


The accompanying maps show the population changes that occurred from 1900 to 1990 in the southwestern United States (Arizona, California, Colorado, Nevada, New Mexico and Utah). The maps represent population and population density by county at decennial intervals. Making generalizations about population trends for the entire region is difficult because local factors affect population change. The small scale of these maps shows the data for the entire region, but not in enough detail to display significant local differences, some of which can be inferred by comparing the total population and population density maps. To be able to compare maps throughout the century, the classification scheme used remains the same for each of the map types.

Geographic Analysis

The Southwest population maps are based upon US Census data and to show population change over time. By arranging the numbers into groups or classes, a visual portrayal of the population values can be created. Several methods are used to show population trends. Growth factors, such as economy or climate, cannot be derived from these models. This type of information needs to be collected on a local basis, taking into account all of the local factors that influence population growth for a specific area.

A small-scale map (1:2,000,000) will show more area in less detail and a large-scale map (1:24,000) will show less area in greater detail. This relationship is important when analyzing data of different scales. Small-scale maps can be used to locate areas of interest requiring more detailed study, and to support local or large-scale modeling and analysis. The value of small-scale mapping is to show the population trends throughout the region.

Because of the small scale and the cartographic generalization of some of the pre-1990 boundary locations, these data sets may not be suitable for large-scale modeling or comprehensive analysis. These boundary locations are not based on legal land descriptions, which could affect the size of a county and render the population density figures inaccurate for local analysis. The scale of the maps also affects accuracy. Exact boundary positions cannot be mapped in detail at this small scale.

Some geographic principles apply when comparing total population with population density. The size of a county will affect how population density is shown when compared to total population. The County of San Francisco is quite small (less than 50 square miles) though its population is fairly high. Given this situation, the population density remains in the high category through the decades, while total population remains in the middle category. When looking at the population density maps, the impression is that San Francisco is a highly populated place, but when looking at the total population it is not in the high category. The same is true for Denver County in Colorado. Its population density is in the high category, but its total population is in the middle category.

Click to view animated maps of TOTAL POPULATION and POPULATION DENSITY by county for the southwestern United States, by decade for the period 1900-1990.

Click any map to view a larger image
pop 1990 pop 1990
pop 1990 pop 1990

In the 1980 and 1990 population density maps, the counties where Las Vegas (Clark) and Albuquerque (Bernalillo) are located tell a similar story. The total population of Las Vegas is larger than that of Albuquerque, yet the population density is higher for Bernalillo County than Clark County. In the 1980 population density map, Maricopa County, Arizona, has a medium population density even though the population of Phoenix is quite high. These examples show how the size of a county affects the population density.

Click any map to view a larger image
pop 1990 pop 1990
pop 1990 pop 1990

Total population or population density by themselves do not give any indication as to where population is concentrated in the county. But, when viewed together, some assumptions can be made. San Bernardino County, California is the largest county in the southwestern United States. The population density maps show the county in the low to medium ranges throughout the decades. The total population maps show the population growing into the high category by 1990. Taking into consideration the population growth of the surrounding counties, an assumption can be made that most of the people in San Bernardino County are concentrated in the Los Angeles metropolitan area.

Click any map to view a larger image
pop 1990 pop 1990
pop 1990 pop 1990

Changing county boundaries can be used to deduce certain characteristics about an area other than the total population and population density. Looking at Colorado in 1900, Arapahoe County has a high population and a lower population density because of its size. In 1910, when Denver County first appears, both the population and the population density classifications increase inside the new county. The assumption can be made that in 1900, before Denver County was formed, most people were concentrated in the western part of Arapahoe County. In 1900, the population density for San Diego County, California was fairly low. In 1910, when Imperial County was formed, the population density increased in San Diego County. This points to the fact that in 1900, before Imperial County was formed, most people were concentrated in the western part of the county near the city of San Diego.

Click any map to view a larger image
pop 1990 pop 1990
pop 1990 pop 1990

The class values show a trend as well. The number of counties in the lower classification ranges decreases and the number in the higher ranges increases with time. Between 1910 and 1920, the numbers stayed the same and growth was slow enough that the classification scheme was unable to pick out any growth for that time period. Between 1930 and 1940, the large increases in the numbers from lower to higher classes suggest a surge in growth.

Click any map to view a larger image
pop 1990 pop 1990
pop 1990 pop 1990

Population Trends

Different economies and climates affect population trends throughout the region. Mining and agriculture have both played a significant role on population growth and decline in the Southwest, but other factors such as availability of water are significant. When viewing the population trends for the entire region, the local factors controlling these trends must be considered individually.

Federal Government policies have played an important role in the development of the Southwest. The Mining Act of 1879 provided initial impetus for the mining industry, and the various Homestead Acts made inexpensive land available for farmers. Public works such as irrigation, electric power generation, road building and water diversion and retention projects helped foster urban and rural growth. For example, the construction of Hoover Dam in the 1930s played a huge role in the development of the Las Vegas area. In the time period immediately preceding and during WWII, the expansion of military bases and ship and aircraft building industries brought many workers to the Southwest. People stayed after the war or moved to the Southwest upon completion of military service. More recently, the use of government lands for resource utilization, such as oil and gas drilling and grazing permits for livestock, has played an important part in the growth of the Southwest. Many local and state governments offer incentives such as low taxes, cheap utilities, and inexpensive land to businesses that relocate from other states.

As mining and agriculture became less labor-intensive and more mechanized, people started moving to urban areas throughout the country, especially in the Southwest, providing the labor pool for manufacturing and industry. Modern transportation, such as better highways, increased rail service and commercial airlines have increased mobility, allowing people to travel and do business over greater distances. Since the end of WWII, tourism and recreation-oriented businesses have become increasingly important in the Southwest. Many people are moving West for better quality of life, more open space, less congestion and increased recreational opportunities. The growing elderly population has been drawn to the warm and sunny climate of the region. Large retirement settlements have been a significant factor in the growth of some regions in the Southwest.

There are many reasons for the huge population growth of the southwestern United States in the 20th century. The Front Range of Colorado, the Wasatch Front in Utah, the Las Vegas, Los Angeles and other metropolitan areas, all exhibit unique population trends. Trends for the future may be based upon different factors yet to exist.


The visual impact of these maps, especially when viewed as an animation, can be a useful tool in understanding population growth and decline. As each decade passes, a pattern emerges that can be seen easily. These patterns can be examined more closely to determine the causes of the population trend for a local area.

The population of the Southwestern United States has increased by approximately 1,500% over the last 90 years, while the population of the United States as a whole has grown by just 225%. In the Southwest, Arizona and Nevada have led the way with increases of 2,880% and 2,840%, respectively. The metropolitan area in Nevada that is responsible for this growth is Las Vegas (Clark County). Clark County had a 90-year growth rate of 22,480%, growing from 3,284 people in 1900 to 741,459 people in 1990. Maricopa County (Phoenix), Arizona, had a 100-year growth rate of 10,275%, with most of that growth occurring between 1960 and 1990.

Considering both the local factors and the general population increase that the southwestern United States has seen over the last 90 years, and assuming that growth will continue, it is easy to see why plans for the future are necessary. Water and natural resources need to be managed to accommodate the future growth and economies need to be examined to ensure a healthy environment.

A brief description of the Geographic Data is available.

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