Watershed

Definition:  An area that receives rainwater or other water input, and accumulates it at its lowest point.  In urban systems the water inputs into watersheds may be from rainfall, household and landscape application, or industrial discharge.  Drainage channels deliver surface water to larger streams.  In dry climates the low point may be a basin where water is lost entirely by evaporation to the atmosphere, while in moist climates, the low point usually empties into a larger stream, lake, or coastal waters.  In urban systems, the drainage networks may be in part or entirely engineered, and include curbs and gutters, storm drain inlets and settling basins for sediments and large debris, and pipes of increasing diameters that finally empty into a creek, river, or bay. 

Examples: Baltimore City comprises four major watersheds; Gwynns Falls, Jones Falls, Herring Run, and the direct harbor drainage.  Each of these watersheds reflects modifications by curb and storm drain infrastructure.  

 

Figure 1.  Diagram of a forested watershed.  The red line shows the area included, in which all water that falls drains to the lowest point.  Water that falls on the outside of the red boundary of the watershed becomes part of the flow of other watersheds not shown.  This diagram isof an ideal, non-urban watershed with no engineered movement of water in or out through pipes.

Figure 2.  The non-engineered watersheds of Baltimore City and thier extensions into Baltimore County.  The flow into these watersheds has been modified by importation of water from reservoirs beyond their boundaries, by storm drain networks that cross the watershed boundaries at some places, and by the piping out of sanitary sewage and storm runoff.  In addition, leakage between the sanitary and storm sewers, and unintentional exchanges between the sewer networks and the streams also complicate the flow of water.

Why important: Watersheds are a crucial feature of natural landscapes.  They are no less important to cities and suburbs, even if they include much built structure.  Watersheds rarely correspond to political boundaries.  Hence watershed management is a political and social process that must cross municipal boundaries, and involve the interest and understanding of many human communities.  Watersheds are a kind of ecosystem, because not only do they include the streams and groundwaters that influence the streams, but they also include the surface and subsurface flow toward the streams, the pipes that channel flow in neighborhoods and commercial developments, and the biological and social elements that occupy these lands.  The watershed as a concept is spatially extensive, three dimensional, and incorporates social, biological, and physical components.

For more information:

• Belt, K. 2000. The Baltimore Ecosystem Study water quality and urban hydrology initiatives - stream studies along an urban rural gradient in the Gwynns Falls and Baisman Run watersheds. Maryland Water Monitoring Council Programatic Coordination Newsletter.
• Black, P.E. 1991. Watershed hydrology. Prentice Hall, Englewood Cliffs.
• Brush, G.S. 2009. Historical land use, nitrogen, and coastal eutrophication: a paleoecological perspective. Estuaries and Coasts 32:18-28.
• Pickett, S.T.A., K.T. Belt, M.F. Galvin, P.M. Groffman, J.M. Grove, D.C. Outen, R.V. Pouyat, W.P. Stack, and M.L. Cadenasso. 2007. Watersheds in Baltimore, Maryland: understanding and application of integrated ecological and social processes. Journal of Contemporary Water Research and Education 136:44-55.