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OCR for page 84
6
Organisms and Biological Processes
INTRODUCTION
Organisms and biological processes were studied through Glen Canyon
Environmental Studies (GCES) from three different perspectives: (1) as
ecosystem components, (2) as resources of specific economic or recreational
importance (trout), and (3) as a means of satisfying the requirements of the
Endangered Species Act (humpback chub, Kanab ambersnail, bald eagle,
southwestern willow flycatcher). These three perspectives, which were com-
bined in the GCES study plan, had somewhat different objectives. Thus, the
efforts of GCES were to some extent divided in three directions. In effect,
ecosystem analysis, which was the unifying theme for GCES, was partially
redirected by special concern over particular species. While the division of
priorities is easily understandable in view of the societal value attached to
trout and the legal requirements attached to endangered species, GCES was
often diverted from its goal of understanding the entire system by strong
focus of its resources on particular components of the system. This problem
is inherent in government studies of ecological systems and can work against
successful ecosystem analysis and prediction, as will be apparent from this
chapter.
The biological work of GCES can be divided into four parts: (1) lake
studies, (2) studies of the Colorado River between the Glen Canyon Dam and
the Paria River (26 km), (3) studies of the Colorado River below the Paria to
Lake Meacl (450 km), and (4) studies of the riparian zone. These components
are connected, of course, but each has distinctive communities and biological
processes.
84
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Organisms and Biological Processes
LAKE POWELL
Characteristics of Lake Powell
85
Impoundment of the Colorado River by Glen Canyon Dam has created a
qualitatively new kind of water source for the Colorado River below. Char-
acteristics of Lake Powell, together with operation of the dam, determine the
temperature, suspended and dissolved solids, nutrients, and organisms
passing downstream. Thus, any comprehensive assessment of the Colorado
River below Glen Canyon Dam must include Lake Powell (Figure 6.1~.
Lake Powell has been studied by several organizations and individuals
over more than 20 years. Stanford and Ward (1991) provide an excellent
interpretive summary of work completed through 1990. The Bureau of Rec-
lamation (BOR) has supported a range of studies on Lake Powell but has
taken particular interest in its effect on the mean salinity of water in the
Colorado River (BOR, 1987~. The National Science Foundation supported a
decade-long system study of the lake that encompassed water quality,
seasonal cycles, productivity, and aquatic community composition. This
work is well summarized by Potter and Drake (1989~. Various individual
projects beginning in the late 1 960 s have dealt with components of the biota
or specific aspects of water quality (Carothers and Brown, 1991~. Thus, the
starting point for GOES with respect to Lake Powell was an extensive but
somewhat amorphous information base. Routine GOES studies of the res-
ervoir did not begin until 1992, when quarterly synoptic surveys were initiated
(unpublished as of 1995) and focused studies were initiated on layering of the
upper water column and vertical distribution of oxygen (Marzolf, 1995~.
Although the filling of Lake Powell began in 1963, the reservoir did not
show strong development of stratification until about 1970 and did not fill to
capacity until 1980. Thus, in a physical sense, the history of the reservoir can
be divided into three intervals: 1963 to 1970, 1970 to 1980, and 1980 to the
present. Optimal operation of the reservoir from the viewpoint of power
production would require all water to pass through the generators at Glen
Canyon Dam, and this was possible until 1980. Since 1980, however,
operations entered a new era in which high runoff can require the release of
water for the purpose of protecting the dam (Chapter 4~. For example, use
of the dam's bypass system was necessary in 1983 in order to prevent water
from passing over the dam. Releases above power plant capacity are called
floods, although they differ in frequency and magnitude from natural floods.
OCR for page 86
86
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OCR for page 87
Organisms and Biological Processes
87
A succession of dry years could, of course, suppress the mean volume of the
reservoir well below capacity. The reservoir might then operate for a long
time with no floods, as it did between 1963 and 1983.
Although Lake Powell receives a very large amount of water from the
Colorado and San Juan rivers, as well as a small amount from the Dirty Devil
River, the large size of the reservoir allows a mean water retention time of
approximately 2 years (Table 6.1~. The residence time of the reservoir is
sufficiently long to allow complete sedimentation of the inorganic material that
enters it at the upper end. Lake Powell receives much sediment from its
water sources (Dawdy, 1991 ) but, because of its great volume, has lost only
a small proportion of its storage capacity through sediment accumulation.
A number of Lake Powell's features are characteristic of reservoirs in
general, while others are more unusual. Like many reservoirs, Lake Powell
has a dendritic shape and is much longer than broad, thus presenting the
possibility of great longitudinal variations in water quality or biotic char-
acteristics. Dendritic shape is accompanied by a high degree of shoreline
development (length of shoreline). Unlike most reservoirs, however, Lake
Powell has an essentially vertical shoreline around much of its perimeter. This
characteristic minimizes the amount of shallow water and the potential for the
development of macrophytes or other littoral communities and reduces the
importance of littoral biogeochemical processes.
As would be expected for most reservoirs, the water level of Lake Powell
fluctuates by several meters per year (8 m on average) and from year to year
(potentially 30 m or more). Because the water withdrawal point for Lake
Powell is fixed, the depth of withdrawal relative to the water surface fluctuates
seasonally and also changed as the lake filled between 1963 and 1980.
Recently, water has been mostly drawn from a zone 50 to 70 m below the
surface, depending on season and year.
Lake Powell is divided throughout all seasons into two chemically distinct
zones separated by a salinity gradient. The lower zone is consistently cold
(about 7°C) and has about 50 percent more dissolved solids than the upper
layer (Figure 6.2~. The upper zone shows a seasonal thermal cycle that is
characteristic of a warm monomictic reservoir with a substantial amount of
water withdrawal. This upper zone is warmest near the surface (exceeding
25°C in the summer) and coolest nearthe junction with the lower layer (about
7°C).
Division of the upper zone into clearly identifiable layers that could be
designated epilimnion, metalimnion, and hypolimnion would probably occur
if it were not for the continuous withdrawal of large amounts of water from the
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88
River Resource Management in the Grand Canyon
TABLE 6.1 Characteristics of Lake Powell Within 50 km of Glen Canyon Dam
Item
Amount Source
Surface area, km2
Maximum depth, m
Mean depth, m
Typical depth of water withdrawal, m
Approximate water retention time, years
Maximum surface temperature, ° C
Minimum surface temperature, ° C
Botto m te m p e ratu re, ° C
Suspended solids, mg/liter
Total phosphorus,,ug/liter
Dissolved phosphorus,,ug/liter
Nitrate-N, ,ug/liter
Soluble silica (S;O2), mg/liter
Total dissolved solids, mg/liter
Primary production, gC/m2/year
Chlorophyll a,,ug/liter
653
171
51
55
2
28
7
15
10
300
7
630
190
Zooplankton abundance, individuals per liter 20
Potter and Drake (1989)
Potter and Drake (1989)
Potter and Drake (1989)
GCES (unpublished)
Potter and Drake (1989)
GCES (unpublished)
GCES (unpublished)
GCES (unpublished)
Stanford and Ward (1991)
Gloss et al. (1980)
Gloss et al. (1980)
Gloss et al. (1980)
Gloss et al. (1980)
Gloss et al. (1981 )
Hansmann et al. (1974)
Paulson and Baker (1983)
and Sollberger et al.
(1 9891a
Sollberger et al. (1989)
°Possibly much higher; see Ayers and McKinney (1995a).
NOTE: Characteristics are for the upper 50 m of the water column unless otherwise specified.
bottom of this zone and the large additions of water at the upper end of the
reservoir. Continual voluminous water exchange of this type often blurs the
boundaries between thermal layers in reservoirs. In Lake Powell the distinc-
tion between metalimnion and hypolimnion istypicallyunclearforthis reason.
The summer epilimnion (mixed layer) is typically well defined, however, and
the overall thermal gradient causes stability of the water column during the
summer. Wind-generated mixing of the entire upper zone (50 to 70 m) occurs
only during winter. Thus, the lake shows summer stratification in the upper
zone, even though it has a broad thermal gradient resulting from water
exchange and a lower zone that does not mix fully at any time with the upper
zone.
Division of the water column into two zones is explained by seasonal
fluctuations in water temperature and salinity. Water entering the reservoir
during the winter is not only cold but also of high salinity relative to the water
derivecl from snowmelt in early summer (Stanford and Ward, 1991~. During
the winter, water entering Lake Powell flows into the bottom zone of the res
OCR for page 89
Organisms and Biological Processes
Temperature, °C
0 10 15 20 25 30
O ~' ' ,
E o
-
a)
O
o
o
-
Q
a)
o
O _
If Feb '95
i
Specific Conductance, ~mholcm
600 700
° ! ' ' '
,
900 1 000 1 1 00
Dissolved Oxygen, mg/liter
0 2 4 6 8
-
o
E
s
c,
o
O
./
f
89
Temperature, °C
o
E o
a)
~ O
0 10 15 20 25 30
l
(
Aug '94
Specific Conductance, ,umho/cm
600 700 800 900 1000 1100
O1 ~ 1 1 1
-
Q
a)
10 0
-
a,
O
O _
it_
AL
O
0, -
Dissolved Oxygen, mg/liter
2 4 68 10
, , ,~
O ~
O _
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-
f
·:
FIGURE 6.2 Profiles of temperature, conductance, and dissolved oxygen for Lake Powell near
the dam at Waheap (unpublished data from the BOR). (umho/cm = micromhosJcentimeter)
ervoir and thus maintains the higher salinity of the bottom zone. During the
summer, the Colorado River brings warm, less saline water derived mostly
from snowmelt. This water enters the upper zone, which matches its density
range. Recent studies by Marzolf (1995 and personal communication) in
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go
River Resource Management in the Grand Canyon
dicate that the summer inflow enters the upper zone at a depth of 15 to 20 m
and remains unmixed with the rest of the upper zone as it passes laterally all
the way to the dam. Winter mixing homogenizes the upper zone, but the
upper and lower zones are so different in density that they do not mix at any
time of the year. Thus, separation of the zones is maintained by seasonal
alternations of density for the incoming water.
Although the upper and lower zones of Lake Powell never mix fully under
the influence of wind, there is a gradual interchange between the two. This
interchange is quite important both to Lake Powell and to the downstream
characteristics of the Colorado River. Slow exchange is caused by the
addition of water to the lower layer during the winter, which increases its
volume. Increase in volume of the lower layer is offset by erosion from the
top of the layer under the influence of wind mixing and by withdrawal of water
through the Glen Canyon Dam outlet structure. If water in the lower layer
were not replaced, or were to be replaced more slowly than at present, the
lower layerwould become anoxic, and anoxic water could pass downstream,
where it could adversely affect trout and other organisms.
Recent surveys of dissolved oxygen concentrations in Lake Powell have
shown that the uppermost 10 m of the lake is often supersaturated (e.g.,
Figure 6.2), as would be expected for most lakes during daylight hours
because of photosynthesis. There is often a metalimnetic minimum (20 to 30
m) of oxygen (e.g., Ayers and McKinney 1995a). Marzolf (1995) concludes
that this minimum is caused by decomposition of organic matter in the water
that enters with the spring and summer inflows at about this level.
The lowermost portions of the bottom layer of Lake Powell show sub-
stantial oxygen depletion with reference to saturation but, because of the
continual replacement of water during the winter, do not become completely
anoxic. Concentrations of oxygen do fall below 3 mg/liter near the bottom
of the lake, however, and thus approach the limiting concentrations for the
sustained presence of most kinds of aquatic life. Even minor changes in the
oxygen dynamics of the reservoir could be significant. At present, however,
water leaving the outlet structure is typically near saturation and experiences
significant reaeration as it is discharged.
The nutrient chemistry of Lake Powell is unusual because of the large
amount of suspended inorganic matter that reaches the lake from the rivers
entering its upper end. Concentrations of suspended sediment in waters
entering the upper end of Luke Powell have a mean of approximately 1,500
mg/liter, although concentrations can increase seasonally to as much as 10
times this amount (Stanford and Ward, 1991~. Because phosphorus is readily
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Organisms and Biological Processes
91
adsorbed onto the surfaces of clay and silt particles, the total phosphorous
load for Lake Powell is quite high. Although now about 20 years out of date,
studies by Gloss et al. (1981 ) indicate that the total phosphorous loading for
Lake Powell would be sufficient to maintain a hypereutrophic condition in the
lake if a significant amount of the total load were in soluble form. In fact,
however, over 90 percent of the phosphorous load is accounted for by
phosphorus adsorbed onto inorganic particles, and this phosphorus is rapidly
deposited as the reservoir sediment. Although Resorption of phosphorus
from sediments is possible even at the bottom of the reservoir, the
interchange between the large phosphorous reserve on the bottom of the
reservoir and the overlying waters is sufficiently slow that phosphorous
concentrations in the water column remain low. Studies by Gloss et al.
(1980), which may still be applicable, indicate that the total phosphorus of the
upper water column typically ranges between 10 and 20,ug/liter within 50 to
100 km of the dam, but may be two or three times higher in the upper water
column of the reservoir arms. The same studies show that dissolved phos-
phorus within 100 km of the dam has a mean concentration close to 10
g/liter in the upper water column.
Concentrations of phosphorus generally decline from the upper end of
the reservoir to the dam. During warm weather, dissolved phosphorus may
be essentially depleted within 50 to 100 km of the dam in the upper water
column. Thus, phosphorus probably limits primary production in Lake Powell
near the dam, while shading caused by turbidity probably limits primary
production in the upper portion of the lake (Gloss et al., 1980~.
Nitrogen also reaches Lake Powell in substantial quantities, but is less
associated with particulate material than phosphorus. Studies by Gloss et al.
(1980, 1981 ) indicate concentrations of dissolved nitrogen in the upper water
column of about 500 ~g/liter, of which approximately half is organic N and
half is nitrate N. Concentrations may be twice this high nearthe bottom ofthe
lake.
Given that nitrate concentrations in Lake Powell are commonly in the
vicinity of 250,ug/liter, significant nitrogen limitation of primary production
seems unlikely. Gloss et al. (1980) did, however, demonstrate one instance
of localized surface depletion of nitrate. Therefore, this issue is not fully re-
solved.
Although the most satisfactory information on nutrients is now quite old,
it suggests that inorganic nitrogen passing through the outlet structure to the
Colorado River is sufficient to support substantial algal growth downstream
of the dam. Autotrophy just downstream of the dam seems to bear this out
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92
River Resource Management in the Grand Canyon
(Ayers and McKinney, 1995b). Dissolved phosphorus in the outlet water
would probably be in the vicinity of 10 ~g/liter, which could support
significant production downstream but could be removed to biologically
negligible concentrations by vigorous algal growth downstream (as sug-
gested by preliminary data from Parnell and Bennett, 1995~.
Concentrations of heavy metals in Lake Powell are potentially of interest
with respect to the biota of the lake as well as aquatic communities down-
stream. Concentrations of mercury in the water column of the lake appear to
fall within the range that might inhibit growth of primary producers and may
have other biological effects downstream (Graf, 1985; Blinn et al., 1977a).
Seleniurr, may also be of biological significance both within the lake and
downstream (Potter and Drake, 1989~. Sources of water for the reservoir
contain significant amounts of radionuclides (Graf, 1985~. The effects of the
lake on concentrations of these substances, and their passage downstream,
are undocumented.
The primary production of Lake Powell has not been estimated recently,
but an estimate that is now approximately 20 years old suggests production
of about 190 9 of carbon per square meter per year, which is consistent with
an oligotrophic or mildly mesotrophic condition (Hansmann et al., 1974; Blinn
et al., 1 977a). Phytoplankton abundance, as indicated by chlorophyll a, was
between ~ and 2 ~g/liters during the 1980s (Paulson and Baker, 1983; Soll-
berger et al., 1989~. This is also consistent with general oligotrophic status,
although concentrations of chlorophyll tend to be higher in the upstream
arms. Algae from Lake Powell are a potential source of food for macro-
invertebrates downstream. Although studies of stable isotopes have clarified
some of the trophic relations downstream of the dam (Angradi, 1994), the
importance of algae in supporting the food web near the dam is still unclear.
Phytoplankton composition of Lake Powell was studied by Stewart and
Blinn (1976) and Blinn et al. (1 977b). Primary production and biomass appear
to be dominated by diatoms, and the lake features a vernal bloom that is
characteristic of oligotrophic lakes. Blue-green algae probably account for
only small proportions of primary production or biomass, but the information
on this subject is now quite old.
Oligotrophic lakes often show significant primary production by attached
algae (periphyton). The potential for periphyton growth in Lake Powell is
reduced by the vertical nature of much of the shoreline, but studies by Potter
and Pattison (1976) showthat illuminated substrates are heavily colonized by
algae and show high production per unit area.
The zooplankton community of Lake Powell was studied by Sollberger et
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Organisms and Biological Processes
93
al. (1989) and more recently by Ayers and McKinney (1 995a). These studies
show an unremarkable species composition that includes copepods, clad-
ocerans, and rotifers. Densities in the upperwater column were in the vicinity
of 10 to 20 individuals per liter according to Sollberger et al. but were found
by Ayers and McKinney (1995a) to be as high as 700 per liter (an extra-
ordinary abundance that needs to be verified) in summer. Trout eat zoo-
plankton that pass downstream (Angradi, 1994~.
The benthic invertebrates of Lake Powell have been studied very little, but
probably are dominated by chironomids (Potter and Louderbough, 1977~.
Fishes have been studied much more extensively (Stanford and Warcl, 1991~.
The fish community is dominated by introduced taxa, including numerous
game species. The fish community has been remarkably unstable, however,
in that the dominant forage species and dominant predator species have
shifted almost continually since the reservoir fish populations were first
established. The lake contains numerous species that are not present down-
stream of the clam. Fish may survive transport through the dam, especially
when some water is bypassing the turbines, but the frequency of fish passage
is unknown.
Influence of Dam Operations on the Lake
The possibilities for influence of dam operations on organisms or
biological processes of Lake Powell are much more limited than the reverse.
The operation of a dam affects a reservoir primarily through changes in water
level, mean depth, and hydraulic residence time. Because the requirements
for delivery of water from Lake Powell are fixed, however, and because the
reservoir is large, the flexibility for managing the water level or mean volume
for any purpose otherthan water delivery is essentially nil. There is at present
no reason to believe that any possible operating plan with the current facilities
would have a distinctive influence on Lake Powell. If new facilities, such as
a multiple outlet withdrawal structure or slurry pipeline, were to be installed,
however, the possible effects of these structures on Lake Powell would
become relevant to dam operations.
Overvi ew of La ke St ud i es
Although some studies of Luke Powell were part of GOES, they were
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94
River Resource Management in the Grand Canyon
added too late and were too fragmentary to serve as an integral part of the
studies. This is unfortunate, given that the water quality, temperature, and
suspended organisms of Luke Powell set the initial conditions for the
ecosystem downstream and may be changed either inadvertently by with-
cirawal at higher points in the water column in the future following a suc-
cession of ciry years or by manipulation through installation of a multiple
outlet withdrawal structure that would allow control of temperature for
environmental purposes (BOR, 1994~.
Changes in the River Caused by the Dam
Withdrawal of water from Lake Powell affects the abundance and
community composition of fishes and other organisms downstream in several
ways, which can be summarized as follows.
· Hydrological changes. The Glen Canyon Dam has drastically reduced
the amount of seasonal change in the flow of the Colorado River. Dam
operations also cause the water level of the river to vary on a daily basis by
an amount that greatly affects the inundation of backwaters and the
distribution of current velocities in the channel (Figure 6.3~.
The original Colorado River was subject to surges in flow and turbidity
corresponding to the flooding of tributaries associated with convectional
storms. Such surges can still occur, especially below the Little Colorado
River, but are reduced in frequency because of the interception of storm flows
above Glen Canyon Dam by Lake Powell. Also, the suppression of floods has
led to net loss of beaches and siltation of backwaters, with associated
changes in habitat for organisms that require these physical features
(Kearnsley et al., 1994~.
· Changes in water temperature. The river does not become warm in the
summer as it did previously (Figure 6.4~. Because water is cirawn from a
depth that lies belowthe upper mixed layer of Lake Powell, watertemperature
is always cold near the dam (7° to 11 °C). Water warms progressively
downstream in midsummer (about 1 °C per 30 river miles (RM); Valdez and
Ryel, 1995) but reaches a summer maximum of only about 1 7°C near Lake
Mead.
· Changes in turbidity. Prior to impoundment, the Colorado River was
very turbid; it allowed very little light penetration. At present, the reach
between Glen Canyon Dam and the Paria River is nearly always transparent.
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Organisms and Biological Processes
less abundant in the lower reach than in the upper reach (Minckley, 1991~.
Tributaries
107
Not only the tributary junctions but also the tributaries themselves
between Glen Canyon Dam and Lake Mead are refugia for the humpback
chub and other warmwater fishes. The Little Colorado River is especially
important in this respect. The tributaries are flooded on an irregular basis
during summer by convectional storms. Such storms can raise the discharge
of the Little Coloraclo, for example, to 20,000 cfs or more in a very short time.
These summer storms are the key means by which coarse debris, sand, and
fine sediments are transported through the tributaries to the main stem. In
adclition, they appear to be related to the welfare of native fishes. Cir-
cumstantial evidence (Minckley and Meffe, 1987; USFWS, 1993 and BOR, un-
published) now suggests that summer floods of tributaries suppress exotic
species, which are not well adapted to these conditions, and thus promote
successful recruitment of native species, which are often suppressed by
predation from exotics. Further study of these phenomena may illustrate the
conditions under which native populations could be reinforced in tributaries
other than the Little Colorado River and may highlight the importance of
hydrological conditions in the Little Colorado River to the humpback chub of
the Colorado River.
Both the Paria and Little Colorado rivers have been drastically altered
since the days of John Wesley Powell. The riparian habitats of both of these
important tributaries have been overgrazed, and this has altered their flow
regimes and physical characteristics. Because of water diversion, the Little
Colorado River is now essentially spring fed (Hubbs 1985) except during
convectional storms, whereas previously it had significant baseflow. The
welfare of the humpback chub and other native species along the Colorado
River will depend on environmental changes not only in the Grand Canyon
but also in tributaries of the Colorado.
Biotic Interactions
Competitive and predatory interactions involving native and exotic
species might well be influenced by the operation of Glen Canyon Dam. The
outcomes of such interactions are notoriously difficult to predict. GOES
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108
River Resource Management in the Grand Canyon
scarcely dealt with this subject, although biotic interactions will likely be
critical in determining the final welfare of humpback chub and other native
species. GCES documented the abundance and distribution of exotic fish
species but primarily as an incidental matter to studies of humpback chub
(\/aldez and Ryel, 1995~. At the community level, outcomes of operational
alternatives are still highly uncertain.
Overview of Biotic Studies on the Main Stem
Studies of the aquatic foodweb by Blinn et al. (1994) and Angradi (1994),
with emphasis on the area near the dam, and studies of the humpback chub
in the main stem by BioWest (\/aldez and Ryel, 1995) are examples of well-
executed projects with specific and useful outcomes. Other studies that have
not yet been finalized may prove to be of this class. The main disap-
pointments of the GCES biotic studies are that they showed uneven coverage
of the biotic resources, were largely still in progress or unreported as of the
end of GCES, were weakly knit together with each other, and did not reflect
a master plan for integration with hydrological and geomorphic studies that
would make the essential connection to operations. However, quite a number
of findings, such as those related to the habitat affinities of the humpback
chub, are directly useful. In fact, a publication by Clarkson et al. (1994) that
discusses the welfare of native fishes in relation to the full range of
management options is a fine example of the kind of bold and broad-ranging
analysis that should have marked the culmination of GCES. Interestingly,
Clarkson et al. published their paper privately-presumably because federal
environmental analysis is still so immature that it cannot tolerate the
publication of views or conclusions that are not already reflected in the plans
of management agencies. The lesson for the future is the necessity for an
integrated view from the earliest planning stages to final synthesis if scientific
studies are to have their fullest value in the service of management.
THE RIPARIAN ZONE
The riparian zone of a river can be loosely defined as the area that is
inundated by a 100-year flood. This zone is affected by the river through
scouring or deposition of sediment. Riparian zones are also influenced by
their close proximity to surface water, which provides habitat and food
OCR for page 109
Organisms and Biological Processes
109
resources that may not be present in surrounding uplands. In addition,
riparian zones can typically support an abundance of vascular plants in arid
climates because of the proximity of ground water to the surface. In general,
riparian zones are centers of biotic diversity, especially in arid climates.
The riparian zone of the Colorado River was changed drastically by the
installation of Glen Canyon Dam (Johnson, 1991~. The old riparian zone
extended far above the mean water level of the Colorado because of the
river's typically strong spring floods, which reached discharges as high as
300,000 ft3/second, or more than 10 times the mean discharge (Stevens and
Ayers, 1993~. Installation of the dam has regulated the Colorado to such an
extent that the new riparian zone, if identified on the basis of high water, is
much restricted; it corresponcisto perhaps 90,000 cts (the 1983 flood was just
over 90,000 cfs). In fact, the riparian zone is qualitatively different now
because spring flooding does not occur at all in most years, whereas strong
spring flooding would have been characteristic of all years under the natural
hydrological regime of the Colorado River. The original riparian zone also
was affected by summer floocis (convectional storm floods) and may still be
under present circumstances below the Little Colorado River (e.g., Stevens
and Ayers, 1993), but not between the Little Colorado and Glen Canyon Dam.
Riparian characteristics associated with proximity to surface water and with
the availability of ground water near the surface are the basis for maintenance
of characteristic vegetation and associated riparian species below Glen
Canyon Dam. In fact, ground water supply to the riparian zone is more
constant now than before 1963 because the operation of the dam has
stabilized the distribution of riparian ground water. This in part explains the
general vegetative enrichment of the riparian zone following closure of the
dam (Johnson, 1991~.
The upper end of the riparian zone is characterized by hackberry, acacia,
mesquite, and other species that were established by and tolerant of
occasional flooding well above the mean annual flood. These species do not
have access to phreatic water because they are too far from the river to reach
the water table. Remaining individuals in this upper zone are old and
probably will not be replaced because of the suppression of flooding. Flood-
intolerant plants appear to be colonizing this zone now (Anderson and
Ruffner, 1987~. Restoration of the conditions leading to the establishment of
this portion of the riparian zone (old high-water zone) are not within the scope
of any feasible operation for Glen Canyon Dam.
The lower portion of the riparian zone contains native species, such as
willow, and exotic species, such as tamarisk. In many locations, tamarisk
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110
River Resource Management in the Grand Canyon
forms a riparian forest close to the water. Tamarisk now provides habitat for
both native and exotic animals and figures importantly in the biology of the
riparian zone. The woody taxa in general are dependent on sandy substrate
deposited by the river and on phreatic water, which is also maintained by the
river. The abundance of tamarisk and the general proliferation of woody
species are attributable to the dam in the sense that the annual floods of the
original riverwould have removed such vegetation, just es the 1983 flood did
(Stevens and Waring, 1985; Pucherelli, 1988~. In addition, fluvial marshes
have developed in places where they would have been swept away by the
natural hydrological regime of the Colorado (Stevens et al., 1994~.
GCES produced information on the composition of riparian vegetation
(Stevens and Ayers, 1994) and lists of resident species of birds (Sogge et al.,
1994), with particular attention to endangered species (Sogge and Tibbitts,
1994a,b). In fact, the majorfocus of GCES in the riparian zone was inventory.
Functional relationships of organisms in the riparian zone and their responses
to the operation of the Glen Canyon Dam were also studied by GCES but less
completely and with a more variable outcome. For example, Stevens and
KJine (1991) concluded that large daily fluctuations may be detrimental to
waterfowl, while Stevens and Ayers (1 993 b) showed that moderation of daily
fluctuations during the period of interim flows probably had no significant
adverse effects on the riparian zone. Controlled or uncontrolled floods are of
particular relevance to the riparian zone, but the forecasting of effects for
such events is weak at present. For the future, responses of the riparian zone
to operation of the dam should be an important consideration of adaptive
management, but the scientific basis for this should be strengthened.
OUTCOMES OF BIOLOGICAL STUDIES
GCES provided much new information on the biotic resources of the
Colorado River below Glen Canyon Dam. The abundances and distributions
of many kinds of organisms were quantified satisfactorily for the first time. In
addition, several kinds of functional relationships, such as the dependence of
rainbow trout on amphipods and Cladophora near Glen Canyon Dam, were
documented. Many functional relationships, however, were not explored
satisfactorily or were not explored at all. Some of these relationships are
critically connected to management options, but studies of them were not
initiated or clicl not come to completion in a way that would be useful to
management. Finally, the integration of biotic components with each other,
OCR for page 111
Organisms and Biological Processes
111
and joint consideration of biological and physical aspects of the environment,
particularly involving sediment dynamics, remained largely undeveloped as
of the end of GCES.
A few examples will illustrate some of the strengths and weaknesses of
the biological component of GCES studies. Present management options
include a range of potential discharge manipulations. Managers might askfor
a list of fishes and other species that might be affected by such manip-
ulations. GCES could easily provide a comprehensive list, along with relative
abundances, probable habitat requirements, and general distribution down-
stream of Glen Canyon Dam. Management might, however, also ask for a
forecast of the effects on particular species. For example, howwould greater
stability of discharge affect species and communities downstream? Here
GCES might prove to be only partially satisfactory. For example, GCES
showed that large rapid fluctuations in discharge interfere with spawning of
trout near Glen Canyon Dam and strand some adult fish. At the same time,
however, such variations appear to dislodge food items that in this way
become vulnerable to trout. What is the relative importance of these two
effects on the trout population? GCES does not provide an answer, although
clearly management neecis at least a qualitative answer to such questions.
In general, the success of GCES was greatest in the area of survey and
inventory, impressive in the analysis of some (e.g., native fishes) but not all
system components, and disappointing in the area of ecosystem analysis.
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Representative terms from entire chapter:
glen canyon
