Seasonal use of the intermittent Virgin River Gorge by protected fishes

              Seasonal Use of the Intermittent Virgin River Gorge by
Protected Fishes
Heritage Grant I95027
R. Craig Addley
Dr. Thomas B. Hardy
Utah State University
Department of Civil and Environmental Engineering
Logan, Utah 84322-4110
1998
DISCLAIMER
The findings, opinions, and recommendations in this report are those of the investigators who have
received partial or full funding from the Arizona Game and Fish Department Heritage Fund. The
findings, opinions, and recommendations do not necessarily reflect those of the Arizona Game and
Fish Commission or the Department, or necessarily represent official Department policy or
management practice. For further information, please contact the Arizona Game and Fish
Department.
1.
INTRODUCTION
The Virgin River extends from its headwaters in southwestern Utah through the northwest corner
of Arizona and eastern Nevada to Lake Mead. Similar to other rivers and streams in the desert
southwest, the Virgin River exhibits extreme flow fluctuations that cover the range from
extremely low clear water base flows during the summer to high base flows during the winter
and early spring and raging sediment laden torrents during the rainy periods or during the spring
runoff (i.e., snowmelt). In addition, the Virgin River consists of both perennial and intermittent
segments. The intermittent segments frequently consist of dry stream bed and/or standing pools
during several months of the summer season, but during other times of the year, they experience
the same range of flows found in the perennial segments of river. The naturally intermittent
segments of the Virgin River include part of the Virgin River Gorge in Arizona and a segment of
the lower Virgin River upstream from Lake Mead in Nevada. This study encompasses primarily
the Virgin River Gorge from near the Utah-Arizona boarder down stream to a large complex of
springs, Littlfield Springs, Arizona (Figure 1).
The native fish fauna of the Virgin River is relatively depauperate, which is partly attributed to
the harsh environmental conditions (Cross 1975,1985). The native fishes consist of two
endangered species, woundfin (Plagopterus argentissimus) and Virgin River chub (Gila
seminuda), and four other species, desert sucker (Catostomus clarki), flannelmouth sucker
(Catostomus latipinnis), speckled dace (Rhinichthys osculus) and Virgin spinedace (Lepidomeda
mollispinis). The Virgin spinedace is currently protected by a multi-agency conservation
agreement with the express goal of removing threats to the species. Up to 14 nonnative species
have been found in the Virgin River system; however, most are rare in the main stem Virgin
River. The most abundant nonnative is the red shiner (Cyprinella lutrensis). Red shiner have
been implicated as a primary cause of the rapid decline in native species, particularly woundfin,
roundtail chub and spinedace, upstream and downstream of the Virgin River Gorge (USFWS
1996). Other species that are frequently collected in the Virgin River include black bullheads
(Ictalurus melas), largemouth bass (Micropterus salmoides), mosquitofish (Gambusia affinis),
channel catfish (Ictalurus punctatus), and carp (Cyprinus carpio).
Water development on the main stem Virgin River for agriculture and domestic use has resulted
in several water diversions in Utah, Arizona, and Nevada, and an off-stream reservoir in Utah
(Quail Creek Reservoir finished in 1985). The long term irrigation diversions and storage of
winter and spring flows in Quail Creek Reservoir with subsequent summer deliveries of this
water downstream of the reservoir to the Washington Fields Diversion have altered flow patterns
in the Virgin River. In addition, water diversions and storage projects on tributary streams (e.g.,
Santa Clara River) have contributed to changes in the flow regime within the main stem. These
alterations have likely reduced the frequency and magnitude of natural intermittent flows within
the Virgin River Gorge although data does not exist to quantify the changes. Diversion
structures along the Virgin River have also created barriers that affect upstream movement of
fish. The closest diversion structures in the vicinity of the Virgin River Gorge are 14 miles
upstream (Johnson=s Diversion) and 14 miles downstream (Bunkerville Diversion), and likely
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have limited impact on fisheries in the Virgin River Gorge.
Fish movement at the top of the Virgin River Gorge in Utah, however, has been impacted by the
approximately 4 foot high Red Shiner Barrier (Figure 1, Plate 1) installed in 1988. The Red
Shiner Barrier was constructed to stop the invasion of red shiner into the upper Virgin River.
Although red shiner currently exist above the barrier, ongoing eradication of red shiner by
chemical treatment remains a priority for recovery actions. In 1988, upstream chemical
treatment was attempted. The treatment was unsuccessful and because of poorly timed
detoxification at the Red Shiner Barrier, the treatment resulted in the loss of an undetermined
number of native fish within the Virgin River Gorge and downstream of the Gorge as far a
Mesquite, Nevada (USFWS 1996). In 1989 another upstream chemical eradication was
attempted (the treatment was detoxified at the Red Shiner Barrier), but was unsuccessful in
removing red shiner from the reaches upstream of the barrier. At the present time, another
upstream chemical treatment is in the preparation stage upstream of the Red Shiner Barrier.
The purpose of this study was to better understand the seasonal use of the Virgin River within
the Virgin River Gorge by native fish species during the summer period when declining flows
result in intermittently dry sections of river. Specifically the goals were to quantify changes in
the use and availability of macrohabitat within the Virgin River Gorge as a result of declining
flows and to quantify movement of fishes in response to the drying conditions. A better
understanding of the use of the Virgin River Gorge by native fishes has potential ramifications
on how both water and fisheries are managed (e.g., flows and fish barriers) in the Virgin River.
It also has the potential of providing a better scientific understanding of fish behavior in response
to intermittent flows and the potential consequences to fish of intermittent flows.
The original objectives and sampling design of this study required alteration during
implementation due to a series of uncontrollable events. Following preliminary notification of
study approval for the 1995 field season, unexpected delays in contract approval precluded
starting field work until late July, 1995. This made the collection of critical early summer field
data impossible. Shortly after field data collections were initiated, flow conditions within the
study reach became highly dynamic and an unusually rapid, but short duration drying of the
stream channel occurred. This required severe modification of sampling methods and goals,
especially in light of the size and access limitations within the study area. Due to these
constraints and limited data to meet research objectives, USU provided additional research
monies from the INSE research program to support the collection and analysis of additional data
during the summer of 1996. Some discussion of these issues and constraints in light of the
original study objectives and sampling design is presented in this paper for completeness,
however, the main objective of the paper is to present the data obtained from this study (altered
as it may be from the original study design) in conjunction with historical sample collections to
meet the original study goals of understanding fish habitat use in the Virgin River Gorge.
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METHODS
Study Site
The primary study segment of the Virgin River where new data were collected and historic data
were analyzed was the 20 mile segment of Virgin River within the Virgin River Gorge (Gorge).
The upstream end of the gorge begins approximately 3 miles upstream of the Utah-Arizona
boarder near the Red Shiner Barrier Dam at River Mile (RM) 59.67 and the downstream end is 5
miles upstream of the confluence of Beaver Dam Wash with the Virgin River in Arizona (RM
39.69 (Figure 1). We also included in the study site historical data collections near the top and
bottom of the Gorge. At the top of the Gorge historic data collected between the Red Shiner
Barrier Dam and the Virgin River Fishes Recovery Team (VRFRT) Atkinville biannual
sampling station near Bloomington, Utah, (RM 66.62) were included in the study. At the bottom
of the Gorge historic data from the VRFRT Near Beaver Dam Wash sampling station (RM
34.95) at the confluence of Beaver Dam Wash were included along with other historical
collections between RM 34.95 and 39.19 (Figure 1).
Habitat within the Gorge varies considerably from the low gradient, wide sandy river channel
found throughout much of the main stem Virgin River in Utah, Arizona, and Nevada. The
gradient in the gorge is steeper (0.55%) compared to other sections of the Virgin River (0.3%),
and within the gorge the narrow, rocky canyon confines the river and allows only limited
development of flood plain features and riparian vegetation, which is dominated by introduced
Tamarisk. Mesoscale habitat within the gorge consists of rocky riffles interspersed with deep
pools created by turbulent scour at the base of rock ledges and only a limited number of long
sandy runs can be found. Both the upper and lower ends of the study site outside of the Gorge
are more typical of main stem Virgin River habitat. Upstream of the Gorge from the Red Shiner
Barrier to the Atkinville VRFRT sampling station the river consists mostly of wider, shallow
sandy runs and little deep water fish habitat (Plate 1). From the downstream end of the Gorge to
the confluence of Beaver Dam Wash the river is wider than in the gorge and consists of a diverse
mixture of riffles, numerous boulder strewn sandy runs, and a few relatively deep pools.
At the present, flows upstream of the top of the Gorge near the Red Shiner Barrier, are typically
perennial, but are extremely low (a few cfs) during most summers. During summer low flows,
the majority of the discharge is made up of return flows from the Washington Fields and Santa
Clara agricultural areas and releases from the municipal wastewater treatment plant (ca. 5 cfs) at
Bloomington. During these low flows the entire river frequently seeps into the streambed and
the river can become dewatered at times from the upper Gorge downstream to the beginning of
the Littlefield Springs at River Mile 42. Inflows from Littlefield Springs consist of numerous
(70) small springs (0.1 to 3.0 cfs) that emerge along a seven mile section of river from inside the
lower end of the Gorge down river to Beaver Dam Wash (Figure 1). The combined total flow of
the springs is 60 cfs (Trudeau 1979; cited in Sandberg and Sultz 1985).
Sampling
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Original Sampling Design--As mentioned previously, due to unexpected delays in contract
approval (late July), sampling difficulties, and an unexpectedly quick drying within the Gorge
during the summer of 1995, our initial one year sampling design was drastically disrupted. In
addition, after initiation of field work in 1995, a better understanding of dynamic flow
conditions, fish habitat issues in the Gorge, and the logistics associated with sampling these
conditions resulted in the realization that some of the original study design was impractical or of
limited value.
The original study design consisted of monitoring changes in macrohabitat availability
throughout the Gorge (reach wide watered and dewatered habitat) and use of this habitat by
fishes prior to, during, and after stream drying. The design also consisted of monitoring
mesoscale habitat (e.g., run, pool, riffle) quality versus habitat use by fishes in four test sections
using field mapping and cross-section monitoring techniques. We also planned to tag a large
number of fish prior to stream drying within the Gorge to record movement of recaptured fish
after drying occurred.
Reasons For Modifying The Original Sampling Design--While considerable effort was
expended in installing head pins and collecting cross-section data for the mesoscale habitat
monitoring in the beginning of the study (actually prior to contract approval), the cross-section
data collection was abandoned. In total, initial data for 35 cross-sections at four test segments of
river (below the Red Shiner Barrier, at two locations in the middle of the Gorge, and one site at
the bottom of the Gorge) were collected. The late starting date combined with rapid late summer
flow fluctuations (declining flows and within day fluctuations) and the logistics associated with
measuring cross-section data and sampling fish under the declining/fluctuating flow resulted in
this component of the original study design being unworkable.
Due to rapid declining flows (including within day drying and wetting of the channel),
manpower was shifted to sampling fish throughout the length of the Gorge to determine densities
in an attempt to understand the large scale effects of habitat de-watering on fishes during and
after the drying. In particular sampling and marking a large number of fish from the area near
the middle of the Gorge was attempted to determine movement patterns of fish based on
subsequent recapture following de-watering of this section of river. Following de-watering and
subsequent re-watering of the Gorge in 1995, field sampling was stopped to reserve a small
amount of financial resources for single re-sampling of the Gorge during the following summer
de-watering event (1996). As noted previously, additional funds from the INSE research
program were used to supplement these efforts.
Modified Sampling DesignBBased on the experiences gained during the initial sampling, the
modified sampling design in effect consisted of sampling at 13 approximately equally spaced
sampling locations throughout the Gorge prior to, during, and after the 1995 de-watering to
determine densities and spatial location of fishes. Sampling sites were chosen to provide a broad
coverage of key locations in the Virgin River Gorge and to take advantage of sites where
previous data had been collected. Initial sampling typically consisted of mapping the mesoscale
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habitat units within each sampling site and sampling habitat units approximately in proportion to
their availability. Rocky riffle habitats were not sampled most of the time, however, because the
bulk of the sampling was undertaken by seining which is not effective in these habitats. Total
sampling effort typically consisted of 5 to 10+ fish samples sites per sampling location.
Following the initial sampling of extensive habitats, each station was sub-sampled. During
declining flows, pools were the most stable habitat type and contained the most fish, so the sub-sampling
concentrated on re-sampling pool and other deep water habitats in the same location
and manner as they were sampled initially. During the 1996 de-watering event, 6 of the original
1995 sampling stations were re-sampled. In addition, one new habitat location was identified as
providing critical deep water habitat conditions and sampled for the first time.
Sampling was accomplished using nylon seines and snorkeling. Seines were made of 3.2 mm
mesh and were 4.5 m long. Seine sampling was accomplished using a combination of single
pass seine hauls and 3 multiple pass seine hauls. The length of the samples was determined by
the length of the habitat units. Typically in open ended pools a blocking net was used to deter
downstream escapement of fish during sampling. All captured fish were identified, enumerated,
measured, checked for tags, and released back into the stream. Snorkeling was used to provide
accurate counts of adult fish in deep water and was only used in instances where water clarity
provided excellent visibility. Young-of-the-year (YOY) fish were not enumerated during
snorkeling.
During initial sampling in 1995 adult fish captured in the middle of the Gorge prior to drying
were tagged with a color latex injection in one fin. The color and fin location of tags was used to
denote capture locations. Subsequent recaptures were noted and the amount of movement from
the initial tagging location was noted.
Historic accounts and records of flows in the Gorge were obtained from the literature and from
the USGS Virgin River gage Near St. George (#9413500) and the Littlefield gage in Arizona
(#9415000). Discharges at the top of the Gorge during the study were obtained from the Near St.
George gage. Discharges at other places in the Gorge were determined using standard gaging
techniques using a Swoffer or Marsh McBirney digital velocity meter. Photographs were used
to document fish kills and physical habitat. Sampling locations and de-watered reaches were
marked on 7.5 minute USGS quads. Habitat mapping at sampling stations was sketched on data
sheets.
Data from historical Virgin River Fishes Recovery Team (VRFRT) sampling sites at Beaver
Dam Wash, Cedar Pockets, and Atkinville Wash were used to show historical fish abundance
trends in the Virgin River Gorge. Additional miscellaneous data collected within the Gorge
since 1992 by the BLM and others were used where applicable to provide baseline information.
Data collected during the 1995 and 1996 summer drying events are used to show fish
distribution during the events. For all of the data analysis, native fish were separated into young-of-
the-year (YOY) and adult size classes based on lengths from Addley and Hardy (1995). The
size separation between adult and YOY was 55 mm (woundfin), 50 mm (speckled dace), 90 mm
(flannelmouth sucker), 90 mm (desert sucker), 68 mm (spinedace), 90 mm (Virgin River chub),
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and 36 mm (red shiner).
RESULTS
Sampling Locations
A combined total of 13 locations where sampled during the 1995 and 1996 field work (Figure 1).
During 1995 12 sampling sites were located at River Mile 39.69 (Below Mouth of Gorge), 40.27
(Above the Station Below Mouth of Gorge), 41.33 (Above Pylon Bridge & Below Second
Bridge), 42.04 (Between Big Ten and Springs), 42.81 (Big Ten), 44.40 (Below Dave=s Rock),
49.10 (Above Campground), 49.78 (Gorge Above Corral), 51.41 (Mile Marker 21 Gorge), 52.89
(Small Bend), 5.08 (North of Top Bridge), and 58.67 (Red Shiner Barrier). Six of these sites
were re-sampled in 1996, River Mile 39.69, 41.33, 49.10, 51.41, 55.36, and 58.67. One
additional site was added at River Mile 40.76 (Cable Across River). Each sampling site was 10-
20+ channel widths long.
Habitat Mapping
Prior to the study, during June 1994, the entire Gorge from the Red Shiner Barrier down river to
the mouth of the Gorge was mapped to determine the linear measurement for each habitat type
(Hardy et al. 1994). Because of the large distance involved, small habitat units (i.e., pocket side
pools) were not delineated. Only habitat units larger than about 1-2 channel width long were
classified. The linear location of runs, pools, and riffles from the Barrier downstream are shown
in Figure 2. In the upper Gorge, above the Cedar Pockets Campground, runs are the dominate
habitat type and pools are relatively rare. In the lower Gorge, from near Cedar Pockets
Campground downstream, a more diverse mix of runs and riffles occurs and pools become much
more abundant.
Habitat mapping at each of our sampling sites in 1995 and 1996 shows this trend in more detail.
Figure 3 shows the percentage of pools, runs, and riffles within each of our main sampling sites.
Clearly most of the habitat in the upper two sampling sites River Mile 58.67 (Red Shiner
Barrier) and River Mile 55.08 (North of Top Bridge), is composed of runs. Beginning at River
Mile 49.1 (Above Campground), pools become more abundant and a relative diverse mix of
pools, runs, and riffles occurs. Copies of sampling station site maps with habitat delineations for
each of the sampling sites are shown in Appendix A (note that glide habitats on the maps were
considered to be shallow runs in the final analysis).
Qualitative/Quantitative Observations of Flow
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Historic Conditions--Historical information on the Virgin River prior to agricultural
development in the late 1800's either doesn=t exist or was unavailable to us. Sketchy data from
the early 1900's, after significant agricultural development began, indicates that the Virgin River
periodically went dry in the Virgin River Gorge. Adams (1903) documented that during May
1902 (a dry year) the river was completely dry A6 miles below Atkinville.@ This is near the
current Red Shiner Barrier. Flow records are available from the Virgin River Near St. George
from 1950 -1956 and 1991- present. This gage is located at the present day Red Shiner Barrier.
Flow records from the 1950s show that the river went dry every year at the gage (Red Shiner
Barrier). This typically occurred between late May and September (Figure 4). U.S. Geological
Survey personnel measured seepage losses near the gage of 36 to 106 cfs in 1952 and 1956
(Sandberg and Sultz 1985), and the U.S. Bureau of Reclamation estimated average annual loss in
this location of 50 cfs (USBR cited in Sandberg and Sultz 1985).
It is obvious that historically the Gorge was frequently dry, likely on an annual basis in all but
above normal precipitation years. The extent of dry conditions through the length of the Gorge,
however, is unknown to us. We suspect that when the river was dry at the gage (near the present
day Red Shiner Barrier), the entire 18 miles of the Gorge down to the beginning of Littlefield
Springs was also dry.
Current ConditionsCThe recent discharge records from the St. George gage reestablishment in
1991 to the present (post Quail Creek Reservoir closure in 1985) show that there is always flow
at the gage, but low flows of 15-30 cfs are common (Figure 5). These limited data indicate that
more flow exists now at the gage than historically and that intermittent conditions have likely
changed in the Gorge. This likely is the result of summer flow releases for irrigation from Quail
Creek Reservoir and the wastewater treatment plant releases near Bloomington, Utah.
Although it appears, that at the present, more summer flow occurs in the Gorge than occurred
historically, sections of the Gorge still typically become intermittent on an annual basis during
the summer. Data collected by the BLM indicate that when flows are in the range of 35-40 cfs at
the St. George gage, sections of the Virgin River Gorge go dry (Stephanie Ellingham, Pers.
Comm.). This was supported during our field work in 1995 and 1996. During August 8-12,
1995, flows went dry in the middle Gorge following several days of flow in the mid 40 and high
30 cfs range at the St. George gage. During June 1-6, 1996, flows went dry in the Gorge
immediately following flows in the low 40 and high 20 cfs range. The six mile section of stream
from the Cedar Pocket Campground downstream to the beginning of the Littlefield Spring
inflows at River Mile 42 was the section of river that typically went dry first and stayed dry the
longest. In recent years, dry conditions or standing water typically has been observed along 12
miles of the river from near the first bridge in the Gorge (river mile 54.5) downstream to the
spring inflows (Stephanie Ellingham, Pers. Comm.).
During the summer of 1995 and 1996 when dry conditions occurred in the Gorge they were
temporally and spatially very dynamic. Typically, when flows first started to dry up, large
sections of river would quickly go from having substantial flows to being dry during a period of
several hours in the early afternoon (about 2 pm). This would occur between the Cedar Pocket
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Campground and River Mile 42 (7 miles). Later in the evening (e.g., 7 pm) or at night as air
temperature declined, flow would start to reappear in an upstream to downstream direction. This
frequently caused temporally isolated pools in the Gorge that were reconnected at night and
during the early morning. During early August 1995, this happened for several days allowing
most fish to work their way out of the drying section of the Gorge with very little mortality.
Later in August, flows in the upper Virgin River increased (due to releases of irrigation water
from Kolob Reservoir in the headwaters of the Virgin River) and continuous flow reestablished
throughout the Gorge.
During 1996, drying in the Gorge happened earlier in the year (June 1-6), occurred more quickly
and persisted longer. The section of Gorge from near Cedar Pockets Campground downstream
to Littlefield Springs remained continuously dry immediately following the first few days of
drying. Some re-watering on the upper end near the campground would occur in the evening,
but the re-watering did not extend very far downstream. Several times we followed water
moving slowly down the channel during the late evening, rechecked the progress of the flows
early the next morning and found that flows had not extended far downstream. The quicker,
more permanent drying during 1996 stranded many fish in the de-watered section.
Some attempt was made to understand quantitatively how flows were behaving throughout the
Gorge, but due to the dynamic flow conditions, long study reach and limited access in many
places, this was extremely difficult. An example of 5 flow measurements taken throughout the
day on the 12th of August, after flows were reestablished throughout the Gorge (following a dry
period in the middle of the Gorge the previous day), shows some of the spatial variation in flow
(Figure 6). About a 60 cfs loss of flow can be observed between the Red Shiner Barrier and the
Below Dave=s Rock (14 miles) station. This flow loss may be slightly exaggerated because the
flow measurement at the Red Shiner Barrier was taken about 2 hours later in the evening than
the measurement at Below Dave=s Rock. This was about the time in the evening flows typically
started to increase as a response to cooling temperatures.
Fish Mortality
During the summer of 1994, Mike Herder (Pers. Comm.) observed dead and stranded fish in the
Virgin River Gorge when the middle section of the Gorge dried up. During drying of the Gorge
in August 1995 only a few scattered individual fish mortalities were observed as part of this
study. During afternoon drying, fish were frequently trapped in isolated pools; however, in the
late evening, night and early morning, flow reconnected the pools allowing fish to move out of
the drying area. During early June 1996, however, rapid, permanent drying in the Gorge trapped
large numbers of adult and young-of-the-year (YOY) fish. Completely dry sections of
streambed and small isolated pools, particularly in the lower half of the dry section from river
mile 45 downstream (Figure 1), contained large numbers of dead and dying native and nonnative
fish (adult, juvenile, and YOY) (Plate 1). No dead woundfin or Virgin River chubs were found.
We attribute this to the rarity of these species in this section of the lower Virgin River. We
observed little selectivity in terms of dead species, except for perhaps that we saw many dead
adult desert suckers and few dead adult flannelmouth suckers in comparison to the relative
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abundance of these two species. We also observed dead fish in the upper portion of the Gorge in
the vicinity of river mile 55. This section of river had continuous running water and live fish in
deep water habitats, but there were small piles of dead fish that had washed into low velocity
sections of the river (Plate 1 and 2). We attributed these dead fish to heat/oxygen stress,
however, some drying may have occurred that we did not observe.
Historic Population Data
Data from the standardized Virgin River Fishes Recovery Team sampling for the Near Beaver
Dam Wash, Cedar Pockets, and Atkinville Wash sampling sites are presented in Tables 1-3.
Table 1 shows the rapid decline in woundfin and spinedace near Beaver Dam Wash beginning in
about 1984 at the same time when red shiner numbers increase rapidly. What appears to be a
similar trend of decreasing numbers of Virgin River chub is also evident in the data, except that
the decline is lagged a few years. Red shiner, although present in the lower river since the
1960's, were captured upstream in Utah for the first time in 1984 (Hickman et al. 1987). Deacon
(1988) hypothesized that perhaps red shiner were first able to make it upstream through the
intermittent Virgin River Gorge during the exceptionally high summer flows of 1983, and that
the establishment of red shiner upstream of the Gorge played a role in allowing red shiner to
become the dominate fish below the Gorge at Beaver Dam Wash in subsequent years. An
alternative hypothesis is that less frequent high flow events during the dry years following 1983
is responsible for the shift in dominance of red shiner (e.g., Leo Lentsch, Pers. Comm.).
The much shorter data set at Cedar Pockets (beginning in 1992) is not useful for observing
trends, but indicates the current relative abundance of native fish and red shiner in the middle of
the Gorge (Table 2). Cedar Pockets is at the top end of the section of river that becomes
intermittent on a regular basis during the summer and it shows low numbers of all native species,
but indicates that native fish are consistently present in this reach of river when flows are
continuous. Red shiner are by far the most abundant fish.
The data from Atkinville Wash show the same trend of increasing numbers of red shiner and
decreasing Virgin River chub and woundfin numbers as occurs at the Near Beaver Dam Wash
site, although the trend is not quite as dramatic (Table 3). Currently low numbers of Virgin
River chub and flannelmouth sucker and sporadically low numbers of woundfin occur at the site.
The one high density sample of woundfin in 1996 likely resulted from salvaged woundfin being
relocated to this reach after chemical treatment operations upstream in the area immediately
below the Washington Field Diversion. Desert sucker numbers in this reach appear to have
remained relatively constant. Large increases in red shiner numbers beginning in 1992 occurred
two years after chemical treatment of this reach to remove red shiners.
1992 Intensive Spatial Sampling
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In the early summer of 1992 intensive spatial sampling of much of the main stem Virgin River
was conducted, including the Gorge study site. A compilation of this data has never been
published to our knowledge. Sampling sites were located approximately every mile through the
Gorge study section. At each site, three samples in representative habitat types were taken.
Every other sample site was either a qualitative sampling site or a quantitative sampling site.
Qualitative sampling consisted of only a single seine haul at each seining location and
quantitative sampling consisted of three seine hauls within a specific habitat site. The average
fish density for the first seining pass at each of these sampling sites is presented in Table 4. No
spinedace were found within the study area which is not surprising because spinedace are
typically a headwater and tributary species and are abundant only in the upper mainstem Virgin
River above Hurricane, Utah, and in tributary streams (e.g., Addley and Hardy 1993). Relatively
low and sporadic densities of the other native species were found throughout the length of the
study reach. Nonnative red shiner were by far the most abundant species throughout the Gorge.
Qualitative / Quantitative Observations of Fish
General Qualitative Observations of Fish Behavior, Summers 1994, 1995, and 1996--During
the summer of 1994, Mike Herder with the BLM sampled through the Gorge study site using
seines, dip nets and snorkeling. During low flows in early June, he observed large fish, likely
suckers or chubs, trying to jump upstream over the Red Shiner Barrier. No fish were successful
in negotiating the Red Shiner Barrier during the observation period. At slightly higher flows,
velocities at the base of the barrier were too high and there was no place for fish to stage in an
attempt to clear the Red Shiner Barrier (Plate 1). Herder also observed while snorkeling that red
shiner were very aggressive and frequently nipped at his bare skin. Shiners were found in nearly
all habitat types. Native fish were found only in what appeared to him to be higher quality
habitat, e.g., deeper water (Mike Herder, Pers. Comm.).
In reaction to the initial stages of drying in the Virgin River Gorge in August 1995, particularly
on the afternoon of August 9th at River Mile 50, we observed large schools fish moving
downstream in what is best described as migration waves. From visual inspection, the fish
appeared to be mostly desert sucker. They moved downstream through shallow water in schools
of 10 to 50 fish, held temporally in deeper water habitats and then moved downstream again. At
the peak of this movement a new school of fish would appear every few minutes. This behavior
was observed only during the first couple of days of the drying period.
Following drying during 1995 and 1996, we observed large schools of adult fish concentrated in
pools at the bottom of the Gorge (at Littlefield springs) downstream of the dry section. We also
observed large concentrations of fish in the few pool(s) immediately below the Red Shiner
Barrier at the top of the Gorge.
At the bottom of the Gorge near the beginning of Littlefield Springs, where flows were still
relatively low (e.g., 20 cfs), schools of fish were concentrated in the deepest water available,
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which were very shallow pools. Plate 3 shows a school of predominately flannelmouth sucker in
a shallow pool. These fish stayed together and were reluctant to move from the relative cover of
the shallow pool. All of these fish were easily captured using seines (Plate 3). Farther
downstream where flows were greater, each deep pool contained similar large schools of adult
flannelmouth sucker, while the faster flowing water in runs and riffles contained large numbers
of desert sucker (see snorkeling discussion below). At the top of the Gorge immediately below
the Red Shiner Barrier the pool(s) contained large numbers of adult Virgin River chub (Plate 1).
One seine pass in the pool adjacent to the Red Shiner Barrier (Plate 1) on August 12, 1995,
yielded 86 adult chub. Sampling in this same pool and two others just downstream on June 6,
1996, yielded an average of 23 adult chub per pool.
During low flow conditions the availability and location of pool habitat and deep run habitat was
the main determinate of the number and location of fish in a reach. At low flows, shallow water
habitats were nearly devoid of fish. All species of fish were confined to pools and deep runs.
Only rarely were fish observed in shallow water habitat and these were typically very small fry.
From our observations, the low flow conditions and the subsequent crowding of fish into low
flow pools with limited space appeared not only to provide the potential for habitat availability
to limit fish numbers, but for significantly enhanced predation and the potential for enhanced
spread of disease. While snorkeling during low flow conditions (discussed below), several adult
chubs were observed making rapid predatory movements toward YOY fish. Although we
observed no successful attacks it appeared that chubs were preying on YOY fish and it appeared
there was relatively little refugia for the YOY fish outside of the pools because of the low flow
conditions. In the confined conditions of laboratory aquaria during some previous studies on
osmoregulation (Addley and Hardy 1995), we lost a number of YOY woundfin due to predation
by larger chubs. In addition, while seining during low flow conditions at River Mile 51.4,
predation by a relatively small juvenile largemouth bass was observed. A captured 110 mm
largemouth bass had a partially digested 55 mm desert sucker in it stomach (the tail of the sucker
was sticking out of the mouth of the bass).
SnorkelingBImmediately following drying in 1996, clear water conditions in the lower portion
of the Gorge provided an excellent opportunity for snorkeling. We snorkeled 324 meters of
stream in the upper Littlefield Springs area at River Mile 40.32 (Cable Across River site) to
accurately enumerate fish species and density (Figure 1). This section of stream appeared to
provide some of the best refugia habitat for fish fleeing the de-watered conditions in the middle
of the Gorge. Habitat in this area consisted of numerous excellent deep water pool habitats
mixed with runs and riffles (Plate 3). Abundant YOY flannelmouth and desert sucker were
present. There were also a lesser number of adult red shiner (YOY red shiner were not present
yet). YOY fish and red shiner were not enumerated while snorkeling, only the adults of larger
fish species were counted. We counted 1639 adult fish--26 Virgin River chub, 514 flannelmouth
sucker, 1080 desert sucker, 2 woundfin, 6 juvenile channel catfish, and 3 black bullheads. There
was a total of 8,149 adult fish/mile or an average 5.7 adult fish/10 m^2. Densities of adult fish
were very high. Typically 200 or more adult fish were crowded into each deep pool (typically
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flannelmouth sucker, Virgin River chub, and desert sucker) or run (typically desert sucker). In
individual habitat units (e.g., pool, runs) densities often exceeded 20 fish/10 m^2. Clearly this
section of river was experiencing severe overcrowding while providing refugia to fish that had
moved downstream out of the de-watered conditions in the middle Gorge.
We also snorkeled some short sections of deep water habitat at the Below Mouth of Gorge site
(RM 39.69) following de-watering in 1996. Several >pools= existed at this site, but only one large
deep water pool existed in this section of river in comparison to the numerous excellent pool
habitats at the Cable Across River site. This pool contained 99 adult flannelmouth suckers, 26
desert suckers, and 1 chub. The density was 8.3 fish/10 m^2. This is a relatively high density,
but only about half of the densities found upstream at the Cable Across River site. In addition,
because no other comparable deep water habitats existing in the area, the total number of fish in
the reach was lower than that at the Cable Across River site.
Seining Fish DensitiesCDuring and following the drying events in 1995 the 12 sampling
locations were sampled to determine spatial distributions and temporal changes in fish density
(Table 5). Because of difficulties mentioned at the beginning of this paper the amount of
sampling time was very compressed and when flows started to decline quickly in early August a
compromise sampling scheme had to be implemented that balanced the time available for
sampling with the need to know the broad spatial distribution of fish throughout the 20 mile
Gorge and with the need to know the specific effects of drying and flow changes at particular
locations. Generally we sacrificed repeated, quantitative samples over time at a few selected
locations for broader more qualitative spatial sampling along the length of the Gorge. With
limited resources we repeated some of the broader sampling initiated during 1995 at the time of
drying in 1996.
Data from the 1995 and 1996 sampling are shown in Table 5 along with additional data collected
in the Gorge from 1992 through 1994 for reference. Maps of each sampling site and fish
sampling locations within each site are shown in Appendix A. Prior to drying, native species
were found throughout the Gorge (also see Tables 2 and 4). Although native fish densities are
relatively low, particularly when compared to red shiner numbers, it is obvious that considerable
use of the entire 20 mile segment of the Gorge occurs when it has flow throughout its length.
Red shiner are by far the most abundant species in the Gorge, followed by desert sucker,
flannelmouth sucker, Virgin River chub, and speckled dace. Woundfin and spinedace were very
rare. Excluding samples immediately below the Red Shiner Barrier, only 20 woundfin and 4
spinedace were captured in the Gorge in over 250 seine passes. Exotic fish were also found in
low numbers. We found channel catfish, black bullhead, and large mouth bass (19), carp (1),
and mosquitofish (1).
Repeat sampling of individual pools at various sampling sites in the middle Gorge prior to,
during, and following drying showed that native fish did not leave habitat in the intermittent
section of the Gorge until being forced out by large scale de-watering. Figure 7 shows an
example of flannelmouth sucker and Virgin River Chub densities from two pools in the section
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of river that eventually went dry in 1995 (one pool at River Mile 49.1 and one pool at River Mile
44.4). Densities were relatively stable until de-watering began to occur on about August 7th.
After de-watering occurred these native fish left the pools even though standing water remained
and at times (typically at night) flow connection between pools was reestablished.
Following drying, fish were found both below the drying section from Littlefield Springs
downstream and in the upper Gorge above the dry section. In the upper Gorge native adult fish
were most abundant immediately below the Red Shiner Barrier (e.g., see sampling dates August
11-14, 1995, and June 4-7, 1995). Figure 8 shows an example plot of the fish densities
immediately following drying on August 11 and 12, 1995. The greatest densities of Virgin River
chub in the watered section of river were at the upstream end of the Gorge at the Red Shiner
Barrier. The greatest densities of desert sucker and flannelmouth sucker were found at the lower
end at Littlefield Springs.
Tagging Results--Prior to drying in 1995 a total of approximately 100 adult fish, flannelmouth,
desert sucker and Virgin River chub, were tagged from locations in the middle of the Gorge.
Most of these fish were originally captured and tagged at the River Mile 49.10 and 44.40
sampling sites. These sites are located in the section of the Gorge that goes dry first, particularly
the River Mile 49.10 site. After drying had occurred in the Gorge, two tagged fish were
recaptured during sampling in the sections of river that remained watered. The low number of
recaptures occurred because of the large study reach and small number of tagged fish. A 420
mm flannelmouth sucker tagged at river mile 44.4 prior to this area drying up, was found three
miles downstream in the Littlefield Springs section at river mile 41.3 on August 11, 1995. A 240
mm Virgin River chub tagged at river mile 49.10 prior to drying was found 1.5 miles upstream at
river mile 51.4 on August 12, 1995.
DISCUSSION
It is clear that native fish use habitat throughout the Gorge when it=s available (seasonally). This
occurs even though on an annual basis, a minimum of about seven miles of the Virgin River
Gorge becomes intermittent and an even larger segment frequently becomes intermittent or
experiences very low flows during the summer. The majority of the pool habitat exists in the
lower portion of the Gorge from about River Mile 49 downstream. This section of river has a
good mix of pools, runs, and riffles. The upper section of the Gorge however is dominated by
sandy run habitat and has fewer pools and riffles. Under normal flow conditions pool habitat is
likely most important for Virgin River chub and flannelmouth sucker, sandy run habitat is
perhaps more suitable for woundfin, and rocky run and riffle habitat is mostly inhabited by
desert sucker. Particularly during low flows, however, the amount and location of deep water
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habitat appears to control the number and location of all native adult fish.
Prior to dry conditions, native fish density was relatively low and scattered throughout the
Gorge. Nonnative red shiner were by far the most abundant species, followed by desert sucker,
flannelmouth sucker, Virgin River chub, and speckled dace. Woundfin and spinedace were very
rare. Other nonnative fish such as black bullhead, channel catfish, mosquitofish and carp were
found in low numbers.
Following low flows and drying in the Gorge, adult fish of all native species were found
primarily in deepwater habitats (mostly pools and deep runs) and fish densities downstream and
upstream of the dry segment were high, particularly downstream at the mouth of the Gorge in the
Littlefield springs section. High densities of fish in pools immediately below the dry section
highlight the potential predation/disease problems associated with low flow crowding (e.g.,
Heckmann 1986) and indicate the extreme importance of habitat in this section of river for
sustaining or providing refugia for the native fish population that seasonally utilizes the middle
Gorge. Although Virgin River chub and particularly woundfin were relatively rare throughout
the entire study area, our results suggests that this reach of river is important potential habitat for
future conservation of these species. Any large scale flow depletions in the Littlefield Springs
reach of river (downstream of River Mile 44) during the summer season could cause serious
consequences for these native fish.
Relatively high densities of Virgin River chub immediately below the Red Shiner Barrier and
observations of Atrapped@ fish trying to jump the barrier, indicate that although the barrier may
eventually be an important feature in eradicating red shiner, at the present it is likely a detriment
to the native fish population by not allowing fish to move upstream into better flow conditions
during the summer. In the absence of the barrier, the Gorge could provide an expanded amount
of fall, spring, and winter habitat that could be exploited by native fish in the upper river. This
would require, however, the behavioral trait by these fish of moving upstream during drying
conditions in the summer. Our tagging data and sampling data were insufficient to determine
how much upstream movement of fish occurred. It does appear from our limited data that many
Virgin River chub were moving upstream. The one tagged chub that was recaptured had moved
upstream and the concentration of chubs in the pools immediately below the Red Shiner Barrier
indicates this same trend.
Conversely, it appeared that a majority of movement of flannelmouth sucker and desert sucker
was downstream based on the large concentrations of these species in the Littlefield Springs area
following de-watering and based on our visual observations of fish moving downstream. There
may also have been some limited upstream movement, however.
Due to the large distances involved and difficulties associated with marking large numbers of
fish for subsequent recapture, the best way to more definitively determine long distance
movements of large native fish (Virgin River chub, flannelmouth sucker, and desert sucker)
associated with drying conditions in the Gorge is to equip fish with radio tags. Information
concerning movement patterns of large adult native fish in the Gorge and throughout the Virgin
Page 15
River will continue to be uncertain, as evidenced by this study, without a concerted effort to
implement a radio tagging and tracking program. For the smaller species, woundfin, spinedace,
and speckled dace, smaller scale marking/tagging and recapture studies appear to be the only
way of monitoring movements because of the inability to implant radio tags in these small fish.
This is very problematic due to the limited number of fish available for marking and the
difficulties associated with recapturing fish.
The disappearance of woundfin in the study reach, beginning approximately in 1984 at Beaver
Dam Wash and the disappearance of Virgin spinedace at the Beaver Dam Wash confluence
coincident with rapid increases in red shiner numbers implicates red shiner in the decline (e.g.,
Deacon 1988) although the causative mechanism for this shift remains indeterminate. However,
the fact that red shiner numbers started to rapidly increase at the same time as their discovery
above the Virgin River Gorge in 1984 implicates the Gorge as being a potentially important
control historically and possibly in the future on red shiner numbers. Large amounts of habitat
exist in and above the Gorge. This gives fish established within and above the Gorge a large
recruitment advantage if there is a dispersal pattern of young fish in the downstream direction.
Deacon speculated that red shiner may have passed through the Gorge during the extremely high
summer flows of 1993. Flow records at the Hurricane and Virgin gages indicate that summer
flows (July and August) in 1983 were much greater (e.g., 50 cfs greater) than the flows at these
gages during the summer of 1995 when we observed the Gorge to only dry up for a short time in
mid-August. It is clearly possible, therefore, that the Gorge did not dry up during 1983 and that
the higher flow conditions made it easier for red shiner to penetrate through the Gorge,
particularly if late spring or summer were, or are, important time periods related to red shiner
upstream movements. It should be noted, however, that prior to 1983 high summer flows within
the Virgin River have existed (e.g., 1967, 1980) without the invasion of red shiner upstream.
Regardless, it seems that summer intermittent conditions in the Gorge historically could have
produced a relatively effective upstream barrier, especially historically, when it appears that long
sections of stream dried up (i.e., 18 miles from Littlefield Springs to the top of the Gorge near
the present Red Shiner Barrier). Higher flow conditions now, as a result of more summer
irrigation return flows due to Quail Creek Reservoir and wastewater treatment plant return flows,
likely have reduced the effectiveness of this natural barrier.
Removal of red shiner from the Virgin River or sections of it will likely remain a major focus in
trying to restore native fish populations in the near future. If red shiner can be removed from the
upper river, the dry summer and harsh high flow conditions in the Gorge provide a natural
barrier to upstream movement of red shiner that should be used to advantage. This barrier could
be enhanced by reducing summer flows and/or installing additional manmade barriers similar to
the existing Red Shiner Barrier in the Gorge.
The existing Red Shiner Barrier at the top of the Gorge is not ideal. At the present, perennial
flows exist immediately below the barrier and even if red shiner can be eradicated upstream, a
continual pool of redshiner still would exist adjacent to the barrier and increases the chance of
the barrier being breached by red shiner (through natural events or human intervention, e.g.,
Page 16
people moving fish above the barrier). A more effective barrier could be created in the middle of
the Gorge downstream of Cedar Pockets Campground where the annual de-watering occurs
(approximately 7 miles of river) by building one or a series of structures. The de-watering
would annually purge redshiner from below any constructed barrier creating a buffer zone and
increasing the barrier=s effectiveness.
An obvious concern with barriers is their potential effects on native fish movements. The
existing Red Shiner Barrier likely traps fish in the upper Gorge in low flow conditions below the
barrier. Although better information on fish movements should be obtained through radio
tracking, this study suggests that a barrier or series of barriers located downstream of the Cedar
Pockets Campground would allow some seasonal use of the Gorge by fish moving upstream
from the Littlefield Springs section of river and seasonal use of the upper section of Gorge by
native fish moving down from the upper river if the existing Red Shiner Barrier at the top of the
Gorge was breached or made passable.
The large number of high quality pools in the lower portion of the Gorge and density of fish in
these pool habitats highlight this section of river (and pools in general) as being important
habitat particularly during low flows. Pools provide cover in terms of depth and velocity refugia
during moderate and low flows. While this study made no attempt to address the quality of
microhabitat as a function of discharge, it was apparent that the 40 to 60 cfs of flow that
occurred in the Littlefield Springs reach during the summer de-watered conditions, provided
relatively good base flow microhabitat. We suspect flows in this range would provide good base
flow microhabitat conditions in pools throughout the Gorge and provide decent deeper water
habitat conditions in the inherently shallower runs and riffle habitats. In some sections of river
upstream of the Top Bridge (RM 54.50) where the river is wider and a somewhat less confined,
higher flows may be needed to provide comparable base flow microhabitat quality.
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Depending on the management objectives for the Gorge two different and conflicting summer
base flow goals could be outlined. If the management objective of the Gorge was to maintain the
historic de-watered conditions in the Gorge and/or use the Gorge as a barrier to red shiners, it
would be necessary to ensure that during parts of the summer months, flows upstream of the
Gorge above the Red Shiner Barrier did not exceed 30 to 40 cfs (and perhaps were much less
than this). If the management objective was, however, to enhance seasonal habitat availability
and use while facilitating fish movement potential throughout the Gorge, then flows of
approximately 60 to 80 cfs would be required at the top of the Gorge. This would ensure
perennial flows throughout the Gorge and provide a minimum of about 20 cfs in the middle of
the Gorge where flow losses of approximately 40 to 60 cfs occur.
REFERENCES
Adams, F. 1903. Report of irrigation investigations in Utah. U.S. Dept. Agriculture,
Government Printing Office, Bulletin No. 124, 330 p.
Addley, R.C. and T.B. Hardy. 1993. The current distribution of spinedace in the Virgin River
Basin. Report to Washington County Water Conservancy District, Hardy, Addley and
Associates, Logan, Utah 84321.
Addley, R.C. And T.B. Hardy. 1995. Virgin river fishes suitability criteria. Utah State
University, Logan, Ut.
Addley, R.C. And T.B. Hardy. 1995. Woundfin, roundtail chub and desert sucker
osmoregulation in natural saline waters of the Virgin River, Utah.. Utah State University,
Logan, Ut.
Cross, J.N. 1975. Ecological distribution of fishes of the Virgin River. Master=s Thesis,
University of Nevada, Las Vegas. 187 pp.
______ 1985. Distribution of fish in the Virgin River, a tributary of the lower Colorado River.
Environ. Biol. Fishes. 12:13-21.
Deacon, J. 1988. The endangered woundfin and water management in the Virgin River, Utah,
Arizona, Nevada. Fisheries. 13:18-24.
Heckmann, R. A., J. E. Deacon, and P. D. Greger. 1986. Parasites of the woundfin minnow,
Plagopterus argentissimus, and other endemic fishes from the Virgin River, Utah. Great
Basin Naturalist. 46 (4):663-676.
Hickman, T. 1987. Study of fishes in the Virgin River (Utah). Annual Report for 1986.
Western Ecosysstems, P.O. Box 1575, St. George, Ut 84770.
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Sandberg, G.W., and L.G. Sultz. 1985. Reconnaissance of the quality of surface water in the
upper Virgin River basin, Utah, Arizona and Nevada, 1981-82. Utah Department of
Natural Resources Tech. Publ. 83:1-69.
U.S. Fish and Wildlife Service (USFWS). 1996. Preliminary draft environmental assessment
for red shiner eradication project, Virgin River, Utah. 27 pp. + Appendices.
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TABLES
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FIGURES
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Figure 1. Map of the Virgin River Gor
starting point near Lake Meade.
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Figure 1 Continued
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Figure 1 Continued
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Figure 1 Continued
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Figure 2. Linear location of run, riffle, an
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Figure 3. Percentage of habitat types in ea
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Figure 4. Historic flow data at the Virgin River near St. George gage (1950-1956).
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Figure 5. Historic flow data at the Virgin River near St. George and Littlefield Gage (1991-
1996).
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Figure 6. Flows at five sites along the Gor
Figure 7. Example of flannelmouth suck
1995 in one pool at River Mile 49.1 and
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Figure 8. Fish densities along the length
1995, immediately following drying.
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Plate 1. Top left: Red Shiner Barr
Bottom Left: Shallow sandy run h
Wash section. Bottom Right: Dea
desert sucker, flannelmouth sucke
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Plate 2. Dead fish in dry section of Gor
sucker, large black bullhead).
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Plate 3. Top left: Shallow pool and
Littlefield Springs area. Top Right: A
Pools and snorkeling in upper Littlef
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APPENDIX A
Habitat maps of each sampling site with fish sampling locations.
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