Assessing long-term fish responses andshort-term solutions to flow regulation in adryland river basinThomas K. Pool, Julian D. OldenSchool of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195, USA

Accepted for publication January 6, 2014

Abstract – Water resource development and non-native species have been cited as primary drivers associated withthe decline of native fishes in dryland rivers. To explore this topic, long-term trends in the fish communitycomposition of the Bill Williams River basin were studied over a 30-year period (Arizona, USA). We sampled 31sites throughout the basin that were included in fish surveys by Arizona Game and Fish in 1994–97 and the Bureauof Land Management in 1979–80. We found that non-native species have proliferated throughout the entire basin,with greater densities in the lower elevations. Native species have persisted throughout most of the major riversegments, but have experienced significant declines in frequency of occurrence and abundance in areas alsocontaining high abundances of non-native species. Next, we assessed the short-term response of the fish assemblageto an experimental flood event from the system’s only dam (i.e. Alamo Dam). In response to the flood, we observeda short-term reduction in the abundance of non-native species in sites close to the dam, but the fish assemblagereturned to its preflood composition within 8 days of the event, with the exception of small-bodied fish, whichsustained lower postflood densities. Our findings demonstrate the importance of natural flow regime on the balanceof native and non-native species at the basin scale within dryland rivers and highlight minimal effects on non-nativefishes in response to short duration flood releases below dams.

Key words: Native fish conservation; non-native species; experimental flood; arid river systems

Introduction

Modification of rivers by means of dams and diver-sions can be a significant form of disturbance influ-encing one or more components of the river’s naturalflow and thermal regime (Poff et al. 2007; Olden &Naiman 2010). In particular, stabilising flows withindryland rivers, which have historically been charac-terised by a high degree of hydrologic variability, canadversely affect native fishes’ survival by changingspawning habitat, altering food resource availabilityand constraining dispersal (Bunn & Arthington 2002;Gido & Propst 2012). The conservation of nativefishes in regulated systems is further complicatedbecause flow regime alteration is often accompaniedby the downstream proliferation of non-native species(Cucherousset & Olden 2011), which exert strongnegative effects via competition and predation (Propst

& Gido 2004; Propst et al. 2008). Furthermore, damsmay influence river fish composition through the cre-ation of reservoirs, serving as hubs for non-nativespecies’ dispersal into upstream reaches (Falke &Gido 2006; Johnson et al. 2008).One approach for mitigating the negative physical

and biological effects of dams on riverine ecosystemsis the release of high flows from dams to mimic par-ticular aspects of the natural flow regime (Konradet al. 2011). In recent years, high-flow experimentshave been performed globally, ranging from discreteevents to multiyear altered flow releases (Olden et al.in press). Applying this method to regulated arid sys-tems by implementing flows that mimic the quantity,quality and timing of the natural flow regime isexpected to benefit native species, whilst potentiallysuppressing non-native species. For example, in theregulated lower Putah Creek of California, it was

Correspondence: T. K. Pool, School of Aquatic and Fishery Sciences, University of Washington, 1122 NE Boat Street, Seattle, WA 98195, USA.E-mail: tpool@uw.edu

56 doi: 10.1111/eff.12125

Ecology of Freshwater Fish 2015: 24: 56–66 � 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

ECOLOGY OFFRESHWATER FISH

found that native species regained dominance overnon-native species in a majority of downstream sitesfollowing systematic dam releases that were designedto mimic the seasonal increases and decreases of theriver’s natural flow (Kiernan et al. 2012). At a largerscale, multiyear changes in annual flow volume andthe implementation of multiple controlled floods fromGlen Canyon Dam produced a reduction in non-nativerainbow trout (Oncorhynchus mykiss) abundance inthe downstream mainstem Colorado River; however,many the displaced trout appeared to simply colonisea downstream tributary stronghold for endangerednative fish. Given the potential of flow manipulationsas an effective management tool, research on thismethod has increased in the last decade, but thereremain relatively few studies that have quantifiedcommunity-level responses (Olden et al. in press).In the American Southwest, the combination of

water development and non-native species threatshave resulted in one of the most endangered fish fau-nas in the world, fuelling conservation concerns thatmany, if not most, of the region’s endemic nativefishes may not persist into the future (Minckley et al.2003; Olden et al. 2008; Pool & Olden 2012). In our

study, we examined fish faunal change in the BillWilliams River basin (BWRB) (Arizona, USA) over a30-year period. Throughout the basin, we identifiedthe native and non-native species responsible for dec-adal shifts in composition, both in the regulated flowregime below the basin’s only major dam (i.e. AlamoDam) and in the free-flowing regimes in the upperbasin. We coupled this long-term study with a short-term assessment of the fish assemblage response to anexperimental flood event below the Alamo Dam. Weevaluated the potential benefit of this flood event todisplace non-native fishes downstream. By integratinga long-term investigation of fish faunal change with ashort-term study of flow management, we provideinsight into the influence of multiple threats on nativefish persistence in dryland river systems.

Methods

Study area

The BWRB drains over 13,000 km2 (4 m3�s�1 aver-age discharge) of terrain in western Arizona and con-stitutes a significant tributary to the lower Colorado

Trout C.

Utah

NewMexico

Mexico

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Francis C.

Burro C.

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0 5 10 20

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Santa Maria

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Fig. 1. Fish survey locations in the BillWilliams River basin of western Arizona,USA. Sample locations are indicated withthe following symbols: Bill WilliamsRiver,○; lower Burro Creek,♢; middleBurro Creek, ; upper Burro Creek,♦; BigSandy River,△; Boulder Creek,■; FrancisCreek,●; Trout Creek,▲; and Santa MariaRiver, . Sites along the Bill WilliamsRiver included: Rankin Ranch (RK),Pipeline Crossing (PL), Swansea Mine(SP), Mohave Wash (MH) and MineralWash (MN).

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Fish responses to flow regulation in a dryland river

River. Vegetation ranges from Sonoran desert scruband Great Basin conifer woodland in the headwaters,to dense riparian forests, dominated by native cotton-wood (Populus fremontii) and willow (Salix gooddin-gii) trees on the floodplains. Additionally, extensivestands of the non-native saltcedar (Tamarix spp.) andnative mesquite (Prosopis spp.) are found on the ter-races lower in the basin (Shafroth et al. 2002). Theupper portion of the basin consists of low-order tribu-taries with a mixture of ephemeral, intermittent andperennial flows that drain into the Santa Maria(3727 km2) and Big Sandy Rivers (5149 km2)(Fig. 1). These unregulated rivers have sections ofboth intermittent and perennial flow (Fig. 2a,b) enter-ing into the Alamo Reservoir, which subsequentlyreleases into the perennial mainstem Bill WilliamsRiver. No perennial tributaries enter the Bill WilliamsRiver downstream of the Alamo Dam. After flowingca. 58 km through a series of canyons and alluvialvalleys, the Bill Williams River reaches its conflu-ence with the Colorado River (in Lake Havasu).Human land use in the basin is fairly limited althoughsome grazing and water extraction for livestock, agri-culture and human consumption occurs (ArizonaDepartment of Water Resources. 2011).The Alamo Dam was constructed in 1968 below

the confluence of the Big Sandy and Santa Maria Riv-ers, creating the Alamo Reservoir (storage capacity of1233 9 106 m3), thereby stabilising river flows forflood control and recreation purposes (Shafroth et al.2010). Prior to construction of the dam, large floods

>1700 m3�s�1 occurred approximately every 10 yearsin the Bill Williams River. The dam is the basin’sonly major impoundment with present-day base-flowreleases typically reaching 0.3 to 1.4 m3�s�1 (U. S.Army Corp of Engineers. 2003) with a > 90% reduc-

(a)

(b)

(c)

Fig. 2. Mean daily discharge for the duration of our study, from 1980 to 2010, in the (a) Big Sandy River (USGS stream gage 09424450),(b) Santa Maria River (gage 09424900) and below Alamo Dam in the (c) Bill Williams River (gage 9426000), Arizona, USA. Recordswere not available for the Santa Maria from 1986 to 1988. High flow events in the Bill Williams River starting in 2005 represent experi-mental flow releases from Alamo Dam.

Table 1. Native (N = 5) and non-native (N = 11) fish captured within theBill Williams River basin (BWRB) during our 30-year study period.

OriginSpecies Code

First observedin the BWRB

Native speciesAgosia chrysogaster, longfin dace AGCH –Catostomus insignis, Sonora sucker CAIN –Catastomus clarkii, desert sucker CACL –Gila robusta, roundtail chub GIRO –Rhinichthys osculus, speckled dace RHOS –Non-native speciesAmeiurus melas, black bullhead AMME 1965Ameiurus natalus, yellow bullhead AMNA 1950Cyprinella lutrensis, red shiner CYLU 1964Cyprinus carpio, common carp CYCA 1963Gambusia affinis, western mosquitofish GAAF 1980Ictaluris punctatus, channel catfish ICPU 1994Lepomis cyanellus, green sunfish LECY 1965Lepomis macrochirus, bluegill LEMA 1994Micropterus salmoides, largemouth bass MISA 1994Pimephales promelas, fathead minnow PIPR 1991Carassius auratus, goldfish† 2008

Notes: Records from the lower Colorado River Aquatic GAP Programme(Whittier et al. 2006), primary literature (Olden & Poff 2005), and stateagency reports were used to estimate the date of non-native species intro-duction into the BWRB.†Two individuals of this species were identified, but they were not includedin the composition analysis.

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tion in the magnitude of high-flow events from histor-ical conditions (Shafroth et al. 2010) (Fig. 2c). Tim-ing of high flows has also changed. Prior to theclosure of Alamo Dam, flows occurred in both win-ter–spring and late summer–autumn; but postdamyears have seen the virtual elimination of the latter.Additionally, beaver (Castor canadensis) have colon-ised throughout the low-gradient reaches of the BillWilliams River; resulting in approximately 104 bea-ver pond habitats (Andersen & Shafroth 2010) thatundoubtedly further alter the river’s hydrology, geo-morphology, chemistry and ecology (Gibson & Olden2014). Whilst beaver historically occurred throughoutthe American Southwest, regular extreme floodevents in the BWRB combined with an absence ofearly reports by trappers and explorers referencing theanimal, suggest that beaver were not common in theBill Williams River prior to the altered flow regime(Shafroth et al. 2010).

Long-term changes in fish faunal composition

Our study examined distributional trends for theentire BWRB fish fauna (five native and 11 non-native species) over a 30-year time period (1980 topresent) (Table 1). Sites throughout the basin wereoriginally sampled in 1979–80 as part of a UnitedStates Bureau of Land Management (BLM) inventoryof aquatic species (Kepner 1979, 1980). The BLMteam collected fish using a combination of single-pass backpack electrofishing (20 to 30m long seg-ments) and seine netting (mesh sizes of 32, 6.4 and3.2 mm) to sample a variety of mesohabitat types ateach site (i.e. water velocities, depths and substratetypes). Approximately 15 years later, in 1994 and1996, the Arizona Game and Fish Department(AGFD) resampled the BLM sites, using the samesampling protocol and effort per site employed bythe BLM team, thereby providing a comparableassessment of the basin’s fish fauna over time(Young et al. 1994; Fresques et al. 1997; Morganet al. 1997). Most recently, in 2008 and 2011(approximately 15 years after the AGFD survey), wesampled 31 sites that had been part of both the BLMand AGFD surveys. We recorded species’ occurrenceand relative abundance at each site, and once again,employed the same sampling protocol and effort asthe previous surveys (Fig. 1). Lastly, records fromthe Lower Colorado River Basin Aquatic Gap Analy-sis Project (Whittier et al. 2006; Pool et al. 2010)and the primary literature (Olden & Poff 2005) wereused to determine that all five native fish species hadhistorically (circa 1900) occurred throughout all ofthe major river segments in the basin.Despite our efforts to mimic the sampling extent

and effort originally employed by BLM, subtle varia-

tion between surveys can influence patterns associatedwith species’ relative abundance. Therefore, species’presence/absence data were used to assess shifts innative and non-native species’ richness, from the his-torical time period to all subsequent time periods. Thisdecision is supported by Jackson & Harvey (1997)who demonstrated that species’ presence/absence issuccessful in elucidating community patterns in situa-tions where it is difficult to obtain reliable relativeabundance estimates. We recognise that this choice ofdata resolution may influence our interpretation ofcommunity change over time (Rahel 1990). The con-temporary fish fauna was then assessed in furtherdetail using the species relative abundance data col-lected during our 2008 and 2011 sampling to comparethe current dominance of native and non-nativespecies throughout the basin.

Short-term fish assemblage response to an experimentalflood event

In 2004, the Sustainable Rivers Project broughttogether The Nature Conservancy and the U.S. ArmyCorp of Engineers to develop an environmental flowplan for the Alamo Dam with the primary goal ofmaintaining endemic riparian cottonwood–willowforests along the river (Konrad et al. 2012). Begin-ning in early 2005, the plan called for one initial lar-ger flow event (204 m3�s�1 peak discharge),followed by smaller high-flow events in March of2006 (69 m3�s�1 peak discharge) and 2007 (max52 m3�s�1 peak discharge) (Shafroth et al. 2010). Asubsequent experimental flood event in March of2008, with a 16-h duration and peak discharge of65 m3�s�1, provided the opportunity to study theshort-term effect of an increased discharge onthe downstream fish community (Fig. 2c). Surveyswere conducted at five sites on the Bill WilliamsRiver – Rankin Ranch, Pipeline Crossing, SwanseaMine, Mojave Wash and Mineral Wash (Fig. 1) –with single-pass electrofishing a week prior to theflow event and 2 days after the flow event, whenbase-flow conditions had reestablished. The siteswere selected because they ranged from 17.7 to44.6 km downstream from Alamo Dam, thereby pro-viding an opportunity to evaluate whether distancefrom the dam (and hence the attenuation of the exper-imental high-flow event) influenced fish assemblageresponse. Further sampling was conducted at theupstream sites closest to Alamo Dam – Rankin, Pipe-line and Swansea – at five and eight days after theflow event. At each site, we sampled four 30-m-longreaches (replicates) using the same collection proto-col utilised in the other river segments of the basin.To assess the short-term change in the Bill Williams

River fish assemblage associated with the experimen-

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Fish responses to flow regulation in a dryland river

tal flood event, we used nonmetric multidimensionalscaling (NMDS) to identify variation in species’ abun-dances across space and time. NMDS is an ordinationmethod that preserves the rank-ordered distancesbetween sample points in ordination space and, forour purposes, provides a useful approach for visualis-ing differences in fish community composition.NMDS uses an iterative approach that rearranges sam-ples in ordination space to minimise a measure of dis-agreement (referred to as ‘stress’) between thecompositional dissimilarities and the distance betweenthe points in the ordination diagram (Kruskal 1964). Adistance matrix based on the Bray–Curtis dissimilaritycoefficient was then used to ordinate the sample sitesin two dimensions, using ten random starts (a compar-ison of stress versus dimensionality supported theinterpretation of two dimensions). Site/day combina-tions with similar community compositions arelocated closer together in multidimensional space,whereas site/day combinations with dissimilar com-munities are positioned farther apart (Kruskal & Wish1978).

Results

Changes in the fish fauna over a 30-year period

In the upper BWRB, fish composition of the majorheadwater tributaries of the Big Sandy River (Trout,Francis and Boulder Creeks) demonstrated consider-ably less change over time in comparison with thelower basin (Fig. 3). Native species’ occurrence inthese creeks remained relatively stable over time,with longfin dace and roundtail chub (Gila robusta)numerically dominating the contemporary fishes(Fig. 4), and only the loss of Sonora sucker (Catosto-mus insignis) in Trout Creek from historical times(Fig. 3a). Non-native species’ richness increased byone or two species in these headwater tributaries,(Fig. 3b) related primarily to the establishment ofgreen sunfish, fathead minnow (Pimephales prom-elas) or red shiner (Fig. 4).The lower reaches of the BWRB, including the

mainstem Bill Williams River below Alamo Dam andthe Big Sandy River, displayed substantial changes innative species’ composition over the last 30 years(Fig. 3a). The Bill Williams River lost its entire suiteof native species by 1980 after the construction ofAlamo Dam, whereas the Big Sandy River has seenits native species’ richness reduced by half in each ofthe time periods examined, caused in part by the lossof native desert sucker (Catostomus clarkii) andspeckled dace (Rhinichthys osculus). Concurrent tothese native declines is the establishment of numerousnon-native species, especially in the Bill WilliamsRiver (now supporting eight non-native species), Big

Sandy River and Burro Creek (Fig. 3b). These sys-tems are now dominated by non-native red shiner(Cyprinella lutrensis), western mosquitofish (Gambu-sia affinis), green sunfish (Lepomis cyanellus) andcommon carp (Cyprinus carpio) (Fig. 4). Exceptionsto this lower basin pattern are in the intermittent lowerreaches of the Big Sandy and Santa Maria Rivers,where the native longfin dace (Agosia chrysogaster)are locally abundant (Fig. 4) and non-native richnessis relatively low (Fig. 3b).

Short-term fish assemblage responses to anexperimental flood

Immediately following the flood event (i.e. day 2),the majority of the Bill Williams River sites experi-enced a reduction in total non-native fish abundance(Fig. 5a) and displayed changes in their overall com-positions (Fig. 5b). The most substantial initial shiftsoccurred in the three upstream sites near the dam (i.e.Rankin, Pipeline and Swansea; Fig. 5a), displaying adecreased mean abundance of western mosquitofish,red shiner and yellow bullhead. Two small-bodiedspecies – red shiner and western mosquitofish –remained in reduced abundances for the remainder ofour sampling period (Table 2). However, 8 days afterthe flood event, the upstream sites had progressedback towards preflood compositions (Fig. 5b) withtotal fish abundances at Rankin, Pipeline and Swan-sea, respectively, only 42%, 19% and 15% lowerthan before the event (note: only the upstream siteswere sampled on days 5 and 8; Fig. 5a; Table 2). Incontrast to other non-native species in our study,green sunfish initially increased in mean abundancein the upstream Bill Williams River sites during post-flood sampling, followed by decreases in abundanceby days 5 and 8 (Table 2).

Discussion

The biogeography of freshwater fishes across theAmerican Southwest has significantly changed in thelast century (Olden & Poff 2005; Moyle & Marchetti2006; Gido et al. 2013), and the BWRB is no excep-tion. Our study illustrates how the local loss of nativespecies, and the spread of non-native species, hasmodified the structure of the fish fauna over a30-year period. Fish assemblage composition in thelower portion of the basin (below and immediatelyupstream from Alamo Dam) has shifted to a non-native species dominated fauna (100% and 90% non-native relative abundance in the Bill Williams Riverand lower Burrow Creek, respectively); whereas theheadwaters have been invaded, but remain primarilynative (79% and 98% native relative abundance cur-rently in Francis Creek and Trout Creek, respec-

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tively). This suggests that the natural flow regimes inthe upper watershed, in combination with greater dis-tance from the Alamo Reservoir, have contributed tothe persistence of native fishes. By contrast, thealtered flow regime below the Alamo Dam andincreased upstream connectivity with the associatedreservoir have supported the establishment and localdominance of non-native fishes both downstream andupstream of the dam.Previous studies suggest that natural flow regimes

can influence the composition of fish communities bypromoting native species’ persistence and providingenvironmental resistance to the proliferation of non-native species (Marchetti et al. 2001; Kiernan et al.

2012; Gido et al. 2013; Mims & Olden 2013). Insupport of this notion, we found that the present-dayheadwaters of the BWRB, which are unregulated anddrain largely unimpacted landscapes, maintained thehighest native species’ richness and abundances overthe past decades. Interestingly, headwater habitats arein fact invaded (every site has been invaded for>15 years), but it appears that under natural flowconditions, native and non-native fishes may be ableto coexist over comparatively long time periods.These results indicate that although maintaining natu-ral flow regimes will not provide complete resistanceagainst the invasion on non-native fishes, the long-term persistence of native fishes within invaded

(a)

(b)

Fig. 3. Radar plots depicting (a) native and(b) non-native species richness withinmainstem reaches and major tributaries of theBill Williams River basin, Arizona, USA.Sites presented clockwise from the BillWilliams River mainstem according toincreasing elevation. Richness estimatesbased on multiple survey locations (seeFig. 1) for each time period. Historical (circa1900) native species richness is assumed torepresent five species and fills the entire plotin panel (a).

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Fish responses to flow regulation in a dryland river

watersheds may be a realistic conservation goal insome unregulated river segments. Notably, nativespecies’ persistence in unregulated river segmentsmay also depend on the identity of the non-nativespecies entering the system (Bunn & Arthington2002). For example, Stefferud et al. (2011) foundthat native fishes experienced the greatest declines indiversity and abundance in association with the intro-duction of smallmouth bass (Micropterus dolomieui)into the minimally impacted and unregulated GilaRiver catchment (New Mexico, USA). Althoughthere have been no basin-wide extirpations of nativespecies upstream from Alamo Dam, native fish in theupper basin should continue to be monitored as localwater resource projects develop, and climate changefurther impacts the magnitude and timing of riverflows in the basin (Sabo et al. 2010).Changes in the fish composition upstream of Alamo

Dam suggest a mutual negative influence of non-native species and reservoir connectivity on the occur-rence and abundance of native species. Although afew non-native species already occurred in the basin

before the dam’s construction (e.g. black bullhead,Ameiurus melas; common carp), Alamo Reservoircontains at least 15 non-native species (Young et al.1994), due in part to stocking for recreational fishingpurposes. In some of the lower-gradient river seg-ments upstream of the reservoir, including lowerBurro Creek and middle Burro Creek, non-nativespecies with ecological traits commonly favoured inlentic environments currently dominate the fishassemblage (e.g. green sunfish, red shiner, yellowbullhead). In a study by Falke & Gido (2006) thatexamined sites upstream of reservoirs throughout theGreat Plains, the authors also found non-native specieswith similar microhabitat and substrate preferences tothe non-native species in our study, indicating thatspecies well adapted for reservoir conditions may bespreading to upstream reaches. Interestingly, inreaches upstream of the reservoir, multiple compo-nents of the natural flow regime (i.e. intermittent flow,flood events) may be protecting native species fromlocal extirpation associated with non-native speciescompetition and predation. In fact, the intermittent

BWLower basin Upper basin

0.00.10.20.30.40.50.6

Rela

tive

abun

danc

e 0.70.80.91.0

BS SM IBU mBU uBU BO FR TR

Fig. 4. Present-day (2008–2011) fish assemblage composition in mainstem reaches and major tributaries of Bill Williams River basin. Theproportional contribution of abundance for each native (solid bars) and non-native (striped bars) species is represented for each major riversegment (species contributing to <5% of the relative abundance across all sites were grouped together as white bars labelled ‘other’). Studyreaches are ordered in increasing elevation from left to right and include the Bill Williams River (BW), Big Sandy River (BS), Santa MariaRiver (SM), lower Burro Creek (lBU), middle Burro Creek (mBU), upper Burro Creek (uBU), Boulder Creek (BO), Francis Creek (FR)and the Trout Creek (TR) (Fig. 1).

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flow occurring at our the Big Sandy and Santa MariaRiver sites may account for the high abundances ofnative longfin dace and relatively low non-native spe-cies’ richness in those areas. Additionally, Rinne et al.(2005) found that significant natural flood events par-alleled a rebound in native fish abundance upstream ofimpoundments in the Verde River system, suggesting

that occasional high flows in our Big Sandy and SantaMaria River sites might also be sustaining native spe-cies’ persistence.In the tailwaters of the BWRB below Alamo Dam,

native species have been replaced with an entirelynon-native species assemblage, highlighting thestrong association between regulated flows and

(a)

(b)

Fig. 5. (a) Total fish abundance within sites (N = 5) along the Bill Williams River following the 2008 experimental flood event. Fish weresampled 2 days after the event at all five sites and on days 5 and 8 at the upstream Rankin, Pipeline and Seapipe (note: sites closest toAlamo dam). (b) Nonmetric multidimensional scaling (NMDS) displaying the change in fish abundance at five Bill Williams River sites(Rankin, Pipeline, Seapipe, Mohave and Mineral) preflood and on day 2 postflood event (Fig. 1). The upstream sites (Rankin, Pipeline andSwansea) were also sampled on days 5 and 8 postflood event. Small arrows identify species contributions to the NMDS labelled with eachspecies’ four-letter code (see Table 1). The two-dimensional solution had a stress of 13.22 (P = 0.009 with 9999 permutations) and anonmetric fit (R2) of 0.916.

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non-native fish occurrence. Despite conservationattempts to repatriate the river with native speciessuch as longfin dace, roundtail chub and Sonorasucker after construction of the dam (Young et al.1994), there has been no record of native species inthe contemporary fish assemblage. These results sup-port the assertion that native fish management strate-gies should embrace opportunities for flowrestoration releases from dams, in addition to moretraditional efforts such as non-native predatorremoval, particularly in dryland rivers where nativefish species are often well adapted for a high degreeof hydrologic variability. Undeniably, efforts toremove non-native predators from throughout thelower Colorado River mainstem have had negligiblesuccess in promoting the recovery of native fish spe-cies (Mueller 2005), providing further support for thenotion that simultaneously addressing the two-partthreat of flow alteration and non-native species’occurrence is critical for native fish recovery in dry-land rivers.Whilst it is promising that some portions of the

basin have maintained long-term (i.e. multidecadal)mixed assemblages of native and non-native fishes,the balance of species coexistence should be regardedwith caution. Numerous examples exist where spe-cies’ extinctions following environmental distur-bances (e.g. Vellend et al. 2006) were considerablydelayed, leading to what are considered extinctiondebts (Kuussaari et al. 2009). For example, in theColorado River mainstem below Glen Canyon Dam(constructed in 1963), despite the introduction of sev-eral predatory non-native species and the alteration offlow, temperature and sediment regimes within theriver, the federally endangered humpback chub (Gilacypha) has persisted in some portions of its nativerange. Unfortunately, diminished reproductive suc-cess and juvenile survival associated with thosethreats now indicate that the future of this endemicspecies is in jeopardy after more than fifty years of

decline. Accordingly, the century-long prospects fornative species within mixed assemblages in theBWRB are unknown.The 2008 experimental flood in the Bill Williams

River provided a unique opportunity to study thecapacity of a single high-flow event to displace non-native fishes. Whilst the event resulted in an initialdecrease in non-native species abundance suggestingthat high-flow experiments may be a viable manage-ment option, only a small subset of the non-nativespecies displayed a sustained reduction in their densi-ties after the flow event. In agreement with previouswork exploring the impact of single high-flow eventson fish assemblages (Thompson et al. 2011), wefound that smaller-bodied species, including red shi-ner and western mosquitofish, were more effectivelydisplaced by the flood compared with larger-bodiedspecies, including largemouth bass and yellow bull-head. Reduced swimming performance and a lack ofappropriate behavioural responses associated withsmaller-bodied non-native fishes may have resultedin this pattern of species displacement (Meffe 1984).This may have been particularly true for small-bodiedspecies in the upstream sites nearest to the dam,where the fish experienced the largest effect of theexperimental flow event. Importantly, the timing andmagnitude of the event were designed to benefitriparian zone vegetation, with little consideration ofresponses by the fish communities (Shafroth et al.2010). This narrowly focused management approachof targeting a single taxonomic group for experimen-tal floods remains a commonplace practice with flowrestoration efforts (Olden et al. in press). More sys-tematic and substantial high-flow events may producemore sustained removal of non-native species, partic-ularly if events target non-native species spawningperiods within the system. Whilst complete extirpa-tion of non-native fishes from the Bill Williams Riveris not a realistic management goal given the abundantsupply of non-native species entering from the main-stem Colorado River, high-flow events combinedwith habitat restoration and native fish repatriationefforts might permit a mixed native and non-nativefish assemblage to exist in this portion of the BWRB.In addition to the natural hydrologic and geomor-

phic features that vary longitudinally along the BillWilliams River, a factor that may have contributed tothe attenuated displacement of fishes from upstream todownstream sites was the presence of beaver damsinterspersed throughout the river. Beaver dams havethe capacity to reduce stream velocity conditions,minimising the flood severity experienced by fishes byproviding refuge habitat during increased flow periods(Gibson & Olden 2014). Consequently, the marginalincreases in green sunfish abundances (i.e. a speciesknown to occupy beaver ponds) may have occurred

Table 2. The mean abundance (standard deviation) of fish species atRankin, Pipeline and Swansea sites (Fig. 1) in the Bill Williams River beforeand after (days 2, 5 and 8) the experimental flood event. All species arenon-native in origin; no native fish were collected in 2008.

Pre-experimental flood

Post-experimental flood

t = 2 t = 5 t = 8

Red shiner 114 (65) 62 (41) 79 (23) 82 (42)Westernmosquitofish

59 (36) 17 (7) 28 (10) 34 (15)

Yellow bullhead 19 (9) 8 (6) 18 (4) 21 (4)Common carp 6 (1) 1 (2) 4 (3) 4 (1)Largemouth bass 4 (1) 1 (2) 2 (2) 4 (3)Green sunfish 3 (3) 9 (9) 5 (3) 3 (4)Channel catfish 2 (2) 0 (0) 1 (1) 0 (0)Bluegill 0 (0) 1 (1) 1 (2) 1 (1)

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because the flood event resulted in short-distance dis-placement of this species from beaver ponds. If beaverdams and ponds do mediate the ecological response toexperimental floods, then it will be critical to under-stand how specific aspects of a flow event (e.g. flowduration, rate of increase to maximum discharge, max-imum discharge rate) relate to beaver dam failure. Cur-rently, our ability to make specific managementrecommendations for instituting flow events in sys-tems with beaver populations is limited, indicating thatfurther work is needed to understand the impact ofexperimental flood events on beaver dam failure(Andersen & Shafroth 2010; Andersen et al. 2011).In conclusion, we demonstrated that the long-term

balance between native and non-native fish species inthe BWRB has significantly changed in the past threedecades, with the most profound shifts in compositionassociated with the basin’s only major dam. Given thestrong connection between a river’s natural flowregime and the range and abundance of its native fishspecies, embracing management approaches thatinclude well-designed flood events in regulated sys-tems may be essential for native species’ persistence.As climate change continues to alter precipitationregimes in many arid systems (Seager et al. 2007),discrete flood events may have to be integrated intomore comprehensive environmental flow efforts thatmimic multiple aspects of the natural flow regime todisplace non-native species and sustain the native spe-cies’ assemblages.

Acknowledgements

We thank Angela Strecker (Portland State University) andMark Kennard (Griffith University) for assistance in the fieldand Andrew Hautzinger (USFWS), Pat Shafroth (USGS),Dick Gilbert (USFWS), Kathleen Blair (USFWS), MitchThorson (USFWS), Doug Andersen (USGS) and Dave Lytle(Oregon State University) for providing logistical support atdifferent times throughout the project. We also thank ClaireHorner-Devine, Jennifer Ruesink, Christian Torgersen and twoanonymous reviewers for their thoughtful comments. Partialfunding support was provided by the USGS National GapAnalysis Program, the USGS Status and Trends Program anda T&E Inc. Conservation Biology Research Grant.

References

Andersen, D.C. & Shafroth, P.B. 2010. Beaver dams, hydro-logical thresholds, and controlled floods as a managementtool in a desert riverine ecosystem, Bill Williams River, Ari-zona. Ecohydrology 3: 325–338.

Andersen, D.C., Shafroth, P.B., Pritekel, C.M. & O’Neill,M.W. 2011. Managed flood effects on beaver pond habitatin a desert riverine ecosystem, Bill Williams River, ArizonaUSA. Wetlands 31: 195–206.

Arizona Department of Water Resources. 2011. Land owner-ship in the Bill Williams Basin (available at www.azwater.gov, accessed 2 November 2011).

Bunn, S.E. & Arthington, A.H. 2002. Basic principles andecological consequences of altered flow regimes for aquaticbiodiversity. Environmental Management 30: 492–507.

Cucherousset, J. & Olden, J.D. 2011. Ecological impacts ofnon-native freshwater fishes. Fisheries 36: 215–230.

Falke, J.A. & Gido, K.B. 2006. Effects of reservoir connectiv-ity on stream fish assemblages in the Great Plains. CanadianJournal of Fisheries and Aquatic Sciences 63: 480–493.

Fresques, T.D., Liles, T.A. & Esparza, F.M. 1997. Native fishsurveys of the Big Sandy, Hassayampa, and Santa MariaRiver drainages - Technical Report. Arizona Game and FishDepartment, Kingman, Arizona.

Gibson, P.R. & Olden, J.D. 2014. Ecology, management, andconservation implications of North American beaver (Castorcanadensis) in dryland streams. Aquatic Conservation, DOI:10.1002/aqc.2432.

Gido, K.B. & Propst, D.L. 2012. Long-Term dynamics ofnative and nonnative fishes in the San Juan River, New Mex-ico and Utah, under a partially managed flow regime. Trans-actions of the American Fisheries Society 141: 645–659.

Gido, K.B., Propst, D.L., Olden, J.D. & Bestgen, K.R. 2013.Multidecadal responses of native and introduced fishes tonatural and altered flow regimes in the American Southwest.Canadian Journal of Fisheries and Aquatic Sciences 564:554–564.

Jackson, D.A. & Harvey, H.H. 1997. Qualitative and quantita-tive sampling of lake fish communities. Canadian Journal ofFisheries and Aquatic Sciences 54: 2807–2813.

Johnson, P.T.J., Olden, J.D. & Vander Zanden, M.J. 2008.Dam invaders: impoundments facilitate biological invasionsinto freshwaters. Frontiers in Ecology and the Environment6: 357–363.

Kepner, W.G. 1979. Aquatic inventory of the upper Bill Wil-liams drainage, Yavapai, and Mohave counties, Arizona -Technical Note 352. U.S. Bureau of Land Management,Phoenix District Office.

Kepner, W.G. 1980. Aquatic Inventory of the Bill Williamsand Hassayampa drainages - Technical Note 354. U.S.Bureau of Land Management, Phoenix District Office.

Kiernan, J.D., Moyle, P.B. & Crain, P.K. 2012. Restoringnative fish assemblages to a regulated California streamusing the natural flow regime concept. Ecological Applica-tions 22: 1472–1482.

Konrad, C.P., Olden, J.D., Gido, K.B., Hemphill, N.P., Ken-nard, M.J., Lytle, D.A., Melis, T.S., Robinson, C.T.,Schmidt, J.C., Bray, E.N., Freeman, M.C., McMullen, L.E.,Mims, M.C., Pyron, M. & Williams, J.G. 2011. Large-scaleflow experiments for managing rivers. BioScience 61: 948–959.

Konrad, C.P., Warner, A. & Higgins, J.V. 2012. Evaluatingdam re-operation for freshwater conservation in the sustain-able rivers project. River Research and Applications 28:777–792.

Kruskal, J.B. 1964. Nonmetric multidimensional scaling: anumerical method. Psychometrika 29: 115–129.

Kruskal, J.B. & Wish, M. 1978. Multidimensional scaling.Sage University Paper series on Quantitative Applications in

65

Fish responses to flow regulation in a dryland river

the Social Sciences, number 07-011. Sage Publications:Newbury Park, CA.

Kuussaari, M., Bommarco, R., Heikkinen, R.K., Helm, A.,Krauss, J., Lindborg, R., Ockinger, E., Partel, M., Pino, J.,Roda, F., Stefanescu, C., Teder, T., Zobel, M. & Steffan-Dewenter, I. 2009. Extinction debt: a challenge for biodiver-sity conservation. Trends in Ecology & Evolution 24: 564–571.

Marchetti, M.P., Light, T., Feliciano, J., Armstrong, T., Hogan,Z., Viers, J. & Moyle, P.B. 2001. Homogenization of Califor-nia’s fish fauna through abiotic change.In: Lockwood, J.L. &McKinney, M.L., eds. Biotic homogenization. New York:Kluwer Academic/Plenum Publishers, pp. 259–278.

Meffe, G.K. 1984. Effects of abiotic disturbance on coexis-tence of predator-prey fish species. Ecology 65: 1525–1534.

Mims, M.C. & Olden, J.D. 2013. Fish assemblages respond toaltered flow regimes via ecological filtering of life historystrategies. Freshwater Biology 58: 50–62.

Minckley, W.L., Marsh, P.C., Deacon, J.E., Dowling, T.E.,Hedrick, P.W., Matthews, W.J. & Mueller, G. 2003. A con-servation, plan for native fishes of the Lower ColoradoRiver. BioScience 53: 219–234.

Morgan, K., Fresques, T.D., Liles, T.A. & Esparza, F.M.1997. Native fish surveys of the Burro Creek drainage andassociated tributaries - Technical Report. Arizona Game andFish Department, Kingman, Arizona.

Moyle, P.B. & Marchetti, M.P. 2006. Predicting invasion suc-cess : freshwater fishes in California as a model. BioScience56: 515–524.

Mueller, G.A. 2005. Predatory fish removal and native fishrecovery in the Colorado River mainstem. Fisheries 30:10–19.

Olden, J.D. & Naiman, R.J. 2010. Incorporating thermalregimes into environmental flows assessments: modifyingdam operations to restore freshwater ecosystem integrity.Freshwater Biology 55: 86–107.

Olden, J.D. & Poff, N.L. 2005. Long-term trends of nativeand non-native fish faunas in the American Southwest. Ani-mal Biodiversity and Conservation 28: 75–89.

Olden, J.D., Poff, N.L. & Bestgen, K.R. 2008. Trait syner-gisms and the rarity, extirpation, and extinction risk of desertfishes. Ecology 89: 847–856.

Olden, J.D., Konrad, C., Melis, T., Kennard, M., Freeman,M., Mims, M., Bray, E., Gido, K., Hemphill, N., Lytle, D.,McMullen, L., Pyron, M., Robinson, C., Schmidt, J. & Wil-liams, J. In press. Are large-scale flow experiments inform-ing the science and management of freshwater ecosystems?.Frontiers in Ecology and the Environment.

Poff, N.L., Olden, J.D., Merritt, D.M. & Pepin, D.M. 2007.Homogenization of regional river dynamics by dams andglobal biodiversity implications. Proceedings of the NationalAcademy of Sciences of the United States of America 104:5732–5737.

Pool, T.K. & Olden, J.D. 2012. Taxonomic and functionalhomogenization of a globally endemic desert fish fauna.Diversity and Distributions 18: 366–376.

Pool, T.P., Olden, J.D., Whittier, J.B. & Paukert, C.P. 2010.Environmental drivers of fish functional diversity and com-position in the Lower Colorado River Basin. Canadian Jour-nal of Fisheries and Aquatic Sciences 67: 1791–1807.

Propst, D.L. & Gido, K.B. 2004. Responses of native andnonnative fishes to natural flow regime mimicry in the SanJuan River. Transactions of the American Fisheries Society133: 922–931.

Propst, D.L., Gido, K.B. & Stefferud, J.A. 2008. Natural flowregimes, nonnative fishes, and native fish persistence in arid-land river systems. Ecological Applications 18: 1236–1252.

Rahel, F.J. 1990. The hierarchical nature of community persis-tence: a problem of scale. American Naturalist 136: 328–344.

Rinne, J.N., Simms, J.R. & Blasius, H. 2005. Changes inhydrology and fish fauna in the Gila River, Arizona-NewMexico: epitaph for a native fish fauna? In: Rinne, J.N.,Hughes, R.M. & Calamusso, B., eds. American fisheriessociety symposium. Bethesda, MD: The American FisheriesSociety, pp 115–126.

Sabo, J.S., Sinha, T., Bowling, L.C., Schoups, G.H.W., Wal-lender, W.W., Campana, M.E., Cherkauer, K.A., Fuller, P.,Graf, W.L., Hopmans, J.W., Kominoski, J.S., Taylor, C.,Trimble, S.W., Webb, R.H. & Wohl, E.E. 2010. Reclaim-ing freshwater sustainability in the Cadillac Desert.Proceedings of the National Academy of Sciences (USA)107: 21256–21262.

Seager, R., Ting, M.F., Held, I., Kushnir, Y., Lu, J., Vecchi,G., Huang, H.P., Harnik, N., Leetmaa, A., Lau, N.C., Li,C.H., Velez, J. & Naik, N. 2007. Model projections of animminent transition to a more arid climate in southwesternNorth America. Science 316: 1181–1184.

Shafroth, P.B., Stromberg, J.C. & Patten, D.T. 2002. Riparianvegetation response to altered disturbance and stressregimes. Ecological Applications 12: 107–123.

Shafroth, P.B., Wilcox, A.C., Lytle, D.A., Hickey, J.T.,Andersen, D.C., Beauchamp, V.B., Hautzinger, A., McMul-len, L.E. & Warner, A. 2010. Ecosystem effects of environ-mental flows: modelling and experimental floods in adryland river. Freshwater Biology 55: 68–85.

Stefferud, J.A., Gido, K.B. & Propst, D.L. 2011. Spatially var-iable response of native fish assemblages to discharge, pre-dators and habitat characteristics in an arid-land river.Freshwater Biology 56: 1403–1416.

Thompson, L.C., Cocherell, S., Chun, S.N., Cech, J.J. &Klimley, A.P. 2011. Longitudinal movement of fish inresponse to a single-day flow pulse. Environmental Biologyof Fishes 90: 253–261.

U. S. Army Corp of Engineers. 2003. Water control manual:Alamo dam and lake, Colorado River Basin, Will WilliamsRiver, Arizona, edn., (available at www.spl.usace.army.mil/resreg/htdocs/Alamo/Alamo_WCM_2003?ALMO_WCM_Scr.pdf, accessed 5 March 2009).

Vellend, M., Verheyen, K., Jacquemyn, H., Kolb, A., VanCalster, H., Peterken, G. & Hermy, M. 2006. Extinctiondebt persists for more than a century following habitat frag-mentation. Ecology 87: 542–548.

Whittier, J.B., Paukert, C.P. & Gido, K. 2006. Developmentof an aquatic GAP for the Lower Colorado River Basin.Gap Analysis Bulletin No. 14 USGS/BRD/Gap AnalysisProgram. Moscow, Idaho.

Young, K.L., Lopez, M.A., Kubly, D.M., Johnson, T.B. &Janisch, J.L. 1994. Bill Williams River fisheries and limno-logical survey - Special Report F748816. Arizona Game andFish Department, Kingman, Arizona.

66

Pool & Olden