Urban Engineered Stream Continuum (UESC)

Definition: The UESC conceptual framework describes comprehensively the way in which urban landscapes alter the transport and fate of organic matter and other constituents within both headwater and downstream aquatic ecosystems.

Explanation: Headwater streams (i.e., 1^st order), as primary sources of organic matter (leaves, wood) have occupied a particularly important position in stream ecosystem theory in that they supply much of the organic matter for downstream food webs.  However, urbanized areas (often without streams in their natural state), have usurped this role through engineered drainage networks, shifting the traditional headwater stream functions upland drainage features which, although organically productive are biologically desolate (e.g., gutters, swales; see “upland riparian zones.”  They have become the dominant, albeit unnatural, source of organic matter (see “gutter subsidy”), as well as water (stormwater) and wastewater (sewage) in addition to providing a matrix for additional mixing of stream flows with shallow groundwater (see “urban karst”).  The increased organic and pollutant loads from these greatly expanded “gutter networks” and underground streams have profound influences on the downstream parts of the network, resulting in aquatic systems which are more heterotrophic (i.e., oxygen starved), more polluted (e.g., metals, pesticides, hydrocarbons) and more disturbed (scour, erosion, and sediment from the many high flow events) than natural streams.

Example: The forested Baisman Run stream flows are dominated by groundwater flows and organic matter inputs by riparian forests.  Conversely similar sized urbanized catchments see greater and more frequent stormflows (with associated contaminants), increased organic matter inputs, and many buried streams with appreciable baseflow.

Figure 1: Additional sources of organic matter, water and pollutants in urbanized headwaters.

Left: Urbanized headwaters, showing gutters and street trees in the Rognel Heights catchment (top), storm drain inlets and catchments with leaves during summer in the Rognel Heights catchment (middle) and baseflow in the buried stream tributary in Spring Branch (left) and stormwater runoff (right) from small headwater catchments.  Right: Natural headwater streams with forested riparian zones. Basiman Run (bottom), with its Pond Branch upstream tributary (top).

Figure 2: Simplified conceptual diagram of an urban stream, with its associated hydrologic systems and water cycle.

 

Why Important: The ways and frequencies with which urban catchments generate stormwater, groundwater and runoff, organic matter, and pollutants have great impacts on both headwater stream structure and function, as well as their downstream aquatic ecosystems; these operate at short and long-term scales and are extremely important to receiving waters such as drinking water reservoirs, aquifers and natural resources.  As such, it is crucial to have an organizing conceptual framework such as the EUSC that reflects the profoundly different way in which these ecohydrologic systems operate and are sustained so that managers, scientists and citizens can relate to the complexities and solutions needed to protect them.

Contributed by BES Co-PI Kenneth T. Belt