24 April 2013

Urban stream syndrome



Definition:
Urban stream syndrome describes the consistently observed ecological degradation of streams draining urban land.  Symptoms of the urban stream syndrome include a flashier hydrograph, elevated concentrations of nutrients and contaminants, altered channelmorphology, and reduced biotic richness, with increased dominance of tolerant species.

Examples:
The most obvious hydrologic changes associated with urbanization are the engineering of stream channels, in which natural features are replaced by concrete channels and streambank stabilization efforts designed to resist increased flood flows.  Extensive piped storm drainage networks often completely bypass riparian zones, channeling large amounts of water from impervious surfaces directly into streams, both quickly and with increased frequency.  A result of this altered hydrology is that incision or “downcutting” is a common feature of urban stream channels.  Downcutting results from large volumes of water scouring out sediment that has accumulated during agricultural activity and/or residential construction in the watershed.  Incision is especially marked in watersheds with old and/or stable urban land use, where there are few sources of sediment to replace material scoured by high flows.  There is therefore tremendous variability in the condition of urban streams, depending on historic patterns of development, redistribution of sediments within streams, and hydrogeologic conditions in the watersheds.  However, we suggest that, over time, urban watersheds move towards stable land use, with large amounts of impervious cover and low sediment production leading to stream incision in most locations.

Why important:
Urban stream syndrome alter multiple aspects of stream ecosystem structure and function; from biodiversity to nutrient retention.

For more information:
·         Groffman, P. M., D. J. Bain, L. E. Band, K. T. Belt, G. S. Brush, J. M. Grove, R. V. Pouyat, I. C. Yesilonis, and W. C. Zipperer. 2003. Down by the riverside: urban riparian ecology. Frontiers in Ecology and the Environment 1:315-321.
·         Kaushal, S. S. and K. T. Belt. 2012. The urban watershed continuum:  Evolving spatial and temporal dimensions. Urban Ecosystems 15:409–435.
·         Walsh, C. J., A. H. Roy, J. W. Feminella, P. D. Cottingham, P. M. Groffman, and R. P. Morgan. 2005. The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society 24:706-723.

Contributed by BES Co-PI Dr. Peter Groffman

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