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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

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:

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.

Click below for more information on specific topics

the urban stream syndrome button urbanization and biotic integrity button catchment vs. riparian urbanization button
Figure 1. Urbanization map of the United States derived from city lights data. Urban areas are colored red, while peri-urban areas are colored yellow.
Image created by Flashback Imaging Corporation, under contract with NOAA and NASA [accessed 7.16.09]. Exit EPA Disclaimer.
Table 1. Common ways of quantifying urbanization
Measure Description
% Total urban area Area in all urban land uses
    % High intensity urban Area above some higher development threshold
    % Low intensity urban Area above some lower development threshold
    % Residential Area in residential-related uses
    % Commercial/industrial Area in commercial- or industrial-related uses
    % Transportation Area in transportation-related uses
% Total impervious area Area of impervious surfaces such as roads, parking lots and roofs; also called impervious surface cover
% Effective impervious area Impervious area directly connected to streams via pipes; also called % drainage connection
Road density Road length per area
Road crossing density # Road-stream crossings per area
Population density # People per area
Household density # Houses per area
Urban intensity indices Multimetric indices combining a suite of development-related measures into one index value [e.g., the USGS national urban intensity index (NUII) Exit EPA Disclaimer, based on housing density, % developed land in basin, and road density]

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