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many shclliish bed closures (now that most point sources have hccn controlled), no systematic attempt has yet been made to relate watershed intpcrviousness to the extent of shellfish bed closures. Taylor"' exami ned the effect of watershed develop- ment on 19 freshwater wetlands in King County, Washington, and concluded that the additional storm- watcr contributed to greater annual water level fluctua- tions (WLF). When the annual WLFexceedcd about 8 inches, the richness of both the wetland plant and amphibian community dropped sharply. This increase in WLF began to occur consistently when upstream watersheds exceeded 10 to 15% imperviousness. Implications at the Watershed Level Tl~e many independent lines of research reviewed here converge toward. a common conclusion- that it is extremely difficult to maintain predevelopment stream quality when watershed development exceeds 10 to 15% impervious cover. What implications might this apparent threshold have For watershed planning'? Should low density or high density development be encouraged? At first glance, it would seem appropriate to limit watershed development to no more than 10% total impervious cover. While this approach may be wise for an individual "sensitive" watershed, it is probably not practical as a uniform standard. Only low density development would be feasible under a ten percent zoning scenario, perhaps one-acre lot residential zon- ing, with a few widely scattered commercial clusters. At the regional scale, development would thus be spread over a much wider geographic area than it would otherwise have been. At the same time, addi- tionalimpervious area (in the form of roads) would be needed to link the community together. Paradoxically, the best way to minimize the cre- ation ofadditional impervious area at the regional scale is [o concentrate it in high density clusters or centers. The corresponding impervious cover in these clusters is expected to be very high (25% to 100%), making it virtually impossible to maintain predevelopmentstream quality. A watershed manager must then confront the fact that to save one stream's quality it may be neces- sary to degrade another. A second troubling implication of the impervious/ stream quality relationships involves the large ex- panses of urban areas that have already been densely developed. Will it be possible to fully restore stream quality in watersheds with high impervious cover? Some early watershed restoration work does suggests that biological diversity in urban streams can. be par- daily [estUled, but Unly alter extenJ'1Ve StUrrnwateC retrofit and habitat structures are installed. For ex- ample, Gsh and macroinvcrtcbratc diversity has hccn partially restored in one tributary of Sligo Creek, Maryland." In other urban watersheds, however, com- prehensive watershed restoration may not be feasible, due to a lack of space, feasible sites, or funding. ParadoxiC A proposed scheme far classifying ccrban stream quality potential ' - ~u I'~J ally, the best way to minimize the creation of addi- tional impervious area at the regional scale is to concentrate it in high density clusters. The thresholds provide areason- ,;,~ able foundation for classifying the potential stream quality in a watershed based on the ultimate amount of impervious cover. One such scheme is outlined in Table 3. It divides urban streams into three management categories based on the general relationships between impervious cover and stream quality: 1. Stressed streams (1 to 10% impervious cover) 2. Impacted streams (1 1 to 25% impervious cover) 3. Degraded streams (26 to 100% impervious cover) The resource objective and management strategies in each stream category differ to reflect the potential stream quality that can be achieved. The most protec- tive category are "stressed streams" in which strict zoning, site impervious restrictions, stream buffers and BMPs are applied to maintain predevelopment stream quality. "Impacted. streams" are above the threshold and can be expected to experience some degradation after development (i.e., less stable channels and some loss of diversity). The key resource objective for these streams is to mitigate these impacu to the greatest extent possible, using effective BMPs. The last category, degraded streams, recognizes thatpredevelopmentchannel stabiIityand biodiversity cannot be fully maintained, even when BMPs or retro- fits are fully applied. The primary resource objective shifts [o protect. downstream water quality by remov- ing urban pollutants. Efforts to protect or restore bio- logical diversity in degraded streams are not aban- doned; insome priority subwatersheds intensive stream restoration techniques are employed to attempt to ,partially restore some aspects of stream quality. In other subwatersheds, however, new development (and impervious cover) is encouraged to take place so as to protect stressed and impacted streams. Watershed-based zocting Watershed-based zoning is based on the premise that impervious cover is a superior measure to gauge ~_ r ~, a i...',... a.... iuc iiitpacisut grvwiu, Cviupareu tv pvpuia~i~u uc+~- sity, dwelling units or other factors. The key steps in ,,~~1 __~~-~V ~Il~t;: ^ '^ ai t~~t!!(~~ +,-1 1X~F~~L~j~, rt_ `s"`,: ~ *-~(1~~ %t _ 107