Land and water planners frequently wrestle with the problems of uncontrolled growth and sprawl and their deleterious impacts on water quality. Fortunately, tools such as watershed management and impervious cover thresholds are available to address these problems. Researchers with the
Water Resources Agency at the University of Delaware, Institute for Public Administration recommends an approach to protect water supplies and contain sprawl in Delaware through land use planning based on natural hydrogeological boundaries - the watersheds. Impervious cover thresholds can
be used to focus growth into watershed zoning districts where development would have the least impact on stream water quality. Since 1991, impervious cover thresholds have been employed within the New Castle County, Delaware zoning code to protect sensitive water resource protection areas
during new development. The authors advocate amending the existing code to establish watershed-zoning districts based on percent impervious cover thresholds in the Christina River Basin of Delaware. By employing this concept in county and municipal zoning codes, smart growth can be concentrated
in areas with existing infrastructure, while protecting the quantity and quality of Delaware's surface water supplies. Land use zoning based on impervious surface coverage should be considered as an effective, measurable, and scientifically defensible technique to protect stream water
quality in the Christina River Basin and other watersheds in the Delaware Valley, the USA, and overseas.
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