USGS Multimedia Gallery: (Video)--The Effects of Urbanization on Stream Ecosystems (extended) Part II: Study Design

Jerry McMahon: Over the last 10 years, the US Geological Survey’s National Water Quality Assessment Program has examined the effects of urban development on stream ecosystems. In this study, the two primary objectives were first of all to look at the physical, chemical and biological responses of streams to urban development. And also to look at how those responses varied across the country. The assessment used a nationally consistent study design in 10 metropolitan areas of the United States: Seattle, Washington; Portland, Oregon; Salt Lake City, Utah; Denver, Colorado; Dallas, Texas; Milwaukee, Wisconsin; Birmingham, Alabama; Atlanta, Georgia; Boston, Massachusetts; and Raleigh, North Carolina.
The colored polygons for each metropolitan area indicates the areas within which we located approximately 30 study watersheds. Together these 30 watersheds reflect a gradient of urban development. That is, across the 30 watersheds, we represented the range of urban development conditions from very low to very high. Before I talk about the study findings, I want to mention three things that we tried to keep in mind when we’re designing and implementing the study. The first thing is that these urban stream ecosystems are made up of complex interacting human and natural systems. And our assumption has been that if we want to meet important societal goals such as building cities that have sustainable ecosystems, we really need to make sure that we account for these interacting human and natural components of stream ecosystems.
These interacting systems are affected by processes operating at several nested special scales: regional, watershed and stream reach. The local stream reach is the most tangible point of contact of stream ecosystems for most people. A stream reach refers to a short stretch or length of the stream that might be viewed when someone is fishing or taking a short walk. At this scale, several stream features are clearly visible.
Hydrology refers to the water flowing in the stream. Habitat refers to stream features such as sediment or rocks on the stream bottom where organisms might live. Stream chemistry refers to chemicals such as nutrients or pesticides found in the stream water. Stream biota such as algae, aquatic insects and fish are examples of the type of organisms that live in streams. The second key design consideration is that a stream’s physical, chemical and biological features will respond to changes in both human and natural characteristics.
In order to isolate and understand the influences of urban development, we had to limit the influence of variation and natural factors such as soils, climate, elevation and slope within each metropolitan study area. We did this by making sure that the study watersheds in each area were located in a single ecoregion. Ecoregions define areas that include a relatively homogenous combination of climatic, topographic, geologic and general land use characteristics. By constraining the study watersheds to a single ecoregion, the study design limited the natural variation within each metropolitan area and also enabled the individual metropolitan studies to be placed in a national context.
Our third key design consideration was an assumption about the response of a stream’s biological community to urban development. That is first, a period of resistance with few changes in the makeup of the biological community over an initial period of development. Second, a period of rapid change in the biological community as development increases. And finally, a state that can be thought of as an exhaustion threshold at which point the biological community is composed mostly of tolerant organisms and little additional changes possible.
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’m Jerry McMahon and on behalf of all my colleagues, I’d like to thank you for your interest in understanding the effects of urban development on stream ecosystems.