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Effects of Urbanization on Stream Ecosystems

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Development can have negative effects on streams in urban and suburban areas. As a watershed becomes covered with pavement, sidewalks, and other types of urban land cover, stream organisms are confronted with an increased volume of storm water runoff, increased exposure to fertilizers and pesticides, and dramatic changes in physical living spaces within the stream itself. In this episode, USGS scientist Jerry McMahon describes two take home messages for managers.




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Jerry McMahon: My name is Jerry McMahon. I am the team leader of the group of U.S. Geological Survey scientists who are studying the effects of urban development on stream ecosystems. 

What is a stream ecosystem? Stream ecosystems include the stream itself, including the water, the stream channel, and the stream banks, the organisms that live in the stream, and the organisms including humans that live on the lands that surround and exert an influence on the stream. Stream ecosystems are complex systems and this complexity is part of what makes them interesting and valuable.01:02Anyone walking next to a degraded urban stream can see signs of the effects of urban development. Over the stream banks, trash hanging  in tree branches, discarded shopping carts and tires and on a warm summer day, mats of algae covering the stream water. 

Urban development alters the hydrology, habitat, and water quality of streams. Roads, parking lots, and buildings increase the impervious cover in urban water sheds, and restrict the infiltration of precipitation into ground water which increases storm water runoff. As a result stream flows in urban areas often rise rapidly after a rainfall event. This rapid increase in stream flow can cause erosion of stream beds and banks and degrade fish spawning and feeding habitats. Common sources of pollution to urban streams include fertilizers and pesticides, animal wastes, seepage from septic tanks, liquids from sewage lines, erosion from construction sites, automobile fluids and vehicle and industrial emissions.02:12Over the last 10 years, the U.S. Geological Survey's National Water Quality Assessment  Program has examined the effects of urban development on stream ecosystems. The assessment used the nationally consistent study design in 10 metropolitan areas of the United States. Urban development significantly affected one or more biological communities in 8 of the 9 metropolitan study areas. Even small levels of urban development had an immediate negative effect. There is no period of resistance to the effects of urban development. A substantial decline in its species richness occurred in all but the Denver, Dallas, and Milwaukee study areas.

Urban development occurred primarily with the conversion of either agricultural or forested lands.   03:00These two land cover types may mask or escalate the influence of urban development on stream ecosystems. So, how can this information be used? First, the condition of stream organisms begins to decline as soon as there is any development within the stream's watershed. A great deal of caution should be exercised in thinking that there is a safe zone of urban development. Second, we now know that streams in different regions of the country respond differently to urban development. Because of these regional differences, management strategies to protect the character of urban streams that work in one part of the country may not be effective elsewhere.

For additional information about  our project, please visit the project website where you can obtain both reports, as well as the data used in the project. I am Jerry McMahon, and on behalf of all my colleagues, I would like to thank you for your interest in understanding the effects of urban development on stream ecosystems. 


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