These materials were developed by Kenneth E. Foote and Katrin E. Molch, Department of Geography, University of Texas at Austin, 1995. These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the authors, Kenneth E. Foote and Katrin E. Molch, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder. These materials may not be copied to or issued from another Web server without the authors' express permission. Copyright © 1995. All commercial rights are reserved. If you have comments or suggestions, please contact the authors or Kenneth E. Foote at firstname.lastname@example.org.
a) The Creation of Heat Islands
Cities are made of concrete, asphalt, brick, stone, and steel. These materials absorb and reflect energy differently than vegetation and soil. They absorb more radiant energy and radiate this energy back into the atmosphere at different times through the day. The result is that cities are warmer than the surrounding countryside, sometimes considerably. Furthermore, cities remain warm well into the night when the countryside has already cooled.
This heat influences air circulation and patterns of precipitation.
b) Changes in Air Quality
Human activities release a wide range of emissions into the environment including carbon dioxide, carbon monoxide, ozone, sulfur oxides, nitrogen oxides, lead, and many other pollutants. Some of these emissions are toxic and have claimed many lives in some cities when concentrations reached dangerous levels as in the "killer smogs" of London in the 1950s and 1960s. Others emissions such as carbon dioxide, act to trap heat in cities. Cities also release quantities of dust into the atmosphere, with effects like those mentioned next.
c) Changes in Patterns of Precipitation
The dust and emissions released into the atmosphere alter patterns of precipitation over the cities and in areas downwind. Cities often receive more rain than the surrounding countryside since dust can provoke the condensation of water vapor into rain droplets. Dust carried downwind from cities and industry can increase rain in city shadows. One of the most notable of these rain shadows, the La Porte anomaly in Indiana, is diagrammed below.
a) Erosion and other changes in land quality
Rapid development can result in very high levels of erosion and sedimentation in river channels.
Pollutants are often dispersed across cities or concentrated in industrial areas or waste sites. Lead- based paint used on roads and highways and on buildings is one such example of a widely dispersed pollutant that found its way into soil. But humans also bury tremendous amounts of waste in the ground at municipal and industrial dumps. These materials can severely contaminate soils. Even such commonplace items like gasoline storage tanks at filling stations have the potential to cause serious contamination. When this problem was discovered a decade ago, hundreds of storage tanks had to be removed and replaced with safer containers. The most extreme cases of industrial pollution, as at Love Canal in Buffalo, New York, have forced the abandonment of large tracts of land.
a) Modification of Habitats
Modifications can take place in many ways. Natural ground cover is replaced with grasses and decorative plants, although many native species will be left in place. Fertilizers spread across lawns finds its way into water channels where it promotes the grow of plants at the expense of fish. Waste dumped into streams lowers oxygen levels during its decay and caused the die-off of plants and animals. Sometimes, cities allow native species, such as squirrels and raccoons, to increase far above their natural concentrations.
b) Destruction of Habitats
In some cases, entire habitats are eradicated by urbanization and native species are pushed out of cities.
c) Creation of New Habitats
New habitats are also created for some native and non-native species. Urbanization has, for example, eliminated many bat colonies in caves, but has provided sites such as bridges for these species to nest. Cities also create habitats for some species considered pests, such as pigeons, starlings, sparrows, rats, mice, flies and mosquitoes.
Sometimes we hardly notice these changes. This view of Austin from the 1890s is notable for the sparsely vegetated landscape it captures. Today, Austin is richly vegetated by live oak, cedar, and many other natural and imported species. The University of Texas campus owes its present lush appearance to a program of tree planting dating back to the 1930s.
The first picture shows the University of Texas at Austin as seen from the Capitol before 1895. On the second picture a part of the University of Texas campus and downtown Austin as it is today can be seen.
a) Flow of Water into Streams
As cities grow, natural groundcover changes dramatically. Natural vegetation and undisturbed soil are replaced with concrete, asphalt, brick, and other impermeable surfaces. This means that, when it rains, water is less likely to be absorbed into the ground and, instead, flows directly into river channels. Not only does more water reach the stream channels, but it arrives far more quickly after a storm. Natural vegetation slows run-off, concrete and asphalt speed the flow. The result diagrammed below indicates how urbanization speeds run-off and produces higher peaks of flow.
b) Flow of Water through Streams
Higher, faster peak flows change streams channels that have evolved over centuries under natural conditions. The result is a spread of the channel vertically and horizontally to carry the extra flow. Rapid erosion of stream banks and down-cutting of stream beds occurs. When the existing stream beds cannot handle the increased flow, they flood the surrounding urban area, particularly development within adjacent floodplains.
Flooding can be a major problem as cities grow and stream channels attempt to keep up with these changes.
Click here to see examples of flood damage in Austin.
i) Methods for Controlling Flooding
Often humans intervene to speed the flow of water through stream channels and to control erosion. Many different methods exist to control flooding and the flow of streams. Banks may be reinforced with plants, rock, or concrete retaining walls. In extreme cases, a stream may be "channelized," that is totally lined with concrete. Such channelization turns the stream into a very efficient culvert. Unfortunately, such channelization has the disadvantage of simply shifting the burden of the extra water flow downstream to other communities.
Click here to see examples of flood control measures in Austin.
It would be nice to find a photo of the Los Angeles River, the diversion of the Trinity River in Dallas, Bogey Creek here in Austin will be fine, though.
ii) Encroachment of Flood Plains
As they grow, cities also tend to encroach on flood plains. These are low-lying areas that can hold and absorb overspill from stream during periods of high water. This encroachment of courses places humans in the way of floods. But encroachment also effects a watershed's ability to manage waterflow naturally. Excess flow may be kept out of floodplains by forcing extra flow downstream at higher speeds, with obvious consequences. Erosion and down-cutting will increase and the danger to downstream communities will increase.
c) Degraded Water Quality
These changes in the flow of precipitation into and through urban watersheds holds a number of consequences of water quality.
i) Increased sedimentation
An increased area of exposed and soils and higher runoff speeds means that more sediments are carried into local and downstream watersheds. This leads to increased rates of sedimentation.
ii) Pollutants in runoff
As water washes across urban surfaces, it dissolves and carries pollutants into streams and rivers. Toxic chemicals, oil, and other pollutants are all deposited in the watershed, sometimes in sediments. Also, most cities use rivers to dispose of the affluent from sewage treatment. Nowadays, it is relatively rare in the United States for untreated sewage to be discharged into waterways since storm and sanitary sewers have been divided for the most part into separate systems, but it does happen from time to time. Chemical tests of water samples are often required to identify pollutants but, sometimes, the color and smell of the water provide clues.
Click here to see examples of pollution of Austin rivers.
Click here to see examples of encroachment of Austin rivers and streams.
The incidence of flash floods is increased because much of Austin rests on thin soils atop relatively impermeable bedrock. This means that less rainfall can be absorbed into the ground. When soils are saturated, water runs into local stream channels in greater quantity and at greater speeds.
A) Divide campus into study sections and assign responsibilities.
B) Acquire a copy of the digital campus map from the anonymous ftp server on austin.grg.utexas.edu in the CAMPUS subdirectory. Alternatively, the file can be obtained from the download directory by clicking on each file while holding down the shift key.
C) Cut your section out of the complete map and compress the resulting file. Do not rotate your section or it will be very difficult to reassemble the sections at the end of the study.
D) Survey your study area on foot to check your digital map and gather direct evidence of the timing and sequence of changes.
E) Gather archival and historical sources including photographs and air photographs that allow you to track and date changes in groundcover. We will try to localize changes to particular decades, rather than years. Good sources of information include the library of the Center for American History (the Barker Texas History Center), the Austin History Center (a branch of the Austin Public Library), and the State Archives. Information is also available from the UT Physical Plant and air photographs can be obtained from many sources including the City of Austin, Texas Department of Transportation, and private providers like Miller Blueprint. PCL library has an excellent set of color air photographs of all of Travis County from 1984.
F) Develop a coding scheme (levels and symbology) for the information you have gathered. We must develop this scheme together as a class so that we maintain consistent coding for all sections of campus.
G) Use the information you have gathered and the coding scheme to map the changes in your study area.
E) Analyze your findings and compare them to the figures we calculate for campus as a whole.
Detwyler, Thomas R. and Melvin G. Marcus, eds. 1972. Urbanization and Environment: The Physical Geography of the City. Belmont, Ca.: Duxbury Press.
Douglas, Ian. 1983. The Urban Environment. London: Edward Arnold.
Goudie, Andrew. 1981. The Human Impact: Man's Role in Environmental Change. Cambridge: MIT Press.
Laurie, Ian C. 1979. Nature in Cities: The Natural Environment in the Design and Development of Urban Green Space. New York: John Wiley and Sons.
Lounsbury, John F. and Aldrich, Frank T. 1986. Introduction to Geographic Field Methods and Techniques, 2nd Ed. Columbus, Ohio: Charles E. Merrill Publishing Co.
Lowry, William P. 1967. The climate of cities. Scientific American 217 (August): 15-23.
McPhee, John. 1989. The Control of Nature. New York: Farrar Straus Giroux.
Marx, Leo. 1964. The Machine in the Garden. New York: Oxford University Press.
Mumford, Lewis. 1953. The Highway and the City. New York: Harcourt, Brace & World, Inc.
Spirn, Anne Whiston. 1984. The Granite Garden: Urban Nature and Human Design. New York: Basic Books.
Last revised 26 January 2000. LNC.