Hydrobiologia 518: 33–46, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Changes in water quality and macroinvertebrate communities resultingfrom urban stormflows in the Provo River, Utah, U.S.A.

Lawrence GrayDepartment of Biology, Utah Valley State College, 800 West University Parkway, Orem, UT 84058-5999, U.S.A.E-mail: grayla@uvsc.edu

Received 10 February 2003; in revised form 14 July 2003; accepted 28 October 2003

Key words: impervious surface cover, macroinvertebrates, substrates, urban runoff, water quality

Abstract

Short-term changes in water quality from 7 summer stormflows and long-term changes in substrates and macroin-vertebrate communities resulting from urban runoff from the city of Provo, Utah, were examined from 1999–2002in the lower Provo River. Stormflows resulted in increased total suspended solids and concentrations of dissolvedcopper, lead and zinc, and decreased conductivity and dissolved oxygen. The degree of change was generally in pro-portion to the magnitude of the storm. However, changes were temporary with water quality parameters returningto pre-storm levels within 12 hours. River substrates showed a trend of increased compaction and decreased debrisdam area downstream through the urban corridor. Macroinvertebrate communities showed trends of decreasedabundance and total species diversity with increasing urbanization. Compared to non-urban reaches, communitiesin urban reaches had few ‘sensitive’ species and were dominated by tolerant species, particularly snails and leeches.Comparisons with previous studies show that changes in macroinvertebrate community composition in the urbanreaches reflected shifts in land use during the past 15–25 years and corresponded to expected threshold levels ofimpact for amount of impervious surface cover.

Introduction

The Provo River is a major trout stream draining thesouthern Wasatch Mountains of northern Utah. Formuch of its length, the river drains National Forestland and other natural areas. Much of the flow ofthe river is highly regulated by large reservoirs andnumerous irrigation diversions.

The final 16 km of the river upstream from itsconfluence with Utah Lake flows through the city ofProvo, Utah. Like other cities in the western U.S.,Provo has experienced significant population growthduring the 1990s. During this time, the city grew atan annual rate of 1.9% and has a current populationof over 100,000 residents (Utah State Governmentcensus data). Concurrent with this increase in pop-ulation has been an increase in roads, housing, andcommercial development of land that was previouslyused for agriculture.

The purpose of the present study was to exam-ine the effects of this urban expansion on the lowerProvo River. Three measures were used: (1) changesin water quality resulting from urban runoff duringsummer thunderstorms, (2) changes in amounts oforganic substrates (debris dams) and compaction of in-organic substrates, and (3) a three-year comparison ofmacroinvertebrate communities along an urbanizationgradient.

The lower Provo River represents a unique oppor-tunity for examining the effects of urban stormflows.Flow in the river is highly regulated, thus there are nolarge, natural floods disturbing the lower reaches fromupstream. Minimum flows are maintained throughoutthe year, thus preventing loss of in-stream habitat dueto drought. In addition, the natural increase in flowresulting from spring snowmelt is maintained as partof the recovery plan for the June Sucker (Chasmistesliorus), an endangered fish found in Utah Lake andthe lower Provo River (U.S. Fish and Wildlife Ser-

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Figure 1. Map showing the study sites along the lower Provo River, Utah. Dashed line indicates the boundary of the urbanized portion of thedrainage basin. Arrows indicate entrance of urban runoff into the river from large storm drains.

vice 1999). No permanent tributaries enter the riverin the urban portion of its channel, and there are nomajor point sources of pollution, such as industrialdischarges or municipal wastewater. Most stormwaterrunoff from the city of Provo enters the river at a fewmajor storm drains, and most of the remainder of run-off not entering these drains is intercepted by irrigationcanals that act as distributaries to the main channel(Fig. 1). Summer thunderstorms are typically intenseand brief, thus runoff resembles flash flooding foundin natural desert streams (Fisher & Minckley 1978).

Given these conditions, three main hypotheseswere tested. First, urban stormflows in the ProvoRiver should increase suspended solids, total dissolvedsolids, and the concentrations of heavy metals, and de-crease the amount of dissolved oxygen (Bryan, 1974;Klein, 1979; Pratt et al., 1981; Liston & Maher, 1986).Second, urban reaches should show an increase infine sediments in the river channel and a decrease inorganic debris dams (Wood & Armitage 1997; Paul

& Meyer 2001). Third, macroinvertebrate communit-ies should show a decrease in overall abundance anddiversity in urban reaches due to the loss of speciesintolerant to the physical/chemical changes resultingfrom urban runoff (Pratt et al., 1981; Garie & McIn-tosh, 1986; Jones & Clark, 1987; Kemp & Spotila,1997).

Materials and methods

Study area

Four sampling sites for substrates and macroinverteb-rates were established along the lower Provo River toprovide a gradient of urbanization and stormflow in-puts (Fig. 1). Site 1, the ‘control’ site, was located up-stream from the city near the mouth of Provo Canyon.This portion of the river channel flows through un-developed land, primarily National Forest. U.S. High-

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way 89 follows the channel upstream to Deer CreekReservoir, and several picnic areas are present next tothe channel. Site 2 was located 3.3 km downstreamfrom Site 1 at the northern end of the city. The areaaround Site 2 has seen significant increases in roads,housing, and commercial development in the past 15years, but some undeveloped agricultural land was stillpresent during this study. Site 3 was located 5.5 kmdownstream from Site 2 near the entrance of severalmajor storm drains. This part of the city has been de-veloped for many years and is primarily surroundedby residential areas. Site 4 was located 6 km upstreamfrom the confluence of the Provo River and Utah Lakenear the streamflow gauging station on the lower ProvoRiver. The riparian area immediately upstream fromthis site is a mix of natural forest and residential areas,whereas the area immediately surrounding the sitehas been developed for housing. All stormflow run-off from the city of Provo enters the river upstreamfrom this site. Common riparian trees along the ProvoRiver include narrowleaf cottonwood (Populus angus-tifolia James), boxelder (Acer negundo L.), river birch(Betula occidentalis Nutt.), and willow (Salix spp.).

The total area of developed land in the city ofProvo draining into the lower Provo River was 32 km2

at the time of this study. Land use around eachsampling site was estimated using aerial photos takenin 2000 and previously from the mid-1970s to mid-1980s, depending upon the site. These estimates weredetermined by manually counting squares on a trans-parent grid overlying each map (1 grid square =40 m2). An area of 5 ha immediately upstream fromthe sampling reach was assessed for each site. Theintent was to estimate the amount of change in im-pervious surface cover (e.g., roads, parking lots, andbuildings).

Elevations at the sites varied from 1487 m at Site1 to 1397 m at Site 4. River gradients were 18,9.6, 6.6, and 3.2 m km−1 at Sites 1–4, respectively.All sites had substrates of predominately armoredcobble with some gravel and finer sediments. Extens-ive growths of Cladophora glomerata (L.) Keutz. andother filamentous algae were present during warmermonths.

Physical/chemical parameters

Measurements of total suspended solids, conductivity,pH, dissolved oxygen, and water temperature duringnormal (non-storm) flows were taken at the time of the

macroinvertebrate samples at all four sites. Stormflowsamples were collected at Site 4.

Suspended solids (photometric) was determinedwith a Hach� model DR/850 portable colorimeter.Conductivity was measured with a Hanna� Instru-ments DiST WP portable conductivity meter, and pHwas measured with an Oaktron� pHTestr 2 portablemeter. Dissolved oxygen was determined by the Wink-ler method (azide modification) on a 300-ml sample(manual titration) using Hach� reagents. Percent sat-uration was adjusted for temperature and barometricpressure as recorded at the National Weather Servicestation 3 km from Site 4. Samples for heavy metal ana-lysis were collected in 250-ml plastic bottles acidifiedwith nitric acid supplied by the Central Utah WaterConservation District, and analyses were conductedby the Utah State Department of Environmental Qual-ity Laboratory. Water temperature was measured witha calibrated field thermometer at the time of sampling.

Inorganic substrates at each site were collectedwith a corer to a depth of ca. 10 cm. Several cores weretaken at each site on each date and combined into onesample. Only materials <64 mm were retained. Afterdrying, the sample was sieved through a set of U.S.standard sieves (#5, 10, 60 and 230) that separatedthe material into pebble (4 to 64 mm), gravel (2 to4 mm), coarse sand (0.25 to 2 mm), fine sand (0.063to 0.25 mm), and silt/clay (<0.063 mm) fractions. Thepercentage of ‘fine’ sediments was calculated as themass of the fine sand + silt clay fractions to the totalmass of sediment. The amount of compaction of graveland finer substrates was measured with a Dickey–John soil compaction meter. This technique alloweda more quantitative measure of compaction than thecommonly-used measure of percent embeddedness(Barbour et al., 1999)

Stormflow discharge was obtained by two meth-ods. For very small stormflows, discharge at Site4 was measured using a Great Atlantic stream cur-rent meter, meter stick, and tape measure. For largestormflows, real-time discharge was obtained from theCentral Utah Water Conservancy District gauging sta-tion located 4 km downstream from Site 4. There areno tributaries or irrigation diversions between Site 4and the gauging station.

Macroinvertebrates

Macroinvertebrates were collected with a Surbersampler (0.20-mm mesh) that enclosed an area of0.1 m2. Three samples were taken in riffle/run por-

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tions of the river at each sampling site on each date(pools made up <10% of total stream channel area atany site). Sites were sampled in mid-late June, early-October, late-December, and mid-March from June1999 through March 2002. A total of 36 samples wastaken at each site.

Macroinvertebrates were preserved in 5% formalinin the field and transferred to 80% ethanol for identific-ation and counting. All specimens were hand-pickedfrom debris. Dry mass for each species was determ-ined after drying the specimens at 80 ◦C for 24 h andweighing to the nearest 0.1 mg. Correction factorswere applied to account for the loss of mass in thepreservatives.

General references used for identification wereMerritt & Cummins (1996), Pennak (1978), Thorp& Covich (1991) and Ward & Kondratieff (1992).Additional references used for specific taxa were asfollows: Ephemeroptera (Allen & Edmunds, 1965;Jensen, 1966; Edmunds et al., 1976; McCafferty& Waltz, 1990), Plecoptera (Gaufin et al., 1966;Baumann et al., 1977; Szczytko & Stewart, 1979),Trichoptera (Smith, 1968; Wiggins, 1977; Peck& Smith, 1978; Alstad, 1980), Elmidae (Brown,1972; White, 1978), Simuliidae (Peterson, 1960),Chironomidae (Epler, 1995), Hirudinea (Beck, 1954;Klemm, 1972), and Gastropoda (Burch, 1982; Cham-berlin & Roscoe, 1948).

Individual taxa were assigned a pollution toler-ance value based on regional studies (Barbour et al.,1999; Northwest (Idaho) listing; Jessup & Gerritsen,2000). Values range from 0–10 with values of 3 or lessindicating a ‘sensitive’ taxon and values of 7–10 indic-ating a ‘tolerant’ taxon. These numerical values werenot based on tolerance to urban stormflows per se,but do incorporate sensitivity to a variety of impactspotentially important in urban stormflows, includinginorganic sediments, heavy metals, and changes insubstrates. Sensitive species included all Plecoptera,Ephemeralla inermis, Rhyacophila coloradensis, Mi-crasema sp., Arctopsyche grandis, Brachycentrus oc-cidentalis, and Pagastia sp. Tolerant taxa includedmost leeches and snails, Caecidotea occidentalis, Or-thocladius sp., Eukiefferiella sp., and small oligo-chaetes. A community tolerance index was calculatedby summing the tolerance values for taxa present at asite and dividing by the total number of taxa.

The study of Pteronarcidae stonefly distributionduring summer 2000 involved a direct census of stone-fly numbers and debris dams along 3, 100-m reachesat each sampling site (ca. 1 ha total area of stream

channel). A kick net (1-mm mesh) was used to collectlate-instar stoneflies, and a fixed 1-hour time periodwas used to count the number of stoneflies in eachreach. Samples were taken in the most favorable hab-itats for the stoneflies, thus maximizing census counts.Before each stonefly census, the total area of riverchannel covered by debris dams (accumulations ofleaves and wood with an area of at least 0.1 m2) ineach reach was measured.

Results

Stormflows

Discharge for the study period that included 7 summerstormflows is shown in Fig. 2. Dates of the storm, rain-fall total and increase in discharge at Site 4 (measuredat peak stormflow), were: (1) 7 July 1999 (5.3 mm,0.28 m3 s−1), (2) 14 July 1999 (4.6 mm, 0.17 m3 s−1),(3) 30 August 1999 (20.3 mm, 3.48 m3 s−1), (4)23 August 2000 (0.3 mm, 0.09 m3 s−1), (5) 26 Au-gust 2000 (17.0 mm, 2.97 m3 s−1), (6) 31 August2000 (17.0 mm, 3.42 m3 s−1), and (7) 14 July 2001(3.6 mm, 0.23 m3 s−1). The increase in discharge waslinearly related to total rainfall (increase in m3 s−1 =0.0203 (mm rainfall) − 0.45; r2 = 0.96). These sevenevents represented all of the summer (June–August)rainfall events during the study period except for a34-mm storm that occurred on 2–7 June 1999 and a2.5-mm storm on 18 June 2000.

Changes in water quality during stormflows inthe Provo River were proportional to the increases indischarge (Fig. 3). During the smaller stormflows (in-crease in discharge <0.3 m3 s−1), changes were slightand within the range of variation found during normalflows (Table 1). Changes during the three larger storm-flows were more pronounced. TSS increased greatly,whereas conductivity and dissolved oxygen decreasedsignificantly. Increases in TSS appeared to be primar-ily caused by suspension of organic detritus fromstream sediments, particularly detachment of decay-ing algae. A ‘flushing’ effect was evident when the 26and 31 August 2000 stormflows are compared. Storm-flow was similar in magnitude for both storms, butparameters showed less change during the 31 Auguststorm.

Short-term changes in parameters during the 26August 2000 storm are illustrated in Fig. 4. Increasesin TSS and metal concentrations and decreases in con-ductivity and dissolved oxygen closely followed the

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Figure 2. Hydrograph of the lower Provo River during the study period (data from a gauging station located 4 km downstream from Site 4).Peaks in the March-May periods represent regulated releases of water to simulate natural spring snowmelt. The decline in the peak dischargeduring spring runoff periods was due to below-normal precipitation in 2000 and 2001.

Table 1. Means and ranges (in parentheses) of physical/chemical parameters at each sampling site during normal(no storm runoff) flows on the lower Provo River for the period June 1999 to March 2002 (N = 12 samplingdates/site). Dissolved concentrations of heavy metals (e.g., lead, copper, zinc) were below detection limits(= 3 µg l−1 for lead, 12 µg l−1 for copper, and 30 µg l−1 for zinc).

Parameter Site 1 Site 2 Site 3 Site 4

Total suspended solids, mg l−1 <1 (0–2) <1 (0–2) <1 (0–2) <1 (0–3)

Specific conductance, µS cm−1 321 (140–483) 337 (140–457) 355 (140–485) 368 (140–502)

pH 8.0 (7.2–8.7) 8.0 (7.4–8.5) 8.0 (7.3–8.7) 8.0 (7.5–8.7)

Dissolved O2, mg l−2 10.1 (8.2–12.1) 10.0 (7.8–12.1) 9.7 (7.2–11.7) 9.7 (7.2–12.5)

Dissolved O2, % saturation 99 (86–110) 99 (84–110) 98 (74–123) 98 (81–114)

Water temperature, ◦C 6.9 (2.0–11.0) 7.3 (2.0–12.0) 8.3 (2.0–14.0) 8.7 (2.5–15.0)

storm surge with maximum or minimum values oc-curring within the first 0.5 h. However, all parametersreturned to pre-storm values within 12 h of the initialstorm surge.

Sediments

Inorganic sediments showed significant downstreamchanges. The proportion of fine inorganic sedimentswas similar at all sites and throughout the study period(Fig. 5). However, compaction of fine sediments wassignificantly higher at Sites 3 and 4 compared to Sites1 and 2 (Student’s t = 13.0, d.f. = 65, P < 0.001).

The quantity of organic detritus, as measured bythe area of debris dams at each site in July 2001,also decreased downstream (Fig. 6). Mean area ofdebris dams was 3.2 m2/100 m2 of channel at Site 1,1.8 m2/100 m2 of channel at Site 2, 1.6 m2/100 m2

of channel at Site 3, and 0.8 m2/100 m2 of channel atSite 4.

Macroinvertebrates

The overall abundance of macroinvertebrates de-creased downstream (Fig. 7). Total density signific-antly declined (F = 6.05, d.f. = 3, 140; P < 0.001),whereas total biomass did not (F = 2.47, P = 0.06).The significant difference in total density betweensites was largely due to the higher density at Site 1compared to the other three sites.

Community composition and diversity showed amarked change between the upstream and downstreamsites (Fig. 8). Sites 1 and 2 were dominated by aquaticinsects in both numbers and biomass, whereas snailsand leeches comprised most of total biomass at Sites 3and 4. Species diversity significantly declined down-stream due to a loss of sensitive taxa as indicated bythe lower EPT index values (Fig. 9). The Jaccard indexof community similarity also indicated the differencesbetween upstream and downstream sites. The Jaccardindex was 0.80 between Sites 1 and 2, but only 0.58–0.59 between these sites and Sites 3 and 4. The indexwas 0.66 between the two downstream sites.

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Figure 3. Relationships between changes in dissolved oxygen saturation, total suspended solids (TSS), and conductivity (as a measure of totaldissolved solids) with changes in stormflow discharge during the 7 storm events sampled at Site 4. Data points labeled ‘a’ and ‘b’ represent thestorms of 26 August and 31 August 2000, respectively.

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Figure 4. Changes in water quality parameters at Site 4 before and during the stormflow on 26 August 2000 event. All parameters returned topre-storm values within 12 h after the initial storm surge.

Among the Ephemeroptera, Baetis tricaudatusDodds was the most common species at all sites,comprising 96% of all mayflies collected. It wasthe only mayfly at the downstream sites. Other spe-cies commonly collected at the upstream sites wereEphemerella inermis Eaton and Heptagenia solitariaMcDunnough, although few of these mayflies werecollected after the first year study.

Common Plecoptera at the upstream sites werePteronarcys californica Newport, Pteronarcella ba-dia (Hagen), Isoperla fulva Claassen, and Hes-

peroperla pacifica (Banks). Uncommon species in-cluded Skwala americana (Klapalek) and Claasseniasabulosa (Banks). Stoneflies were rare at the down-stream sites in Surber samples.

I. fulva was occasionally collected at site 3, andnone were found at Site 4 during the period of study.Pteronarcid nymphs were not collected in any Surbersamples at the downstream sites.

The apparent lack of pteronarcid nymphs at thedownstream sites prompted more extensive samplingfor these nymphs during July 2001. Sampling of 3 ha

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Figure 5. Amount of fine sediments (0.25 mm and smaller) and compaction of sediments (in force units) at sampling sites on the lower ProvoRiver. Vertical bars represent ±1 standard error.

of stream channel at sites 3 and 4 yielded only 4nymphs compared to 54 nymphs at Site 1 and 50nymphs at Site 2. Overall, the downstream reduc-tion in pteronarcid nymphs was correlated with theobserved reduction in debris dams (Fig. 6).

Trichoptera collected at all 4 sites were Hydro-psyche oslari (Banks), Brachycentrus occidentalis(Banks), and Hesperophylax sp., although these wereconsiderably less abundant at the downstream sites(Fig. 8). Uncommon species at the upstream sitesand rare or absent at the downstream sites wereArctopsyche grandis (Banks), Cheumatopsyche sp.,Rhyacophila coloradensis Banks, and Micrasema sp.

Other common aquatic insects included the elmidbeetles Optioservus quadrimaculatus (Horn), Sim-ulium arcticum Malloch, and chironomids Orthocla-dius sp., Eukiefferiella sp., and Pagastia sp. Thediamesid Pagastia sp., the only one of these in-sects considered to be a ‘sensitive’ species (Barbouret al., 1999), was present at all sites but in decreasingnumbers downstream (mean density = 707 individu-als m−2 at Site 1, 402 individuals m−2 at Site 2, 175individuals m−2 at Site 3, and 74 individuals m−2 atSite 4).

Common non-insect invertebrates were snails,leeches, and crustaceans. The operculate snail Am-nicola limosa limosa (Say) comprised 99% of allsnails collected and reached its highest mean dens-ity at Site 3 (2090 individuals m−2). Two leeches,Helobdella stagnalis L. and Nephalopsis obscura Ver-rill, were found at all 4 sites and were the most

abundant leeches. Dina dubia Moore & Meyer wasrare at Sites 1–3, and Glossophonia complanata (L.)was uncommon at Site 3. Remaining biomass (lis-ted as ‘other’ in Fig. 8) was mainly comprised of theisopod Caecidotea occidentalis (Williams) and smalloligochaetes.

The distribution of individual species in the lowerProvo River closely reflected their pollution tolerancevalues. Sensitive species comprised 42% of all speciesat Site 1, 37% at Site 2, and 6% at Sites 3 and 4.Tolerant species comprised 17% of all species at Sites1 and 2 and 44% of all species at Sites 3 and 4. Thecommunity tolerance index was 3.8 at Site 1, 3.9 atSite 2, 6.2 at Site 3, and 6.1 at Site 4.

Impervious Surface Cover (ISC)

Significant changes in land use and the amount of ISCat each sampling site have occurred during the past15–25 years (Fig. 10). Site 1 had a 5% increase in ISC(relative to total area), mainly due to the constructionof a picnic area and small parking lot. At Site 2, theISC increased by 43% and the area of riparian forestdecreased by 10% as a result of the construction of ashopping mall in 1997. Sites 3 and 4 had increases inISC of 25–27% relative to total area, primarily from anincrease in new residential housing. Overall, the datafrom these 4 sites show that ISC in the lower ProvoRiver has increased from 17% to 49% of total area inthe past 15–25 years.

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Figure 6. Relationship between abundance of stonefly nymphs (Pteronarcella badia and Pternonarcys californica) and area of organic debrisdams in the lower Provo River. Three channel reaches were sampled at each of the 4 sampling sites.

Discussion and conclusions

The hypothesis that urban stormflows in the ProvoRiver should increase suspended solids, total dissolvedsolids, and the concentrations of heavy metals, anddecrease the amount of dissolved oxygen was con-firmed. Concentrations of suspended solids and metalsincreased with stormflows and dissolved oxygen de-creased in all cases. Total dissolved solids, however,decreased with increasing stormflows, in contrast tostudies in other watersheds (Paul & Meyer, 2001).

The hypothesis that urban reaches should show anincrease in fine sediments in the river channel was notsupported. Fine sediments were similar in amounts atall sites, although compaction increased significantlyat the downstream sites. The lack of change in finesediments is likely due to the increased discharge tosimulate natural spring runoff. When sufficient wa-ter is present in upstream storage (as in 1999–2000),this increase in discharge serves to “flush” the riverof finer sediments and prevent long-term deposition inthe river channel. Furthermore, the intensity of largersummer stormflows may also act to remove fine sed-iments. Changes in sediment texture to more coarsesediments downstream have been observed in otherurban streams (Finkenbine et al., 2000).

The hypothesis that macroinvertebrate communit-ies in the lower Provo River should show a decreasein overall abundance and diversity in urban reachesdue to the loss of sensitive species was confirmed.

Upstream sites had much higher total species diversitythan downstream sites due to the presence of a numberof sensitive species which was reflected in the higherEPT values and community tolerance value. Over-all, the downstream sites showed a typical responsein terms of reduced macroinvertebrate abundance andincrease in tolerant species, particularly snails andleeches (Pratt et al., 1981; Garie & McIntosh, 1986,Jones & Clark, 1987, Kemp & Spotila, 1997).

Changes in water quality resulting from urbanstorm runoff do not appear to have a significant effectin causing differences between upstream and down-stream benthic communities. Water quality at all siteswas similar during normal flows (Table 1), and thechanges caused by stormflows were generally slight orwithin the normal range of variation experience in thestream. Dissolved oxygen, for example, was reducedduring stormflow events, but the magnitude of change,even during large storms, was not much greater thanduring normal flows, particularly when extensive algalgrowths were present. Metal concentrations increasedduring stormflows, but peak concentrations were or-ders of magnitude less than that reported elsewherein urban runoff (e.g., Bryan, 1974; Garie & McIn-tosh, 1986; Liston & Maher, 1986) and are unlikelyto be toxic to benthic organisms (Johnson & Finley,1980). Furthermore, due to the brief nature of thestorm events, water quality changes were also briefwith conditions returning to normal within 12 h aftereven large storm events.

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Figure 7. Total density and biomass (dry mass) of macroinvertebrates at sampling sites on the lower Provo River. Vertical bars represent 95%confidence limits from ln-transformed data (N = 36 samples/site).

Changes in the characteristics of the river sub-strates between upstream and downstream reaches area more likely cause for at least some of the changesin community structure. The reduction in debris dams,likely due to the reduced riparian forest and routineremoval of logs at irrigation diversion structures, wasaccompanied by a reduction in pteronarcid stoneflydensities. These stoneflies are typical shredders (Cum-mins, 1973), feeding on allochthonous detritus, thusthe reduction in debris dams also means a loss of foodresources for these species. Pesacreta (1997) foundhigher mortality of Pteronarcys dorsata in an urbanstream in North Carolina compared to a non-urbanstream in a series of enclosure experiments, althoughthis effect was hypothesized to be due to either siltdeposition or an unknown toxicant. The increasedcompaction of substrates at the downstream sites ef-fectively reduces the total habitat volume availablefor macroinvertebrates, particularly small species andearly instars of larger species for which the interstitialspaces in the substrates are a primary habitat (e.g., thereduction in densities of Pagastia sp.).

A one-time quantitative sampling of macroinver-tebrates communities in riffle habitats of the lowerProvo River was conducted in summer 1975 (Winget,1977) using a Surber sampler with 0.28-mm mesh.One of the sites sampled, corresponding to Site 4 ofthis study, had community values within the range en-countered in this study, i.e., a total density of 2313

individuals m−2 and a total biomass of 0.7 g DM m−2.Seven EPT taxa were collected (compared to 4 in thisstudy), and the community tolerance index was 4.2(compared to 6.1 in this study). There were severaldifferences in the species present in 1975 comparedto this study. The stonefly Isoperla and the mayflyEphemeralla inermis were common in 1975 (86 and635 individuals m−2, respectively), whereas no snailsor leeches were found. In this study, neither Isoperlanor E. inermis were collected at Site 4, and snails andleeches had mean densities during the study period of620 and 121 individuals m−2, respectively.

Mangum (1991) sampled riffle macroinvertebratesat Site 4 in October 1989, April 1990, and July 1990using a Surber sampler with 0.28-mm mesh. By thistime, snails and leeches had reached densities of 43and 265 individuals m−2, respectively. As in Winget’s(1977) study, 7 EPT taxa were collected. The com-munity tolerance index, however, had increased to 5.1.Like this study, neither Mangum (1991) nor Winget(1977) found any pteronarcid stoneflies at Site 4.

Changes in land use, particularly ISC, has beenused as a predictor of water quality degradation inurban watersheds. Arnold & Gibbons (1996) statedthat 10% ISC was the threshold when degradationfirst occurs, 10–30% ISC represents ‘impacted’, and>30% ISC means the stream has been ‘degraded’.Other studies of macroinvertebrate communities inurban streams have indicated significantly changes oc-

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Figure 8. Contribution of macroinvertebrate taxa to total density and biomass at sampling sites on the lower Provo River (combined data forentire sampling period).

cur when the ISC exceeds ca. 15% (e.g., Klein, 1979;Jones & Clark, 1987).

The threshold in ISC noted in previous studiesgenerally applies to the lower Provo River. The 25%increase in ISC at Site 4 corresponded to a decrease of2 units in the community tolerance index. Decreasesin the tolerance index at Site 4 were due to the lossof sensitive EPT taxa and an increase in tolerant taxa,particularly snails and leeches. In contrast, the ISC atSite 1 was at the 10% ISC threshold level of Arnold &

Gibbons (1996), and its macroinvertebrate communitymetrics had changed little since 1975.

Prior macroinvertebrate data are not available forSites 2 and 3. The ISC at Site 2 followed nearlyidentical increases as occurred at Site 4 (Fig. 10), andboth sites have similar macroinvertebrate communitiesat present. Although Site 3 showed the greatest changein ISC, this change is recent. Thus, Site 3 has not yetreached the levels of substrate compaction or loss ofdebris dams experienced at the downstream sites. Con-

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Figure 9. Contribution of Ephemeroptera, Plecoptera, and Trichoptera (EPT) to total species, total density, and total biomass at sampling siteson the lower Provo River.

Figure 10. Land use from aerial photographs at each of the study sites (before 2000, photos were not taken of all areas at the same time).‘Riparian’ refers to forested areas along the river channel; ‘Fields’ refers to agricultural pastures, parks, or native shrubs.

sequently, the macroinvertebrate community is stillmost similar to that of Site 1, although with lower totaldensity. The long-term changes in the macroinverteb-rate community at Site 4 suggest there may be a timelag of a decade or more for changes in river habitats as-sociated with urban development to be readily appar-ent in the macroinvertebrate community composition.The area around Site 3 is undergoing rapid commercial

and residential development, so stormflows are likelyto affect it more in the future.

Overall, changes in water quality resulting fromurban stormflows in the lower Provo River are relat-ively slight and short-term in nature. The long-termchanges in substrates and land use (particularly theincrease in ISC) with increasing urbanization are thelikely cause behind the changes in macroinvertebratecommunity composition, and the changes at Site 4 are

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a useful indicator of the potential for changes in thefuture with increasing development.

Changes in the substrates and macroinvertebratecommunities of the lower Provo River have occurredgradually during the past two decades as the degreeof urbanization has increased. The extent of thesechanges may be regarded as a minimal baseline fromwhich to compare other urban river systems where ad-ditional, confounding variables are present, such asadditional pollution from industrial activity, channel-ization or wastewater discharges. In addition, changesin the lower Provo River show that urban storm-flows have an impact even in semi-arid regions wherestormflows are relatively infrequent.

Acknowledgements

Dr Reed Oberndorfer, Central Utah Water Conserva-tion District, provided discharge data and facilitatedthe metal analyses. Melissa Davis, UVSC biologystudent, collected the data on pteronarcid abundanceand debris dams. Dr Walter Dodds, Kansas StateUniversity, and Dr Sam Rushforth, UVSC, kindly re-viewed drafts of the manuscript. The UVSC FacultyDevelopment Fund provided financial assistance forequipment.

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