These cyclic and cumulative effects mean that urban ecological relations can become particularly complex. One change leads to another and another in a complicated, spiraling series of feedback loops. Sometimes it is easy to see how urbanization shapes the environment, particularly in peripheral areas were large suburban developments may consume hundreds of acres per year. But the impacts of urbanization begin early. Sometimes, the period of initial urban settlement produces the most radical changes in watersheds as tracts of land are cleared for the first time.
Encroachment continues long beyond initial settlement as an almost continuous process.
Click here to see examples of encroachment of Austin rivers and streams. These changes in urban environments have obvious consequences for their human populations. Increased levels of airborne and waterborne pollutants have adverse effects on human life. Large cities and cities in mountain basins are particularly prone to air quality problems stemming from the burning of fossil fuels for transportation, electrical generation, and heat. Urban water supplies have to monitored carefully for increased levels of toxins.