GRAND CANYON MONITORING AND RESEARCH CENTER 

THE STATE OF NATURAL AND CULTURAL RESOURCES 

IN THE COLORADO RIVER ECOSYSTEM

2001

SCORE REPORT

May 15, 2002 

Draft - Do Not Cite, Photocopy, or Distribute 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

1 
TABLE OF CONTENTS

PURPOSE.......................................................................................................................................1 

WATER RESOURCES............................................................................................................. 2-8

 Reservoir Characteristics.....................................................................................................2 

Colorado River Hydrology ..................................................................................................4 

Unregulated Tributary Flows...............................................................................................5 

 Thermal Characteristics.......................................................................................................7 

AQUATIC BIOLOGICAL RESOURCES............................................................................ 9-19 

Aquatic Foodbase Studies....................................................................................................9 

 Trout Investigations...........................................................................................................12 

 Predation Issues .................................................................................................................16 

 Population Trends..............................................................................................................16 

Other non-native fish species.............................................................................................18 

TERRESTRIAL WILDLIFE AND HABITAT RESOURCES......................................... 19-25 

 Avifauna.............................................................................................................................20 

 Mammals............................................................................................................................21 

 Herpetofauna......................................................................................................................22 

 Terrestrial Invertebrates.....................................................................................................23 

 Vegetation..........................................................................................................................24 

ENDANGERED AND SPECIAL STATUS SPECIES....................................................... 25-32 

 Humpback Chub ................................................................................................................25 

 Flannelmouth Sucker .........................................................................................................29

 Southwestern Willow Flycatcher.......................................................................................30 

 Kanab Ambersnail .............................................................................................................31 

LITERATURE.............................................................................................................................33 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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SCORE REPORT 

2001 

Purpose – 

The mission of the Grand Canyon Monitoring and Research Program (GCMRC) is to 

provide decision makers with current and objective information needed to maintain the Colorado 

River Ecosystem (CRE) as a sustainable large river ecosystem given the complexities of multiple 

resources and conflicting resource values exist. Providing accurate and objective information on 

the status and trends of these physical, biological, and cultural resources has been envisioned as a 

critical component of the adaptive approach used in managing the CRE. The purpose of this 

report, “The State of Natural and Cultural Resources in the Colorado River Ecosystem”

(SCORE) is to present quantitative findings and descriptive information about the condition of 

the CRE for Glen and Grand Canyons. The report includes but is not limited to status data 

collected in recent years, but where possible includes trend information based on previous 

research and monitoring activities. Thus we seek to provide information on the status and trends 

of key CRE resources. 

Much of the data being collected by the GCMRC are directed toward documenting 

resource attributes and components of this ecosystem. Several issues have complicated 

developing an effective monitoring program to assess the ecological health and functioning of 

the Colorado River. The issues are, 1) frequency of counts and measurements required to assess 

all habitats and species contained within this river ecosystem. These efforts can be cost

prohibitive, and in many circumstances comparative data for long-term resource trends do not 

exist owing to either changes in methodologies, discontinuation of measurements, or the lack of 

an effective index to characterize a resource. 2) Introduced species add to the species richness of 

an ecosystem and in some cases are highly desirable due to their recreational or aesthetic value 

(e.g., trout); yet conversely, these same species may compete or prey on native species; 

consequently, are considered undesirable in parts of the CRE. 3) Typically, endangered species 

induce much public attention yet are difficult to monitor owing to their spatial or temporal rarity 

in a system. 4) Lastly, an effective program must provide a consistent set of indices that 

accurately measure the different resources of interest. 

Stakeholders and managers need to accurately articulate their underlying questions and 

conceptualize a framework they will use to assess the resource to management actions. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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Obviously there are numerous parameters that can be measured or estimated; yet selecting a suite 

of parameters that appropriately reflect specific resource responses is a necessary step. 

Presently, managers are beginning to establish an ecosystem management framework that 

prioritizes levels of resources as management targets. This SCORE report is a summary of the 

status and trends of certain resources, identified as major management goals previously specified 

in the environmental impact statement Glen Canyon Dam (GCD) operations (1), and the GCMRC 

long-term monitoring and research strategic plan (2). We have reduced the number of parameters 

to reflect selected resource trends based on GCMRC’s perception of their importance to 

stakeholders and the CRE. From this draft SCORE report GCMRC hopes it will elicit 

constructive responses and generate requests for greater specificity in data presentation from

management and stakeholders. Additional information regarding specific resources can be 

obtained from reports and publications cited in the text; as well as a bibliographic listing of 

previous studies available at the GCMRC web site (http://www.gcmrc.gov/). 

Water Resources 

 Reservoir Characteristics - Water year (WY-2001), physical and water quality characteristics 

of Lake Powell reservoir responded to climatic changes in the Upper Colorado River Basin, 

evidenced by reduced unregulated inflow equaling 6.96 MAF (3). This represented a 29% 

decrease from the reservoir’s average unregulated inflow (Fig 1). Reservoir elevation varied in 

response to inflow and descended from a maximum 1121 to 1117 m. The physio-chemical 

processes occurring in the reservoir regulate the water quality characteristics entering the 

downstream section of the CRE. Conductivity is a water quality parameter that has 

demonstrated considerable utility. Previous findings specific to the downstream section of Glen 

Canyon have correlated higher conductivity with higher nutrient concentration (4) and greater 

water clarity (5). Conductivity in 2001 began to increase after a decade long trend of reduced 

salinity (3), in response to reduced reservoir levels and inflow volume (Fig 2). There is also some

evidence to suggest that nutrients (i.e., orthophosphate, nitrate and nitrite) are becoming elevated 

in the reservoir (3). Additionally, other studies indicate a positive correlation between optical 

water clarity conditions and conductivity (5). Because of the present physical and chemical 

conditions in Lake Powell and the positive correlation existing between increased conductivity 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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with nutrients and light availability we would project that environmental conditions would be 

more conducive for autotrophic production in Glen Canyon. Monitoring data for the past decade 

specific to limnological changes occurring in the Lake Powell reservoir can be obtained from the 

GCMRC Integrated Water Quality Program(3). 

Outflow

1 -O c t

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'01 Inflow

Avg Inflow

1921-2001

Outflow

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Figure 1, hydrographic time series showing daily averages at Lake Powell Reservoir between 1 

October 2000 to 30 September 2001, data series represents surface elevation, unregulated inflow 

discharge, regulated outflow and mean annual inflow discharge (1921-1963) of the Colorado 

River. 

L a k e S t a g e ( m - a m s l)

Lake stage

L a k e S t a g e ( m - a m s l)

Lake stage

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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975

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Depth in Elevation (m)

Conductivity (μS/cm)

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1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

0

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penstock depth

jet-tube depth

Figure 2, vertical conductivity (μS cm-1) profile measured at the Wahweap forebay station above 

Glen Canyon Dam. Time series are based on quarterly sampling that extends from September 

1990 to September 2001. 

Colorado River Hydrology - 2001 was an unusual year with the enactment of emergency 

criteria at Glen Canyon Dam (GCD), where Western Area Power Authority responded to a series 

of irregular electrical shortages in California. It was observed that flow releases departed a 

number of times from the established ramp rates as stipulated in the GCD-EIS (1) normal 

operating criteria. A total of 20, 1-hr periods exceeded the 4,000 cfs/hr ascending rate, with a 

median up-ramp rate of 4,125 cfs/hr (SE 490), and with a maximum rate of 8,280 cfs/hr. 

Alternately, a total of 420, 1-hr periods were observed to have exceeded the established 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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descending ramp rate of 1,500 cfs/hr. The median down-ramp rate was 1,560 cfs/hr (SE 116) 

with a maximum descent of 2,230 cfs/hr. Scheduled daily discharges varied seasonally as 

indicated in Fig 3. No discharge flows exceeded normal EIS operating criteria (maximum

25,000 cfs, and minimum 5,000 cfs). The hourly maximum, minimum and median flow releases 

observed for WY-2001 were 21,090 cfs, 6,010 cfs, and 11,610 cfs, respectively. The WY-2001 

annual release volume from GCD was 8.23 MAF (3). 

 

Glen Canyon Dam Daily Discharge

WY-2001

DATE

DISCHARGE (cfs)

2000

6000

10000

14000

18000

22000

26000

1-Oct

31-Oct

30-Nov30-Dec

29-Jan

28-Feb30-Mar

29-Apr

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MEAN

MIN

MAX

Figure 3, annual hydrograph for the 2001 water-year (October 1, 2000 - September 30, 2001) 

representing the daily mean, minimum and maximum discharge release from Glen Canyon Dam.

Unregulated Tributary Flows - The relationship of flow frequency, duration and magnitude 

for the primary tributaries in Grand Canyon are fundamentally significant to the ecological and 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

5 
 

physical processes occurring in the Colorado River ecosystem (7, 8). For purposes of spatial 

reference all sampling locations referred to in this SCORE report are in relationship to the 

distance from Lees Ferry (0 RM). Values are positive in the downstream direction from Lees 

Ferry, and negative in the upstream direction toward Glen Canyon Dam. 

The frequency and magnitude of tributary discharge determines the extent to which 

suspended sediment are supplied to the system and available for transport, channel and bar 

deposition, degree of water clarity conditions for autotrophic production, habitat availability for 

rearing larval-young fish and spawning adults, and potential piscivory by visual sight-feeders. 

The cumulative discharge frequency represents the mean daily discharge values drawn from the 

U.S. Geological Survey streamflow records for two gaged tributaries (Paria River, 1.0 RM; and 

Little Colorado River (LCR, 61.5 RM)), and reflects the frequency of time when flows are less 

than or equal to a specific discharge (cfs). For comparative purposes we have graphed the 

cumulative flow frequency for the period of record for each of the two major gaged tributaries. 

Mean daily discharge records extend from 1923 to present for the Paria River, and from 1946 to 

present for the LCR. 

Frequency analysis provides a means to compare recent flow events for the last two water 

years WY-2000 and WY-2001 (i.e., water year (WY) and extends from October 1 to September 

30 the following year), relative to the probability distribution for a specific tributary based on the 

entire period of record. Tributary discharge frequency for the past two years was considerably 

less than the average distribution for the entire period of record (Fig 4). The cumulative flow 

frequency for the Paria River in WY-2001 was slightly less than the cumulative average for the 

period of record. Alternately, the LCR flow frequency in WY-2001 was slightly greater for 

lower volume flows and less frequent for flows greater than 80 cfs. For WY-2000, we should 

have observed considerably less sediment supplied to the river system from either tributary 

source. Additionally, WY-2000 was associated with experimental low summer steady flow 

(LSSF) (9), which resulted in greater water clarity for phytobenthic production (10), and perhaps a 

potential for a higher incidence of piscivory for susceptible sized fish. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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Tributary Discharge Frequency

 

Cumulative Frequency

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Figure 4, Cumulative frequency analysis of the two primary gaged tributaries, Paria River, 

26.5-RKM (USGS 09382000) and the Little Colorado River, 124.5-RKM (USGS 09402000) of 

the Colorado River, encompassing the period of record and the two most recent water years 2000

& 2001 (October 1 - September 30). 

 

Thermal Characteristics – The thermal characteristics for the Colorado River mainstem are 

influenced by the hypolimnetic releases originating from Glen Canyon Dam. For WY-2001, 

mean daily temperatures from GCD were stenothermically cold, and annually averaged 9.2EC, 

and varied little between seasons. Although, little seasonal variation occurred, the warmest 

releases were during the winter period (maximum, 10.4EC) and coolest in late spring (minimum, 

8.5EC). Additionally, longitudinal warming was observed 390 km downstream from GCD at 

Diamond Creek (225 RM) and attained a maximum daily temperature of 19.6EC (Fig 5). 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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Colorado River Mainstem Daily Temperature

TEMPERATURE C

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22

10/1/99

10/31/9911/30/9912/30/99

1/29/002/28/003/29/004/28/005/28/006/27/007/27/008/26/009/25/00

Glen Canyon Dam (-15 RM)

Diamond Creek Gage (224 RM)

WY2000

DATE

TEMPERATURE C

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10/31/0011/30/0012/30/00

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WY2001

Colorado River Mainstem Daily Temperature

TEMPERATURE C

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10/1/99

10/31/9911/30/9912/30/99

1/29/002/28/003/29/004/28/005/28/006/27/007/27/008/26/009/25/00

Glen Canyon Dam (-15 RM)

Diamond Creek Gage (224 RM)

WY2000

DATE

TEMPERATURE C

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10/1/00

10/31/0011/30/0012/30/00

1/29/012/28/013/30/014/29/015/29/016/28/017/28/018/27/019/26/01

WY2001

Figure 5, thermal characteristics of the Colorado River, demonstrating seasonal and daily 

variability (mean, minimum, and maximum) at two sites, Glen Canyon Dam (0-RKM) and 

Diamond Creek gage (390-RKM). The annual thermograph for the two most recent water years 

(October 1 - September 30); Fig-6A, WY2000, and Fig-6B, WY2001. 

In evaluating summer thermal characteristics for WY-2001 (i.e., extending from June 1 

through August 31; 92-days), the mean daily temperature was 9.1EC at GCD, and 16.9EC (SE 

0.1) at Diamond Creek, representing a 7. 8EC longitudinal increase (Fig 6A). The maximum 

water temperatures attained downstream were just at or below levels (16 - 22EC) considered 

suitable for spawning (6) by most native fish. In comparison with the previous year (WY-2000), 

marked by experimental low summer steady flows (8,000 cfs), mean daily temperatures for the 

same time period (June 1 - August 31, 2000) were 9.44EC at GCD and 19.46EC at Diamond 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

8 
 

Creek (Fig 5B). This longitudinal increase in temperature downstream (10.02EC) over the 

summer period represented a 2EC difference compared to 2001. 

Aquatic Biological Resources 

Aquatic foodbase studies – As predicted, the phytobenthic community has responded 

positively to flow modifications by having increased in biomass. This response is primarily a 

function of establishing a higher minimum flow level, which has increased the total wetted area 

and reduced the frequency of atmospheric exposure that often led to desiccation (13, 15). 

Cladophora glomerata, a green filamentous and branched alga, structurally supports a diverse 

assemblage of diatoms(11, 12, 13), and provides habitat for aquatic macroinvertebrates(14, 15). This 

alga/epiphytic relationship has been recognized as an essential trophic element in this aquatic 

ecosystem (13, 14, 16). For this reason, maintaining or increasing this filamentous alga or other 

aquatic foodbase has been considered an important stakeholder objective. 

Although, overall phytobenthic biomass has increased in this system, C. glomerata is no 

longer the dominant autotrophic component, nor has it increased in overall biomass since 

instituting flow regulations. Instead an alternate phytobenthic assemblage, referred to as 

miscellaneous algae, macrophytes and bryophytes (MAMB) (13, 17) has become the dominant 

autotrophic assemblage. Prior to 1996, this same assemblage had not been previously observed 

in significant quantities. Whether or not the causal response is principally due to flows is 

unknown. This same autotrophic assemblage is known for being better adapted to lower nutrient 

conditions than C. glomerata (4, 15). Additionally, Oscillatoria sp., a blue-green alga, is more 

prevalent in the mid- and lower portions of Grand Canyon and is associated with increased 

turbidity (8). Although this algal component has recently increased in this system it is not been 

known for its trophic importance (15). 

A total of six sites are annually monitored (0.5, 1.5, 61.3, 69.1, 128.9, and 205.5 RM), and 

depending on seasonal conditions, phytobenthic growth in the lower downstream riverine 

sections of Marble and Grand canyons are strongly variable due to sediment inputs from the 

major gaged and ungaged tributaries (8). For this reason, annual trends derived from the 

monitoring data are displayed only for the early summer period (June-July) because of typical 

high water clarity conditions for autotrophic production. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

9 
 

0

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Mean AFDM - g / m 2

Oscillatoria

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1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR

Mean Density - # / m 2

SNAILS

INVERTS

Figure 6, trend in total mean summer (June-July) phytobenthic biomass and densities in the 

Colorado River; Fig 7A, mean summer autotrophic biomass (g m-2); and Fig 7B, mean summer 

macroinvertebrate densities (# m-2). Samples were collected during June at six monitoring sites 

located at 0.5 RM, 1.5 RM, 61.5 RM, 69.1 RM, 128.9 RM, and 205.5 RM. 

The graphic representation shown in Fig 6, represents the mean quantity of autotrophic 

biomass and macroinvertebrate densities for the entire CRE during the early summer period (18). 

There has been three-fold or greater biomass and densities in the autotrophic and 

macroinvertebrate community. The phytobenthic data represents standing biomass and densities 

at or slightly below 8,000 cfs (227 m3

s

-1) varial zone (zone of stage fluctuation). These standing 

crop estimates do not necessarily reflect biomass and densities in the deeper mid-channel section. 

The frequency of tributary flows varies between years and may in part explain differences in 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

10 
 

biomass and density values between years. As observed in the winter and early spring period of 

1998-1999, flood frequency was higher for LCR and Paria River, which may explain some of the 

variation in phytobenthic biomass observed between years. 

Recent taxonomy based on internal and shell morphology has identified this small aquatic 

snail as Potamopyrgus antipodarum. This species is native to freshwater lakes and streams of 

New Zealand; however, has recently expanded its distribution world-wide (Europe, Asia, and 

North America). This snail was originally detected in the mid-1980s in Idaho, and has since 

dispersed to other states (i..e., Montana, Wyoming, California), most recently Arizona (Colorado 

River, Lees Ferry tailwater) and possibly Utah (Green River, Flaming Gorge tailwater). There is 

considerable concern regarding whether this particular invasive snail species may exert a strong 

influence on the overall phytobenthic community through competitive grazing pressures (18). 

Although the initial period of encroachment into CRE are presently unknown., attempts are 

being made to reconstruct arrival time using specimens from voucher collections. Recent 

GCMRC findings would suggest a potential decrease in densities of other aquatic invertebrates 

(10). 

Gastropods (snails) have now become the dominant invertebrate in this system; however they 

appear to be more commonly found at highest densities in the Glen and Marble canyons. The 

distributional pattern of snails indicate that they are almost absent in the lower and western 

Grand canyons (Fig 7). During early summer, the highest snail densities were observed in the 

Glen and Marble Canyon sections. In WY-2000, during the LSSF the highest snail densities 

were observed in the Lees Ferry section (10, 18), and represented over 95% of the 

macroinvertebrate composition (10). Overall mean estimates in recent years for the entire CRE 

show snail densities (numeric count m-2 ) ranged between 3,700 to 1,000 m-2 (Fig 6). It appears 

that their establishment and population expansion in the CRE followed the 1996-beach habitat 

building flow (BHBF) (18). Yet, it is unknown whether or not the BHBF and/or the stabilized 

flow conditions were responsible. However, we do known that these snails have continued to 

persist in this system since, and appear to favor stable flow and high clarity conditions. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

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0

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0.8 3.1 98.62 109.61 202.9 328.8

AFDM - g / m2

Oscillatoria

MAMB

Cladophora

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SAMPLING SITES

Density - # / m2

Snails

Misc.

Chironomids

Simulids

Oligoch

Gammarus

Limbricids

Fig 8, WY2001 longitudinal trend in standing phytobenthic biomass and 

macroinvertebrate densities distributed throughout the Colorado River. Fig 8A,

represents mean summer autotrophic biomass (g / m2 ); and Fig 8B, mean summer

aquatic macroinvertebrate densities (# / m2 ). Samples were collected during June

at six monitoring sites located at 26.3 rkm, 30 rkm, 123.5 rkm, 134.6 rkm, 228 rkm,

and 354 rkm.

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

12 

Trout investigations - Monitoring efforts have emphasized understanding the population 

size, dynamics and distribution of rainbow (Onchorynchus mykiss; RBT) and brown trout (Salmo 

trutta; BNT) in the Colorado River. From a management perspective, RBT represent an aquatic 

resource of significant importance in the Colorado River for its recreational value upstream in 

Glen Canyon; however, downstream from Lees Ferry (0-RM) both trout species are of concern 

due to their potential risk to native fish resources in Grand Canyon. Since management 

objectives vary spatially we have separated the monitoring efforts to reflect these spatial 

differences. 

Lees Ferry Fishery - The size-class distribution of electrofishing catch indicates that over 

35% of fish are equal to or less than 150 mm in length. The desirable management goal of 

having a high proportion of wild-spawned fish has been attained. Presently, all RBT in Glen 

Canyon are derived solely from natural spawned stock since stocking of hatchery-reared fish was 
 

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1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

PSD

hi

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Mean

Figure 8, Proportional Stock Density (PSD) is the proportion of fish larger than 406 mm (16 in.) 

total length (TL) contained within the catchable sized population (>305 mm; 12 in. TL). The 

annual trend in PSD indicates that the overall proportion of fish >406 mm TL has significantly 

decreased since 1991. 

discontinued in 1999. This signifies a major change in the fishery from the mid-1980's when 

over 70% of RBT were hatchery reared fish. Estimates of actual population size in Glen Canyon 

made in 1998 have indicated that the quantity of catchable sized fish greater (>305 mm; 12 in.) 

have increased significantly from 1990, from 98,000 (19) to over 260,000 (20). This has 

represented a 2.6 fold increase over the target population level (100,000). 

Graphically represented are the proportional stock density (PSD) estimates for RBT (1990-

2001) in Glen Canyon (Fig 8). The PSD is a useful metric to quantify the proportion of large 

RBT (> 406 mm; 16 in.) contained within the catchable sized population (>305 mm; 12 in) of 

RBT. The annual trend shown in Figure 8 would indicate that the proportion of fish larger than 

>406 mm has significantly decreased since 1991. Additionally, the decline in PSD appears to be

negatively correlated with RBT abundance (Figure 9). This observation, coupled with the 

recognition that this is primarily a catch and release fishery for all size classes of fish, suggests 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

13 
 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

14 

Fig 10, represents both observed and predicted angler catch effort of RBT in the Lees Ferry, 

Glen Canyon Reach, from 1980 to 2001. These catch efficiency data (CPUE) reflect both 

0

1

2

3

4

5

6

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

Catch per Minute

Figure 9, mean catch-per unit effort (CPUE, 95% CI) of rainbow trout caught in Glen Canyon 

using electrofishing from 1991 to 2000. Mean CPUE is equivalent to the number of fish (RBT)

caught per minute, based on nine sampling sites over three sampling periods per year. 

that the decline in RBT PSD is likely being driven by sustained large recruitment and density 

dependent growth affects. This phenomenon, called conservation of biomass, is typical of 

fisheries experiencing continued large recruitments and low harvest rates. 

In response to modified flows, population densities of RBT initially indicated a positive 

increase in relative abundance (electrofishing CPUE; Fig 9). However, since 1997 sampling 

trend has shown that relative abundance (mean CPUE) has stabilized and perhaps is decreasing. 
 

e irical data collection efforts, and the recent development of a AGF-RBT Stock Synthesis 

Model (20) (RSSM) that simulates inter-annual angler catch rates. RSS Model was paramete

mp

rized

igure 10, represents both observed and predicted angler catch effort for rainbow trout in the 

ees Ferry, Glen Canyon Reach, from 1975 to 2001. These angler catch efficiency estimates 

CPUE) reflect both empirical data (AGF creel surveys), and predicted catch efficiency based on 

n determining the present RBT and BNT distribution and population size in the Colorado River. 

using stock assessment parameters, GCD flows, and calibrated to observed angler catch data 

collected by AGF creel surveys. Although, this data has indicated a sizeable RBT population 

available to recreational angling, these results suggest that predicted angler catch rate has 

decreased (Fig 10), and in the near future may reflect qualitatively on angler satisfaction. 

0

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1.2

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1975 1980 1985 1990 1995 2000 2005

CPE (fish/hour)

Predicted Angler CPE Observed Angler CPE

F

L

(

the AGF - RBT Stock Synthesis Model used to simulate inter-annual angler catch rates. 

Grand Canyon - Recent non-native fish monitoring (21) as of WY-2000 have focused efforts 

o

Present results have indicated that these two species are widely distributed throughout the entire 

extent Grand Canyon. Total population estimates for RBT and BNT were 743,000 (CI.95% ± 

250,000 (excluding Glen Canyon)) and 56,000 (CI.95% ± 35,000), respectively. Population 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

15 
 

estimates were based on an extrapolation method that relates single pass electrofishing efforts

absolute fish density estimates derived from depletion sampling (21). Notable, are the differ

in the distributional patterns for these two salmonid species. RBT abundance indicates a 

longitudinal decline as a step-function with distance from Glen Canyon Dam. Highest 

abundance estimates are in Glen Canyon (20) averaging 11,000 fish km-1, decreasing to 6,0

km-1 in Marble Canyon, and 2,000-600 fish km-1 in the lower reaches of Central and We

Grand canyons. Whereas, BNT has shown the lowest abundance in the upper and lower portions 

of Grand Canyon, it reaches highest densities in the upper Granite Gorge (76-110 RM). BNT 

distributional pattern is correlated with the number of tributaries (Clear Creek, Bright Angel 

Creek, and Shinumo Creek) available for spawning. Although RBT demonstrates a longitudina

decline in abundance, this species still remains one of the most abundant fishes found through

the CRE. 

Predation - Results indicated that the frequency of piscivory was highest for BNT, averaging 

8% (n = 15

to

ences

00 fish

stern

l

out

4) in comparison to < 0.5% for RBT (n = 241) (25). Additionally, significant variation 

p

nce

 CPUE) are the best trend indicator for RBT and BRT population 

nsity, as

the

uld be noted

tly

in BNT predation occurred in response to seasonal prey availability and spatial proximity to the 

LCR (25). Although BNT appear to be the most significant piscivore in this system, cumulative 

predation by RBT may have as much of an affect owing to this species overall abundance. The 

recent attern in salmonid piscivory corresponds with previous field assessments (6, 26) and 

anecdotal observations (23). 

Trout population trends - At present, site-specific estimates of relative abunda

(electrofishing arithmetic mean

densities. This index parameter appears to respond in proportion to the actual stock de

reported above. The trend analysis (1984 to 2001) represents the combined data sets from

previous investigations and monitoring efforts(6, 22, 23, 24, 27)

using electrofishing as a method to

monitor fish in the Colorado River. Although electrofishing methods were used throughout

period of this trend analysis (1984-2001), it sho that the electrofishing equipment 

changed after the mid 1980’s (i.e., boat, voltage pulsator unit, and electrical field). Because of 

this change, CPUE reported from sampling activities during the mid 1980’s may not be a direc

comparable with index of relative abundance with present sampling efforts. With this caveat in

mind, annual population trends (Fig 11) for both RBT and BRT based on CPUE indicate that 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

16 
 

relative abundance for both species observed at the LCR Inflow (55-65 RM) and Bright Angel 

Creek (85-94 RM) areas decreased during the early to mid-1990's. Subsequently, population 

densities for both trout species have increased approximately 400-500% in recent years (1995-P

This response mirrors the response of RBT in the Lees Ferry Reach (-15.5 to 0.0 RM). 

The overall trout population response is strongly correlated to flow modifications, as part of 

the EIS preferred alternative (1)

. During the mid-1980's in the Bright Angel Creek area

). 

trout

electrofishing sampling would indicate that trout composition was composed predominantly of 

RBT (represented 73% by composition) (assuming no gear selectivity differences between

species). In comparison, over the last 10-years a compositional shift has occurred where BRT 

now represent 72% of the all trout found adjacent to the Bright Angel Creek area (based solely 

on electrofishing). Additionally, although the CPUE data suggests that the Bright Angle Creek 

reach has switched from being dominated by to being dominated by BNT, species composition 

in LCR inflow reach has remained relatively constant with the actual proportion of BNT to RBT

representing less than 3% of the total (Figure 11; 2nd y-axis). 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

17 
 

LITTLE COLORADO RIVER INFLOW

 

RBT CPUE (# fish / 10 hr)BNT CPUE (# fish / 10 hr)

0

2

4

6

8

10

12

14

16

18

20

22

0

200

400

600

800

1000

1200

1400

1600

1800

Rainbow Trout

Brown Trout

BRIGHT ANGEL CREEK

 RBT CPUE (# fish / 10 hr)

0

 BNT CPUE (# fish / 10 hr)

200

400

600

800

1000

1200

1400

1600

1800

0

200

400

600

800

1000

1200

1400

1600

1800

Dec-1983Dec-1985Dec-1987Dec-1989Dec-1991Dec-1993Dec-1995Dec-1997Dec-1999

Rainbow Trout

Brown Trout

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

18 

Figure 11, trends in relative abundance for rainbow trout and brown trout (1984- 

2001) based on electrofishing effort (arithmetic mean CPUE) specific to A) the Little 

Colorado River inflow (55-75 RM); and B) Bright Angel Creek (85-105 RM). 

Other non-native fish species - The distribution of other non-native species are confined 

predominantly to the middle and lower portion of the CRE (140-225 RM), or certain tributaries 

and specialized habitat (e.g., backwaters, springs and littoral shoreline) (28, 29)

. The overall 

abundance of this warm-water fishery is considerably less than the non-native trout species. Due 

to a greater emphasis being placed on native fish sampling; as well as gear type limitations 

(electrofishing), we are unable to confidently assess relative abundance of these larger (carp and 
 

catfish) and smaller bodied fish (fathead minnows, red shiners) persisting in the CRE. In 

general, these other non-natives species have increased in tributaries. Yet, in recent years there 

has been a compositional shift with increasing non-natives fish caught in the LCR (Fig 12). The 

LCR drainage, and the flow frequency and magnitude of hydrological events appear to function 

as the source of origination and as the primary mechanism for dispersing small-bodied fish into 

the CR mainstem. Once in the mainstem successful reproduction and recruitment of these nonnatives occurs in other tributary confluences; as well as along near shoreline habitat 

(27, 28)

. 

Warm, stable, and low velocity habitats are conducive for high rapid production of many smallbodied fish as was observed in WY-2000 during the LSSF (30)

. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1991199219931994199519961997199819992000

NON-NATIVE

Speckled Dace

Humpback Chub

Flannelmouth

Sucker

Bluehead Sucker

Figure 12, estimated species composition from USFWS sampling in the Little Colorado River 

using hoopnets, during 1991-1995, and 1998-2000. 

Terrestrial Wildlife & Habitat Resources 

Beginning in WY-2001, an integrated effort was initiated to census and monitor 

invertebrates, mammals and herpetofauna concurrent with the avifauna and terrestrial vegetationGCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

19 
 

monitoring program. The overarching purpose has been to provide ecological information on the 

terrestrial community. Ground dwelling arthropods can be an effective and an easily monitored 

group to measure faunal responses to change in habitat quality. Additionally, using an integrated 

approach assists in relating vegetational structure and trophic links to the distribution and 

abundance of the vertebrate (i.e., birds, mammals, herpetofauna) and arthropod communities. 

The data presented here for this first-year terrestrial monitoring effort are based on spatial 

comparisons made between the up-stream and downstream sections of Grand Canyon (i.e., 

between RM 0.0 to RM 61.5 and the lower Grand Canyon RM 61.5 to RM 225), and three 

riparian habitat zones along the CRE (i.e., the hydrologically active zone, HAZ; new high water 

zone, NHWZ; and old high water zone, OHWZ) (31). The predominant vegetative composition 

consisted primarily of Coyote willow (Salix exigua), horse-tail (Equisetum arvense), brikellia 

(Brickellia longifolia) in the HAZ; tamarisk (Tamarix ramosissima), and arrow-weed (Pluchea 

sericea) in the NHWZ; and honey mesquite (Prosopis glandulosa), catclaw acacia (Acacia 

greggii), and net-leaf hackberry, (Celtis laevigata) in the OHWZ.

 Avifauna - Surveys to determine the status of breeding birds detected (visually or auditory) a 

total of 1787 birds, at 55 monitoring sites (31). This represented a total of 48 bird species found 

along the river corridor. Patterns in breeding bird composition and their relative proportion were 

similar to those found in previous studies (35, 36). The average number of birds encountered 

during walking count transects for all zones was 8.5 (SE 0.6). The most common species of 

birds observed were Lucy’s warbler (44.2%, LUWA), house finch (9.2%, HOFI), Bell’s vireo 

(8.1%, BEVI), and blue-gray gnatcatcher (8%, BGGN). The distributional pattern of neotropical 

migrants and year-long residents occupying riparian zones in the CRE indicated that the highest 

avifauna richness (# of species) was in the NHWZ, although absolute bird abundance was 

significantly higher in the OHWZ (Fig 13). Waterfowl surveys were not conducted last year, but 

were commenced as of this year (WY-2002). 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

20 
 Bird Abundance per Pat h

0

5

15

New High Water

Old HighWater

Marble Canyon Grand Canyon

Bird Richness per Plot 0

1

2

3

4

5

NHW Richness

OHW Richness

Marble Canyon Grand Canyon

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

21 

 c 10

Figure 13, mean avifauna relative abundance and species richness based on walking transects 

conducted in Marble and Grand Canyons for two hydrological zones (new high water zone, 

NHWZ; and old high water zone OHWZ). 

Mammals - Results for small mammal trapping efforts measuring relative abundance of 

rodents occupying the three different habitat zones has indicated that nearly one half of all small 

mammals were captured in the OHWZ; whereas the remainder were equally distributed between 

the HAZ and NHWZ (31)

. Inclusive of all habitat zones, observed trap success (captures/trap set) 

was very high (27.6%) along the river corridor, indicative of high mammal densities. A total of 

20 native mammal species were identified, and represented 77% of the total number of mammals 

to have previously been documented to occur along the CRE. Differences were observed for 

mean rodent relative abundance among the three different hydrologic zones with the highest 

rodent abundance occurring in the OHWZ. Reach comparisons made for Marble Canyon and 

Grand Canyon indicated that rodent abundance increased in the OHWZ in the mid- and lower 

western Grand canyons (Fig 14). 
 

Mammal Abundance per Transect

0

5

10

15

20

Water's Edge

New HighWater

Old High Water

Marble Canyon Grand Canyon

Ma

m

mal Richness per Transect

0

1

2

3

4

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Figure 14, mean relative abundance and species richness based on small mammal trapping 

efforts in Marble and Grand Canyons for three hydrological zones (hydrologically active zone, 

HZA; new high water zone, NHWZ; and old high water zone OHWZ). 

Herpetofauna - During WY-2001, a total of seventeen species of herps were observed, of 

these the most common taxa were the Western whiptail, Cnemidophorous tigris; desert spiny 

lizard, Sceloporous magister; side-blotched lizard, Uta stansburiana; tree lizards, Urosaurus 

ornatus; and Woodhouse’s toad, Bufo woodhousei (31). Differences were observed in herp 

relative abundance by hydrologic zone, with the greatest abundance occurring in the OHWZ. 

Additionally, reach comparisons made between Marble Canyon and Grand Canyon showed that 

the mean herp abundance (#/transect) increased for both OHWZ and NHWZ in the downstream

reach (61.5 - 225 RM). Overall, the highest abundance in herps was observed within the 

OHWZ; although, species composition was found to be distinctly different between the habitat 

zones (Fig 15). 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

22 
 

Herp Richness per Transect

0

1

2

3

4

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Mean Herps per Transect

0

5

10

15 Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Figure 15, mean abundance and species richness for herpetofauna observed within Marble and 

Grand Canyons along a linear transect (# / 100 m) for three hydrological zones (hydrologically 

active zone, HZA; new high water zone, NHWZ; and old high water zone OHWZ). 

 Terrestrial invertebrates - This program has begun to establish a voucher and reference 

collection. This will be an ongoing project, and currently over 2200 specimens have been sorted, 

preserved and partially identified. Preliminary results have indicated that abundance and species 

richness of terrestrial arthropods are higher for sites below the LCR than found in Marble 

Canyon (Fig 16) (31). Yet, comparisons made of average abundance between the three 

hydrological zones showed no significant differences. The overall pattern in ground dwelling 

arthropods demonstrated an increase from the wetted shoreline up into the OHWZ. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

23 
 Arthropod Abundance per Transect

10

100

1000

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Arthropod Richness per Transect

0

4

8

12

16

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

24 

Figure 16, mean arthropod relative abundance and species richness based on sampling efforts in 

Marble and Grand Canyons for three hydrological zones (hydrologically active zone, HZA; new 

high water zone, NHWZ; and old high water zone OHWZ). 

However there were distinct differences observed in the arthropod composition that resided 

within each of the hydrologic zones. Several taxa appear specialized to specific habitats with 

little distributional overlap observed between zones. For the 127 taxa so far identified, 82 were 

found to reside in only a single habitat zone. Indicating that ground dwelling arthropods were 

responding to a gradient of soil texture and or soil moisture across zones. There appears to be 

some compositional relationship between birds and ground-dwelling invertebrates and the 

vegetation in the monitoring sites, rather than the degree of productivity and biomass associated 

with the NHWZ. 

Terrestrial vegetation – Vegetational transect data from the monitoring sites has indicated 

that differences existed in total vegetation volume (VV) between OHWZ and NHWZ. The 

highest VV was observed in the NHWZ and lowest in HAZ (31). The vegetational cover (VC) 

and plant species richness was strongly influenced by the stage discharge relationship. VC was 

lowest in the 15,000 cfs zone, highest between 25,000-35,000 cfs and lowest in the upper 45,000 

cfs zone. The plant species found in the 15,000 cfs zone were on average facultative wetland 

species. The highest total VV measured by zone, showed that species richness for woody plant 
 

species was highest in the NHWZ for Marble Canyon; yet, in Grand Canyon species richness 

shifted toward plants favoring the OHWZ (Fig 17). 

TVV per Transect

0

50

100

150

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Plant Richness per Transect 0

1

2

3

4

Water's Edge

New High Water

Old High Water

Marble Canyon Grand Canyon

Figure 17, mean total vegetation volume (TVV) and species richness based on linear transects in 

Marble and Grand Canyons for three hydrological zones (hydrologically active zone, HZA; new 

high water zone, NHWZ; and old high water zone OHWZ). 

Endangered & Special Status Species 

Humpback chub - Earlier research and monitoring studies were centered on a descriptive 

characterization of life history of humpback chub (HBC), Gila cypha. Although this information 

has been valuable it has not allowed until recently, the reconstruction of HBC population 

dynamics and trends in the Grand Canyon. In the last two years we have restructured and 

changed the emphasis of the overall monitoring approach and instituted a sampling effort to 

estimate HBC abundance and recruitment success. We can presently estimate abundance and 

recruitment only for the LCR population because the numbers of fish captured elsewhere is too 

small. 

The transient nature of HBC residence within the LCR poses significant sampling design 

problems in attaining unbiased estimates of abundance. Typically the portion of the adult 

population residing in the CR mainstem exhibits seasonal immigration into the LCR (earlySpring) to spawn. Following the spawning period, these spent adults disperse back to the CR 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

25 
 

mainstem by late June-July. The remaining and apparently small proportion of the total adult 

population continues to reside in the LCR as yearlong residents. The reproductive contribution 

and spatial fidelity of these two adult sub-components remains largely unknown. Lastly, young 

of year fish and juveniles from the previous reproductive year are occasionally dispersed into the 

CR mainstem by monsoonal flooding or density dependent factors. 

Cornerstone to GCMRC’s recent shift to monitoring abundance and recruitment of HBC are 

the utilization of techniques collectively termed mark-recapture methods. Many of the models 

developed from these methods rely on the concept of geographic closure during and between 

sampling periods. To account for the fish movement issues described above, a serial sampling 

design using the two-event Chapman-Petersen abundance estimator (66)

 was incorporated into the 

GCMRC native-fish monitoring program beginning in 2000. This design includes a total of two 

sampling periods within the LCR (Spring and Fall) and one within the LCR inflow reach of the 

CR (Summer). Because the movement and distributional pattern of adult HBC in the LCR 

population are variable among years, this design also allows for a combination of estimators to 

be used strategically depending on how HBC movement patterns are manifested during any 

particular year. The spring sampling period was chosen to avoid violating assumptions of 

closure due to immigration. Sampling began in the LCR (May & June), followed by sampling in 

the CR-mainstem (August & September) and then repeated once again in the LCR (October & 

November). The sampling area was confined spatially and included the CR mainstem (55.6 to 

68.3 RM), and the LCR, which extended from the confluence upstream to 14.2 km. 

During the fall 2000, a total of 1,590 (CI 95% ± 992-2,552) individuals >135 mm TL 65

 were 

estimated within the LCR. Because this estimate was conducted during the fall, the majority of 

these fish are sub-adults. During 2001, attempts were made to increase the precision of the 

estimator and reduce the sampling bias attributed to minimal mixing and immigration timing of 

adult fish. Preliminary estimates from 2001 suggest that approximately 2,090 (CI 95% ± 1,611-

2,569) individuals >150 mm TL were present in the LCR during Spring (USFWS IN REVIEW), 

1,044 (CI 95% ± 446-1,644) individuals >200 mm TL were present in the CR mainstem (55.6 to 

68.3 RM) during Summer (SWCA IN REVIEW), and 1,106 (CI 95% ± 934-1,179) individual 

>150 mm TL were present in the LCR during Fall. Preliminary examination of the Spring 2001 

data further indicate that the estimated abundance of HBC > 150 mm TL in Spring 2001 may be 

biased low by an unknown amount because a substantial number of adult fish may have already 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

26 
 

spawned and exited the LCR before the initiation of sampling. With these preliminary estimates 

in hand and considering what is known about typical timing of adult movement (Valdez and 

Ryel 1995), the best abundance estimate for fish >150mm for the entire LCR population is the 

sum of the Summer CR abundance estimate and the Fall LCR abundance estimate. Under this 

assumption, the total abundance is approximately 2,150 (CI 95% ± 1,511-2,885) >150 mm TL. It 

is of interest to note that this preliminary estimate is nearly identical to the abundance of HBC 

>150 mm TL found in the LCR during Spring 2001. The reader should explicitly note that the 

2001 estimates are preliminary and subject to change as 2001 analyses are subjected to peer 

review and subsequent revision. 

A stock assessment model referred to as HBC Super-Tag has been in the process of 

development to evaluate population dynamics in HBC (42). The stock assessment method uses 

statistical and mathematical calculations in estimating past and current population abundance and 

production. This model uses a combination of estimated parameters (average recruitment, 

recruitment anomalies, and adult mortality) and other specified parameters (growth, vulnerability 

schedule, von Bertalanffy growth equation) (43, 44)

. It is hoped that in the future this stock 

assessment approach will allow one to construct quantitative predictions about fish population in 

response to alternative management choices. 

The modeling suggests that recruitment among brood years for the 1st year age-class (70-140 

mm) have significantly decreased (Fig 18). Combining independent data sets from previous 

annual sampling efforts (1987-2000) (37, 38, 39) in the LCR indicate that catch rates (# fish/hour) 

for age-1 and unmarked age-2 fish have substantially declined corresponding to the 1992-1998 

brood years. These independent analyses provide further evidence that the declining trend for 

HBC recruitment is real and suggest that recruitment following the 1991 brood year is lower than 

previous brood years contained in the period of record. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

27 
 Estimates of Age-1 Recruitment by Brood Year for the Little Colorado River Population of

Humpback Chub 

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Brood Year

Abundance

Supertag

AGFD

USFWS

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

28 

Figure 18, graphical representation of recruitment trends for the Little Colorado River population 

of humpback chub, reflecting the abundance of age-class 1 for a particular brood year. The trend 

analysis represents inter-annual variation in CPUE data from independent sampling efforts 

performed by both the Arizona Game and Fish Department and the US Fish and Wildlife Service 

as compared to estimated trends from the Super-Tag model. 

Furthermore, given that the model accurately estimated both recruitment and mortality rate, 

the modeled results would indicate that HBC abundance (>150 mm) has declined substantially 

between 1993-2000 (Fig 19) (37, 38, 39, 40). Although the actual causal mechanism for the 

recruitment decline is unknown, there are a number of hypotheses that could be responsible for 

the inter-annual trend. These include: 1) predation and competition occurring either in the CR 

and/or the LCR, 2) tributary hydrology, 3) GCD operations (research & interim flows 1990-01), 

and 4) parasitism (Asian tapeworm) (34). Although at first glance one might tend to conclude 

that predation by non-native salmonids are the single causal factor responsible for the 

recruitment decline due to the extraordinary increase in their abundance beginning in the mid 

1990’s, recognize that the increase in trout abundance in the CRE appears to follow rather than 

precede the observed decline in HBC recruitment (See Fig 11 & 18). However, it is likely that 

any of the above mentioned factors might act as a dominant controlling mechanism within any 
 

particular year. Therefore, it is critical that alternative hypotheses be evaluated (e.g. predation or 

parasitism within the LCR). 

Estimated Abundance of the Little Colorado River Population of Humpback 

Chub

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

HBC Abundance (>150 mm)

Supertag

Douglas and Marsh (1995)

Kaeding and Zimmerman

(1982)

USFWS (in Review)

Figure 19, graphical representation of the abundance of humpback chub > 150 mm (Total 

Length) contained within the Little Colorado River Population as estimated by Kaeding and 

Zimmerman (estimate germane for fish > 200 mm TL), Douglas and Marsh (using the null model 

from Program CAPTURE), USFWS (Chapman-Petersen Model), and Program Super-Tag. 

Flannelmouth sucker - A project to develop a stock assessment model for the flannelmouth 

sucker (FMS), Catostomus latipinnis, has been recently initiated by AGFD. Preliminary results 

have indicated that recruitment occurring in the LCR, although variable has not declined like the 

pattern observed for HBC. Fig. 20, shows that between 1993-1995, recruitment was 

considerably higher in the LCR tributary. Unlike HBC these fish are known to move 

considerable distances throughout the CRE, and actively spawn in a number of tributaries (28, 33). 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

29 
 

Even though this species is not limited to the LCR tributary for reproduction (32), it remains 

unknown whether or not adult FMS have high spawning fidelity to their natal streams. 

0

500

1000

1500

2000

2500

3000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

RECRUITMENT TO 15Omm

Figure 20, graphical representation of recruitment trends for flannelmouth sucker in the Little 

Colorado River. Trend reflects the abundance of a size class < 150 mm for a particular brood 

year. 

Southwestern Willow Flycatcher - The southwestern willow flycatcher (SWFL); Empidonax 

trailii extimus, is a neotropical migrant, that is an obligate riparian insectivore (47), preferring 

habitat near open water (46). The historical breeding range of the SWFL includes Arizona, New 

Mexico, southern California, and southern portions of Nevada, Utah, and perhaps southwestern 

Colorado, and extends east into western Texas. It probably winters from Mexico to Panama, 

with historical accounts from Colombia (48). The SWFL is distinguished from other subspecies 

by its distribution, morphology and color, nesting ecology (46, 47, 49, 50), as well as singing 

behavior (45). 

Surveys conducted during 2001, between Lees Ferry and Diamond Creek (0.0 to 225 RM) 

detected six individual birds at four different sites (31). Yet, of the SWFL’s surveyed this year 

only one breeding pair was confirmed. The remaining birds were suspected to be migrants or 

unsuccessful in establishing nesting territories. The single successful nesting pair was located at 

RM 50.4L downstream and contained two SWFL eggs and one brown-headed cowbird egg. 

Brown-headed cowbirds are brood nesting parasites that are implicated as being one of the major 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

30 
 

causes in the decline of this species. Fledgling success for this year was unknown. Since 1997, 

the inter-annual trend for SWFL nesting has been a single breeding pair per year (Fig 21). From

1982-2001, all of the willow flycatcher nests detected along the Colorado River corridor in 

Grand Canyon have occurred from RM 46 to RM 72 (31, 35, 51, 52, 53, 54, 55, 56). 

Southwestern Willow Flycatchers

Confirmed Nests

YEAR

NESTS

0

2

4

6

8

10

12

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Fig 21, number of confirmed southwestern willow flycatcher (SWFL) nests detected along 

the Colorado River corridor in Grand Canyon from 1982-2001. No surveys were conducted 

between 1988-1990. 

 Kanab ambersnail – The Kanab ambersnail (KAS), Oxyloma haydeni kanabensis (57), was 

listed in 1992 as a federally endangered landsnail (58). The extant populations of KAS are 

described from only a few localities: (1) Three Lakes, near Kanab Utah; (2) in seeps and springs 

along Kanab Creek, near Kanab, Utah; (3) at Vaseys Paradise, a spring in Grand Canyon, 

Arizona (59, 60), and 4) populations on the east slope of the Canadian Rocky Mountains in 

Alberta. In Grand Canyon, Vaseys Paradise (31.5 RMR) is a fast-flowing, cool, dolomitic-type 

spring, with abundant wetland and phreatophytic vegetation (e.g., crimson monkey flower, 

sedge, smartweed, and poison ivy, and non-native watercress). Although this site is 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

31 
 

recreationally popular, it is limited by access due to dense cover of poison ivy (Toxicodendron 

rydbergii) and vertical terrain (62). 

Natural over-wintering mortality is considered approximately 20-80% of the population. 

Seasonal population estimates made for low-zone habitat (below 100,000 cfs) are derived using a 

mean bootstrap estimates (Fig. 22). 

KANAB AMBER SNAIL - VASEY'S PARADISE (RM 31.5)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Mar-95

Jun-95

Sep-95Dec-95

Mar-96

Jun-96

Sep-96Dec-96

Mar-97

Jun-97

Sep-97Dec-97

Mar-98

Jun-98

Sep-98Dec-98

Mar-99

SAMPLING PERIOD

ABUNDANCE

KAS

Figure 22, Kanab Amber Snail 

For mitigative purposes, translocation efforts were conducted to establish and monitor 

secondary populations of KAS at three sites (Keyhole Spring, Upper Elves Chasm, and Lower 

Deer Creek) in Grand Canyon (61). An additional captive breeding population was established at 

the Phoenix Zoo. Translocation and success in establishing a self-sustaining population is 

presently being monitored. Considerable taxonomic debate has centered on this species, where 

conventional taxonomy based on internal and shell morphology is being revisited using 

molecular genetic techniques (64). Recent genetic findings (64), would suggest that large genetic 

distance exist between the Vaseys Paradise population and all other Oxyloma populations, yet 

taxonomic studies reveal certain populations are morphologically indistinguishable. This 

taxonomic debate has not been resolved. 

GCMRC – 2001 SCORE REPORT (Draft, May 15, 2002) 

32 
 

Literature Citations - and Bibliography for additional information (5 pgs.) 

1 U.S. BOR. 1995. Operation of Glen Canyon Dam, final environmental impact statement. 

U.S. Dept., Interior, Bureau of Reclamation. Salt Lake City, UT. 337 pp. 

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15 Blinn, D.W., J.P. Shannon, L.E. Stevens, and J.P. Carder. 1995. Consequences of 

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