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As the populations of southwestern cities increase, the expanse of concrete, asphalt, and other impervious heat-retaining materials also grows. Negative consequences of an increasing urban footprint include greater flooding, runoff of pollutants (e.g. oil, grease, heavy metals), rising local temperatures (i.e. urban heat island effect), and habitat loss. The increased concentration of runoff and non-point source pollution into channels can significantly impair channel stability as well as the quality of water and biological functions of rivers and ephemeral streams. Simple and pragmatic methods to address these issues include the development of bio-retention basins, urban forestry, and other Green Infrastructure practices.
Impervious heat retaining materials such as asphalt and concrete can raise temperatures as much as 10 degrees Fahrenheit above surrounding areas (Image courtesy of: Mollie Walton).
Hydrocarbon products (e.g. oil) are one of many road toxins that can end up polluting water sources. Other common pollutants include heavy metals, fertilizing nutrients, and de-icing salts.
Concentrated runoff from upstream impervious areas has allowed for higher peak discharge (i.e. greater flooding) and erosion in downstream sections of southwestern streams and arroyos.