IMPERVIOUS COVER, AQUATIC COMMUNITY HEALTH,AND STQRMWATER BMPs: IS THERE A RELATIONSHIP?

Richard R. Horner, Christopher W. May, Eric H. Livingston and John Maxted

ABSTRACT: Research during the past 10 years has showed that the health ofaquatic biological communities declines with increasing levels of watershedimperviousness. The hypothesis tested in this project was that the use of structuraland nonstructural stormwater BMPs will allow a healthier biological community athigher levels of imperviousness. To test this hypothesis bioassessments wereperformed in Montgomery County, Maryland, Austin, Texas, Vail, Colorado, and thePuget Sound area of Washington. This paper will report primarily on the findingsfrom the Puget Sound area where lowland salmon spawning and rearing streamsand their watersheds were studied to identify the linkages between watershedconditions, specifically urbanization, and the habitat elements and biologicalresponses. The study’s intent was to produce a knowledge base for managing landwith reference to ecological protection goals. Measures of benthicmacroinvertebrate and fish community integrity declined from the lowest levels ofurbanization without exhibiting a threshold effect, although retention of naturalriparian buffer partially ameliorated the decline of invertebrates. The study produceda set of conditions necessary to preserve the highest levels of biological integrity oravoid the lowest. A follow-up study is in progress to assess the influence ofstructural and nonstructural best management practices (BMPs) on the sameecological communities. Results to date demonstrate that retention of a wide, nearlycontinuous riparian buffer in native vegetation has greater and more flexible potentialthan other options to uphold biological integrity when development increases.Upland forest retention also offers valuable benefits, especially in managing anydevelopment occurring in previously undeveloped or lightly developed watersheds.Structural BMPs have less mitigation potential than the non-structural BMPsassessed and should not be regarded, as they so often are, as the leading or eventhe single strategy. Still, they have their place in management, especially inmoderately and highly developed watersheds, to help prevent further resourcedeterioration, and, in dense networks along with non-structural means, in lessdeveloped basins of relatively high ecological value and sensitivity. None of theoptions is without limitations, and widespread landscape preservation must beincorporated if we are to keep the most productive aquatic resources in the PugetSound region. While circumstances differ in other settings, the methods used andgeneral conclusions likely have wide applicability.

BACKGROUND

The effects of watershed urbanization on streams are well documented. Theyinclude extensive changes in basin hydrologic regime, channel morphology. andphysicochemical water quality. The cumulative effects of these alterations producean in-stream habitat considerably different from that in which the natural biologicalcommunity evolved. Klein (1979) was one of the first to observe thatmacroinvertebrate diversity drops sharply in urban streams in Maryland whenwatershed imperviousness exceeded 10 to 15 percent. Similar reductions inmacroinvertebrates and fish have been reported by numerous researchers aroundthe country (Table 1).

Table 1. Summary of studies examining the relationship between watershedimperviousness and biological community health.

1 B i o l o g i c a l h e a l t h) Declines rapidly after 10%

insects/fish i m p e r v i o u s n e s s .Steward 1983 Seattle Salmon Coho reduced at 1 O-l 5%Jones and 1987 N. Va. Aq. insects ‘Declines rapidly at 15-25%Clark imperviousness. Steedman 1988 Ontario Aq. ‘Insects Decline begins at 10%

Booth 1991 Seattle Fish, Channel stability and fish habitatchannels declined rapidly after 10%

imperviousness.Scheueler/Galli 1992 Maryland Fishlaq.

insectsLuchetti 1993 Seattle Salmon

10-I 5% imperviousnessBlack/Veatch 1994 Maryland Fishlaq. Poor fish/aq. Insects > 30%

Insects imperviousness.I Shaver et. a!. 1995 Delaware Aq. Insects

Horner 1997 Seattle Fishlaq.Insects imperviousness- -

In addition, development pressure has a negative impact on riparian forests andwetlands. which are intimately involved in stream ecosystem functioning. Muchevidence of these effects exists from studies of urban streams in the PacificNorthwest (e. g., Richey 1982. Steward 1983, Scott et al. 1986, Booth and Reinelt1993).

What has been missing, however, is definition of the linkages tying togetherlandscapes and aquatic habitats and their inhabitants strong enough to supportmanagement decision making that avoids or minimizes resource losses. Lackingthis systematic picture, management efforts have not been broadly successful infulfilling the federal Clean Water Act’s stipulation to protect the biological integrity ofthe nation’s waters; and it is unlikely they would be any more successful in protectingendangered salmon and aiding their recovery Effective management needs wellconceived goals of what biological organisms and communities are to be sustainedand at what levels, and then the foundation for judging what habitat conditions theyneed for sustenance and, in turn, watershed attributes consistent and inconsistentwith these conditions.

INVESTIGATING THE FRAMEWORK

The hvoothesis

To investigate the relationship between aquatic community health and urbanization,two projects have been completed or are well advanced. The first was funded bythe state of Washington Department of Ecology’s Centennial Clean Water Fund within-kind support from cities and counties. Its intent was to establish attributes of thee

2

watersheds contributing to selected stream reaches and then to make a number ofmeasurements of the riparian zones. in-stream habitat. and biological conditionswithin those reaches. The concept underlying the study. simply illustrated asfollows. was that watershed and riparian characteristics determine habitatconditions, which, in relation to evolved organism preferences and tolerances. setthe composition of biological communities:

Watershed and Riparian s Habitat 2 AquaticCharacteristics Conditions Biota

Data analysis was directed at establishing the linkages represented by the arrows,as well as the less direct connections between watershed characteristics and biota.Final analysis identified an optimum set of variables representing each componentthat can be measured cost effectively in future monitoring. Details on this finishedstudy can be found in publications by May (1996). Homer et al. (1997). and May etal. b) and in the graduate student theses cited below.

The second project, funded by the U. S. Environmental Protection Agency. is beingundertaken by the Watershed Management Institute in cooperation with localstormwater management programs in four areas of the United States. Its focus isthe efficacy of the best management practice (BMP) tools that urban water resourcemanagers can apply in attempting to achieve aquatic resource protection andrecovery goals. Its intent is to determine to what extent different types of BMPsameliorate the negative effects of urbanization documented in the first study.Specifically, this project is testing the hypothesis that the use of structural andnonstructural BMPs will allow a healthier aquatic biological community to exist athigher levels of watershed imperviousness. In addition to assessing the benefits ofstructural BMPs. this project also is examining the effectiveness of non-structuralBMPs. specifically the retention of forest areas within a watershed and the existenceof a 30 meter wide riparian buffer along first order streams.

Study Desiqns

In the initial study, 22 watersheds were selected to represent a range ofdevelopment levels from relatively undeveloped (reference sites) to highly urbanized.Because of its integrative nature, total impervious area (TIA) was chosen as theprincipal measure of urbanization. Attributes of the stream catchments wereestablished using standard watershed analysis tools and methods, includinggeographic information systems, aerial photographs, basin plans, and field surveys.Stream flow was continuously monitored by local agencies on 10 of the studystreams. The research group modeled hydrologic characteristics on the remainder(Cooper 1996). Physical and chemical water quality was monitored during baseflowand storm runoff, and sediment metals contents were measured at 23 sites on 19 ofthe streams (Bryant 1995). Sediment particle size distribution was determined at 46sites on 20 streams (Wydzga 1997). Extensive surveys of in-stream physical habitatand riparian zone characteristics were conducted on 120 reaches along all 22 studystreams (May 1996). The broadened coverage allowed watershed-scale habitatcharacterization: associations between habitat and other variables were investigatedin reaches with common data. There was only a rudimentary accounting of BMPs in

this study. Most structural BMPs in place at the time of the study’s start (1994) weredesigned according to old drainage standards that have since been upgraded.

Biological assessment focused especially on benthic macroinvertebrates, whichwere collected in 31 riffles on 21 streams and taxonomically identified. Data wereused to develop a benthic index of biotic integrity (B-IBI; Kleindl 1995) a communityindex comprised of groups with specific life strategies or trophic functions or certaintaxa shown to be relatively sensitive or insensitive to the stresses produced byurbanization. Salmonid abundance data were obtained from public, private, andtribal sources. Data used in the study were all collected using widely accepted andconsistent electrofishing methods (May 1996). Fish were incorporated into theanalysis using the ratio of juvenile coho salmon to cutthroat trout abundance. Coho(Oncovnchus kisutch) is, arguably, the salmonid most vulnerable to urban runoff,being the only species whose juveniles over-winter in streams and because of itsreliance on pools that often disappear in urban streams. Cutthroat trout(Oncorynchus clarki) is a species with far less social and economic importancearound Puget Sound, probably related to the relatively small size it attains as a full-time resident in the nutrient-poor waters. It seems to gain a competitive advantageover coho in urban streams that are relatively dynamic hydrologically and insediment transport.

The follow-up study added 21 watersheds and 39 stream reaches to provide moreability to distinguish BMP effects. The same methods used in the first study wereemployed for these catchments and reaches in watershed analysis, riparian andhabitat characterization, and benthic macroinvertebrate sampling and analysis.Some additional fish data were obtained from the same sources. Local stormwatermanagement agency data bases were consulted to locate BMPs and, as far aspossible, identify their design and as-built characteristics and maintenance histories.This search yielded more than 2500 structural BMPs. To date the analysis hasconsidered these BMPs only in the aggregate in terms of their coverage per unitarea of watershed and per unit impervious area. Future work will investigate thequality of their construction and maintenance and further investigate their effect onaquatic resources in terms of quality. A system has been developed to quantify theirpotential performance in storm runoff water quantity and quality control with respectto BMP quality. Future work will also expand the hydrologic data base to as many ofthe new watersheds as possible.

The present data base drawn from both studies consists of 76 reaches on 40streams with full watershed, riparian, habitat, benthic macroinvertebrate, and BMPdata. Fish data obtained with consistent and widely accepted methods exist for 30reaches. All sites are on streams of third order or smaller and are located inwatersheds ranging in area from 0.7 to 60 km2 , with headwaters at elevations lessthan 150 meters above sea level. Total impervious areas range from 1.2 to 60.6percent. It should be noted that the highest TIA is very close to the actual maximumthat exists in low-order stream watersheds in the region. The small streams in moreurbanized watersheds have been almost entirely Culver-ted below ground. All streamgradients are under 4.5 percent (most are <2 percent). The study watershedsrepresent the two general types of geologic and soil conditions found in the PugetSound region, the more prominent poorly drained glacial till and the moderately welldrained glacial outwash formations.

4

The second study also established and monitored 45 sites in three other locations:Montgomery County, Maryland; Austin. Texas: and Vail. Colorado. In all fourlocations, 20 sites were selected on small streams that received runoff fromdevelopments that did not have stormwater LIMPS, 20 sites were selected on smallstreams that received runoff from developments with stormwater BMPs, and fivereference sites were selected to provide background conditions. The 20 test andcontrol sites represented a range of watershed imperviousness.

RESULTS AND INTERPRETATIONS

The results of the sampling programs are best exhibited by a series of graphsdepicting relationships between biological community health, watershedimperviousness, and habitat quality. Examples of some of the data from each of thestudy sites will be presented. Unfortunately, at this time, only the data from thePuget Sound area is available for detailed analysis and interpretation within this papaper.

Maryland

00 .*

00

00.*

0 0 l.

-0l . 0000 m0

0.

0

..

0.

. .

0

~pijij%q 10 20 30 40 50 60 70

% Impervious Cover

5

Cl0 0..03 .

no:. .. . mo0 DO 0,’0 .. . .

.

l

.

.

10 20 30 40 50 50 70 0 20 40 60 80 103

Impervious Cover ( % ) Habitat Quality ( % )

Preliminary Results from Montqomery County, Maryland

The results for the Montgomery County, MD data contribution to the overall studyreveal three major findings. These finding show that the IBI scores for benthics arenot dependent on having a high habitat for the test groups; BMPs are controllinghigh imperviousness flows from affecting the surrounding stream habitat; and thereare elevated temperatures below BMPs compared to control sites.

When comparing the percent habitat score to the benthic IBI score for the test andcontrol groups independently, they produce two distinct conclusions. The test sitesshows a higher mean habitat score, but a lower mean IBI score in comparison to thecontrol sites. This result is peculiar because the trend is that as habitat scoreincreases, so does the IBI score. Normally a steam free of impacts can contain ahealthy stream habitat that will support a healthy benthic community. Having a highhabitat score with a lower benthic community rating can point to a water quality issueproblem, in this instance it may be water temperature increases by the BMPs. Whenexclusively comparing test and control sites with imperviousness of l0-20% or 20-40%, the data for sites with BMPs shows that as the habitat increases, the IBI scoreslightly lowers; and for control sites that as habitat increases, there are higher IBIscores. This points to the preliminary conclusion that BMPs do not have a positiveor negative impact on the downstream benthic community, but they are benefitingthe downstream habitat. Stormwater management may benefit and improve thedownstream physical and channel habitat by decreasing storm flow; but at the sametime is not improving benthic community diversity or health, but also is not impactingthe benthic community health. The benthic community may not be as high of qualityas its supporting habitat because of the temperature increase from the BMPdischarge.

Comparing the percent imperviousness of the drainage area to the percent habitatscore reveals a significant finding in support of the BMPs improving stream habitat.As the percent imperviousness increases for the test sites, the habitat remainsconstant. In comparison, as the imperviousness increases for the control sites, thehabitat lowers. This indicates that the BMPs are controlled high imperviousnessstream flows from negatively affecting the stream habitat. The more impervious thecontrol sites are, the more the high runoff during storms is hurting the habitat.

Overall, the biological health of the macroinvertebrate community declined aswatershed imperviousness increased. However, the decline was not as steep or asrapid in the sites with BMPs. It appears that sites with BMPs mitigate the impacts ofincreased imperviousness within the 12 to 23% imperviousness range. Above thislevel, the decline in biological condition continues for both the test and control sitesat similar rates.

The second finding is the downstream temperature effect of the stormwater ponds.Comparing the four control sites to the four test sites for water temperature, all thetest sites are 2-8°C warmer on average than the control sites. Observed storms on6/27/97 (I .O inches), 7128197 (.2 inches), 8/l 7/97 (1.2 inches) and 8/20/97 (2.2inches) are the only times when the control sites were briefly warmer that the testsites. This shows how the BMP was able to minimize the peak water temperatures,while the steams without BMPs experienced a quick thermal impact of water

7

B-IBI and coho salmon/cutthroat trout ratio both declined with TIA increase withoutexhibiting a threshold of effect; i. e., declines accompanied even small levels ofurbanization (Figure 1). Only in reaches with TIA 6 percent was B-IBI 232(maximum possible = 45). 8-181 scores between 25 and 32 were from reaches with<10 percent watershed TIA. with eight notable exceptions. all from two streams.

Falling in the range 25-35 percent TIA. these reaches still had more of theirupstream riparian zones in intact wetlands than all but one of the remaining streamsAll but one had at least 70 percent of the upstream riparian area in wetland or forestextending a minimum of 30 meters from each stream bank. These observationssuggested that maintenance of a relatively wide, nearly continuous. natural riparianbuffer may mitigate some of the effects of more distant urbanization. It appearedunlikely that streams draining relatively highly urbanized basins (TIA >45 percent)can maintain B-IBI above 15 (minimum possible = 9).

Figure 1. Benthic Index of Biotic Integrity (B-IBI) and Coho Salmon/Cutthroat TroutRatio Over a Gradient of Watershed Total Impervious Area (% TIA) (from May et al.

Coho salmon dominance over cutthroat trout at ratios >2:1 was seen only with verylittle urbanization (TIA <5 percent). At some point under 20 percent TIA ratios fell

below 1. These results confirmed the findings of earlier regional studies (Steward1983, Scott et al. 1986, Lucchetti and Fuerstenberg 1993).

Key Habitat Factors:

Reasons for the observed declines in biological integrity were investigated byconsidering the associated habitat characteristics. Concentrations of potentiallytoxic metals were well below regulatory criteria until TIA rose above 40 percent.Sediment concentrations only began to rise at that point but did not come close tostate guidelines. Therefore, neither water nor sediment quality could explain thepatterns and rates of loss of biological integrity.

Hydrology and physical habitat conditions associated with it exhibited more linkagewith watershed characteristics on the one hand and biological attributes on theother. Hydrologic fluctuation and its stress were represented by the ratio of 2-yearfrequency peak flow rate/winter baseflow rate, a measure proportional to relativestream power. The ratio was almost always <20:1 with TIA <15 percent, and no B-IBI score 235 occurred in a reach with a higher ratio. Large woody debris (LWD) is acrucial habitat component in Pacific Northwest streams, dissipating flow energy,protecting stream banks and beds, and providing cover and habitat diversity, in partthrough its role in pool formation (Bisson et al. 1988). Numbers of pieces of LWDand their volumes both dropped sharply with urbanization, from highs of 650/km toalways <300/km with >5percent TIA, except at a few sites were wood had beenadded in stream restoration projects. Still, LWD was low even in many unurbanizedreaches, seemingly because of historic timber harvest and stream “cleaning”.

Dissolved oxygen was measured in the sediments (intragravel dissolved oxygen,IGDO) in redds built like those constructed by salmon to deposit eggs. Sampleswere drawn from buried aquarium airstones. Effectiveness of oxygen transport tothe eggs was expressed in terms of the ratio IGDO/DO in the water column. In allbut one case, the mean ratio was >0.8 in the reaches draining catchments with <5percent TIA, while it usually fell well below 0.7, to as low as 0.3, with TIA >10percent. Exceptions again were in segments with riparian corridors in relativelyintact forest cover and wetlands.

How Good is Good, and How Bad is Bad?

Figure 2 portrays the strong relationship observed between biological integrity andthe combination of low urbanization and extensive riparian buffer. This graphic wasa start to answering fundamental questions essential to aquatic ecologicalmanagement. These observations suggest a set of stream quality zones similar tothose proposed by Steedman (1988). Further to this analysis, the findings werereviewed to propose a set of conditions that appear to be necessary, though not bythemselves sufficient, for a high level of biological functioning. These include:1. >70 percent of upstream riparian zone in forest cover or wetlands >30 meters

wide,2. 2-year peak flow rate/winter baseflow rate <20,3 . <1 5 percent of surface bed material composed of fine sediments (<0.85 mm),4. pebble count DIO >5 mm,5 . IGDO/DO >0.8,

9

6. LWD frequency >2 per bankfull width (BFW).7. >40 percent of LWD with diameter >0.5 meter,8. >50 percent of surface area in pool habitat.9. < 2 BFWs between pools. and10 >50 percent cover over pools.

Figure 2. Relationships Among Biological Integrity, Watershed Urbanization,and Riparian Corridor (from May et al. 1997a)

Assessment of BMP Influence on Salmonid Stream Bioloqy

General Assessment:

Figures 3a and b illustrate the variation of B-IBI and coho salmon/cutthroat trout ratio(CS/CT), respectively. with and without the presence of structural BMPs in thewatersheds. Both curves decline less sharply than those in Figure 1. plotted with thefirst study’s data only. There are not conclusive signs that the mere presence ofstructural BMPs raises biological measures. a trend agreeing with the one found byMaxted and Shaver (1997) in a similar investigation. In part. conclusiveness ismissing because of the dearth of points representing watersheds without BMPs inthe medium-high levels of urbanization. In Figure 3a the elevated scores in the 25-35 percent TIA interval are the same ones with good riparian zones describedearlier. At the highest imperviousness (>43 percent) B-IBI was usually below 15 andalways below 20, and CSICT was well under 1, even with BMPs. Still. Figure 3bgives the first evidence of CS/CT ratios >l, and even >2 sometimes. at any but thelowest levels of urbanization; and the sites involved are served by structural BMPs.

Ability to demonstrate conclusive structural BMP effect is probably compromised bylight coverage of developed area with installed practices. Of the 23 catchmentshaving BMPs and falling in the medium range of urbanization (15-40 percent TIA).the average BMP coverage was 4.1/km2, and only nine had more than 2 BMPs/km2.Many of these facilities are below-ground peak rate control vaults without waterquality benefits or oil spill containment manholes with very limited capability.

1 0

Figure 3. Benthic Index of Biotic integrity (B-IBI) and Coho Salmon/CutthroatTrout Ratio Over a Gradient of Watershed Total Impervious Area (% TIA)

With and Without Structural BMPs

(a) B-IBI

3 5z2 3 0

UY 25

s 2 2 0 1 0 1 5

i 50

%Total impervious Area

(b) CohoCutthroat

n w/ BMPs

0 w/o BMPS

4

0 10 2 0 3 0 40 50 6 0 7 0

%Total Impervious Area

11

Assessment of Structural and Non-structural BMP Potentials--Methods:

For further investigation of structural BMP influence, an index of coverage, thestructural mitigation variable (SMV), was computed: SMV = (No. BMPs)/(% TIA l X).The quantity X is an arbitrary number selected to be larger than the maximum No.BMPs/% TIA found in the survey and is used so that the index works numerically inthe remainder of the analysis (X = 15 was used). The index represents only a countof all structural BMP types, including peak rate control surface detention andsubsurface vaults, oil spill containment manholes, infiltration basins, wet waterquality ponds, and others. It takes no account of facility capabilities or quality ofimplementation, subjects for future investigation. It was desired to construct anindex of structurally mitigated urbanization (SMI) as a function of % TIA and SMVthat would decrease as SMV increases. The form SMI = (% TIA) * (1 - SMV) fulfillsthis criterion. The intention was to investigate if higher levels of urbanizationcompensated by BMPs (SMI < % TIA) would produce ecological measures higherthan the same amount of urbanization if unmitigated (SMI = % TIA). Left for futurework is a similar examination of how in-stream habitat variables are affected bymitigation.

Non-structural BMP influence was investigated in a similar fashion. As pointed outearlier, the most effective non-structural BMPs retain natural soil and vegetationcover, which in the Puget Sound region means the forest that invariably coversundisturbed landscapes. Intact upland forest and riparian zones in forest cover orwetlands were regarded as de facto non-structural BMPs. A forest retention variable(FRV) was taken as the fraction of forest cover in the watershed, and an index ofurbanization mitigated by forest retention (FRI) was computed according to FRI = (%TIA) * (1 - FRV). The inverse form was used to make FRI decrease with FRVincrease and be consistent in form with SMI. Likewise, a riparian retention variable(RRV) was taken, according to the study’s findings, as the fraction of upstreamriparian corridor in forest cover or wetlands at least 30 meters wide measured fromeach stream bank. An index of urbanization mitigated by riparian retention (RRI)was computed according to RRI = (% TIA) l (1 - RRV). Measures of biologicalintegrity were analyzed relative to each index.

Assessment of Structural and Non-structural BMP Potentials--Results:

Figures 4a-f illustrate various mitigation influences on B-IBI and CS/CT. Signs ofeffective mitigation would be allowance of greater urbanization while preserving thesame level of biological integrity. The graphs specifically look at opportunities toensure high levels of biological function (B-IBI >35 and S/CT >3 always) and toreduce the tendency of functioning to decline to low levels (B-IBI <20 and CS/CT <1always) with relatively high impervious cover. The vertical lines represent theoreticalcutoffs of regions where high integrity is ensured, in the case of the left-hand line,and where low integrity is inevitable, in the case of the right-hand line, based on thedata that were collected and indexed as described. It must be emphasized, and willbe in the ensuing discussion, that there are some practical limits to reaching inreality the levels of mitigation represented by these theoretical lines. Figures 5a andb, respectively, depict forest and riparian retention in relation to % TIA. These plotswere used as guides in judging actual potentials to apply these non-structuralpractices.

1 2

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--

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A

--*,

.,

.

T 8

..*

.

W---.

.

l .

l .

I.

Table 2. Analysis of Structural and Non-Structural Potentialto Mitigate Urbanization Effects on Stream Biological integrity

MITIGATION

W MaintainCS/CTm--p+

(3b)

<20Prevent

Structural(4a)

CS/CT<1.0

MaintainB-IBI235

(4d) MaintainCS/CT

m--l-sB-IBI<20

(4d) PreventCS/CT

retention((4b)

(4e)

B-IBI135

MaintainCS/CT23.0

(4b) PreventB-IBI<20

EQUATION--

-- -- 7.8 7.8

--

% TIA =215/(1 ~-NO.BMPs/%TIA)% TIA =81/(15-No.BMPs/%TIA)% TIA =650/( 1 ~-NO.BMPs/%TIA)% TIA =575/( 1 ~-NO.BMPs/%TIA) 1% TIA =10.6/( 1-FractionForest)% TIA =2.9/(1-FractionForest)% TIA =38.9/( 1-FractionForest)

THEOR. PRAC.INDEPENDENT MAX. MAX.

VARIABLE TIA TIA-- 14.9 14.9

-- 45.4 45.4

-- 38.8 38.8

1.4” 15.8 15.82.6b 17.3 17.31 0.3b 45.7 -20c

1.4b 6.0 6.02.6b 6.5 6.5

1 0.3b 17.2 -8’

1.4” 47.8 47.82.6b 52.4 52.4

1 0.3b >I00 -55c

1.4” 42.3 42.32.6b 46.4 46.4

1 0.3b >lOO -50c

0.30 15.1 15.10.50 21.2 (21 .2)c0.60 26.5 -20

>

0.80 14.5 -8(-1 o)c

0.10 43.2 43.20.20 48.6 -45

(-50)c

Table 2 continued

MITIGATION(FIGURE) GOAL’

Forest Prevent-retention CSlCT(4e) <1.0

Riparianretention(4c)

(4f)

(4c)

(4f)

MaintainB-IBI23.5

Forest)% TIA = I 0.607.6/(1- 0.70Fraction >30 0.80

MaintainCS/CT23.0

m)% TIA = 0.701.5/(1- 0.80Fraction >30 0.90

PreventB-IBI<20

PreventCS/CT<1.0

m)% TIA = 0.4019.7/(1- 0.50Fraction >30 0.60

* CS/CT = Juvenile coho salmomcuttnroat trout ratlo; tneor,theoretical/practical maximum.

THEOR. PRAC.MA.Ka MAXa

TIA TIA34.9 34.939.3 39.344.9 -40c

19.025.338.0

5.07.515.0

_-

_.-48.854.962.7

19.025.3-30

l$sjL

7.5-10

%F54.9-60’

32.839.449.3

.~~_32.839.4-4o=

lrac. max.--

y Median, mean, and maximum, respectively, at sites with structural BMPs.‘Judgment based on Figures 5a orb; number in parentheses, if given, based onjudgment of what could be accomplished with a concerted effort to retain forest ornatural riparian zones with special land use planning or regulation.

Figures 3a and b show that the maximum TIA with high biological integrity as definedabove was 14.9 percent for invertebrates and 7.8 percent for fish. The question foraquatic ecosystem management is, “Can the various forms of mitigation, applied atthe highest practical level. raise these percentages?” To pursue an answer, simplealgebraic equations were derived from the graphs to compute TIA for given levels ofstructural or non-structural mitigation. For example, in Figure 4c the left-hand verticalline falls at RRI = 7.6 and represents the equation (% TIA) l (1 - Fraction riparian >30m) = 7.6. Solving, (% TIA) = 7.6/(1 - Fraction riparian >30 m). Table 2 givesequivalent equations from all graphs and some impervious levels that couldconceivably be mitigated using the various techniques in theory and, consideringlimitations. in practice.

Structural BMPs appear to have the realistic potential to mitigate no more than about5 percent TIA increase over the Figure 3a maximum (-20 versus 15 percent) and stillattain the highest B-IBI levels, and would not allow any more urbanization than theFigure 3b maximum if CS/CT ~3.0 is to be maintained. if the highest level ofcoverage found in the survey were applied (-10 BMPsl% TIA), fish might be