What is urbanization?

Urbanization refers to the concentration of human populations into discrete areas, leading to transformation of land for residential, commercial, industrial & transportation purposes. It can include densely populated centers, as well as their adjacent periurban or suburban fringes (Fig 1), and can be quantified in many different ways (Table 1). Example definitions used to classify areas as “urban” or “developed” include:

• Core areas with population density ≥ 1,000 people per square mile, plus surrounding areas with population density ≥ 500 people per square mile (U.S. Census Bureau, for 2000 Census )
• Areas characterized by ≥ 30% constructed materials, such as asphalt, concrete, and buildings (USGS National Land Cover Dataset )

Why does it matter?

• Urban development has increased dramatically in recent decades, and this increase is projected to continue. For example, in the U.S. developed land is projected to increase from 5.2% to 9.2% of the total land base in the next 25 years (Alig et al. 2004).
• On a national scale, urbanization affects relatively little land cover, but it has a significant ecological footprint—meaning that even small amounts of urban development can have large effects on stream ecosystems.

Key pathways by which urbanization alters streams

Riparian/channel alteration – Removal of riparian vegetation reduces stream cover and organic matter inputs; direct modification of channel alters hydrology and physical habitat.

Wastewater inputs – Human, industrial and other wastewaters enter streams via point (e.g., wastewater treatment plant effluents) and non-point (e.g., leaky infrastructure) discharges.

Impervious surfaces – Impervious cover increases surface runoff, resulting in increased delivery of stormwater and associated contaminants into streams.

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