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Figure 2: Channel stability as a function of imperviousness (Booth and Relnelt, 1993) z.5 z.n t.5 1.0 0.5 0.0 o °~; ~~ o O STAB LE CI[ANNELS NN(LS ° a:,p a; m X UNSTABLE CHA ° L1.ROE~I.AKE SUBCATCItMENTS O E'er a~ o °~ : ° CENL'R.~tLLYSTA©LE CHANNELS $ o: C>~° p ° 1 0-yr lorested dtscharpe a~x:,~ ~ ~ o: ° 2-yr current dtscharpe • O ~~ x x ~ ~~ x X x x x CF.NERAI,L~UNSTAD/.E CHANNELS [0 20 30 40 50 60 PERCENT IMPERV70US AREA IN CATCHMENT years.10•"•Z' Where these tools have been applied to urban streams, they have consistently demonstrated that. a sharp threshold in habitat quality exists at ap- proximately 10 to 15% imperviousness.°•"•'~ Beyond this threshold, urban stream habitat quality is consis- tently classified as poor. Imperviousness and water quality Impervious surfaces collect and accumulate pot lut- ants deposited from the atmosphere, leaked from ve- hicles or derived from other sources. During storms, accumulated pollutants are quickly washed off and rapidly delivered to Habitat assessment tools have aquatic systems. consistently demonstrated that a sharp threshold in habitat Monitoring and modeling studies quality exists at approximately have consistently indicated that urban 10 to 15% imperviousness. pollutant loads are directly related to ~ watershed imperviousness. Indeed, imperviousness is the key predictive variable in most simulation and empirical models used to estimate pollutant loads. For example, the Simple Method assumes that pollutant loads are a direct func- tion ofwatershed imperviousness'-', as imperviousness is the key independent variable in the equation. Threshold limits for maintai~ting backgrott~td pollutant loads Suppose that watershed runoff drains into a lake that isphosphorus-limited. Also assume that the present background load of phosphorus from a rural land use amounts to 0.5 lbslac/yr. The Simple Method predicts that the postdevelopment phosphorus load will exceed background loads once watershed imperviousness (I) exceeds 20 to 25% (Figure 3j, thereby increasing the risk of nutrient overenrichment in the lake. Urban phosphorus loads can he reduced when urban best management practices (BMPs) arc installed, such as stonnwater ponds, wetlands. filters or infiltra- tion practices. Performance monitoring data indicates that BMPs can reduce phosphorus loads by as much as 40 to 60%, depending on the practice selected. The impact of this pollutant reduction on the postdevelopmcnt phosphorus loading rate from the site is shown in Figure 3. The net effect is to raise the phosphorus threshold to about 35%-60% impervious- ness, depending on the performance of tl~e BMP we instal(. Therefore, even when effective practices are widely applied, we eventually cross a threshold of imperviousness, beyond which we cannot maintain predevelopment water quality. Imperviousness and stream warming Impervious surfaces both absorb and reflect heat. During the summer months, impervious areas can have local air and ground temperatures that are 10 to 12 degrees warmer than the fields and forests that they replace. In addition, the trees that could have provided shade to offset the effects of solar radiation are absent. Water temperature in headwaterstreams is strop` ly influenced by local air temperatur::. ~Jalli`' reported that stream temperatures throughout the summer are increased in urban watersheds, and the degree of wann- ingappears to be directly related to the imperviousness of the contributing watershed. He monitored Five head- water streams in the Maryland Piedmont over asix- month period, the streams having differing levels of impervious cover (Figure 4). Each of the urban streams had mean temperatures that were consistently warnler than a forested reference stream, and the size of the increase (referred to as the delta-T) appeared to be a direct function of watershed imperviousness.. Other factors, such as lack of riparian cover and ponds, were also demonstrated to amplify stream warming, but the primary contributing factor appeared to be watershed impervious cover.`' Imperviousness and stream biodiversity The health of the aquatic ecosystem. is a strong environmental indicator of watershed quality. A num- ber of research studies have recently examined tl~e links between imperviousness and the biological di- versity in streams. Some of the key findings are sum- marized in Table 2. Aquatic insects The diversity, richness and composition of the benthic or streambed community has frequently been used to evaluate the quality of urban streams. Not only are aquatic insects a useful environmental indicator, ~__a ~...:.. ,.. but They also form the base of the stream ~~~~ ~~_..,,~ ~~~ most regions of the country. 102 i 1aYf ~~,~~iL-1~iL~~~7~(~ttaia;i_r~-.aairt~ia}`~, !_; `~l~rl~, lay[~1~:°~`iJ l.•,~1~41~:__~~,_1