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OCR for page 75
s
Limnology of Lake Powell and the
Chemistry of the Colorado River
JACK A. STANFORD, University of Montana, Poison, Montana
JAMES V. WARD, Colorado State University, Fort Collins, Colorado
INTRODUCTION
Lake Powell is a large, dendritic reservoir that fills the Glen Canyon
segment of the lower Colorado River. Operation of Lake Powell in turn
controls the hydrodynamics of the Colorado River through the Grand Can-
yon National Parl: to Lake Mead. The limnology of the reservoir is greatly
influenced by (1) the morphometry and geology of the canyon it fills, (2)
the design and operation (including the economics of hydropower produc-
tion) of the dam, (3) the hydrodynamics and chemistry of the inflowing
rivers, and (4) climatic variability. The limnology of the reservoir, coupled
with dam operations, therefore determines the limnology of the Colorado
River downstream from Lake Powell. The purpose of this paper is to re-
view the limnology of Lake Powell in relation to the volume and quality of
influent and effluent waters and to discuss the ramifications of reservoir
operations on the biophysiology of the Colorado River.
Since our review of the ecology of the Colorado River (Stanford and
Ward 1986a-c; Ward et al., 1986), other detailed syntheses have been forth-
coming: Graf (1985), Bureau of Reclamation (1987), NRC (1987), Carlson
and Muth (1989), and Potter and Drake (1989~. Data and interpretations in
these lengthy documents are pertinent to this paper. Potter and Drake (1989)
is required reading for anyone interested in the developmental history and
limnology of Lake Powell, as it summarizes results of a 10-year, multidisciplinary
study funded by the National Science Foundation. However, Potter and
75
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76
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
Drake (1989) is rather popularly written, and where possible we cite herein
the various technical reports and published papers that were forthcoming
from the study. Few papers containing original data pertaining to the lim-
nology of Lake Powell and its effects on the riverine environment down-
stream have been published since the study summarized by Potter and Drake
(1989) was concluded in 1980.
MORPHOMETRY AND HYDRODYNAMICS OF LAKE POWELL
Glen Canyon Dam
Glen Canyon Dam is an arch structure with a 107-m (350-ft) base and
476-m (1,560 ft) crest and is composed of 3.6 x 109 m3 of concrete. It
plugs sheer walls of Navajo sandstone bedrock incised over the millennia
by the Colorado River. The dam has a crest elevation at 1,132 m (3,715 It)
above sea level, which is 216 m (710 ft) above bedrock and 164 m (583 ft)
above the river channel. Eight penstocks discharge a maximum of 943 m3/s
(33,000 ft3/s) to generators rated for 1.3 MW of hydropower production at
full capacity. The penstocks draw water from an elevation of 1,058 m
(3,471 It). The authorized full pool elevation is 1,127.76 m (3,700 ft)
above mean sea level. Four bypass tubes and two concrete-lined spillways
that release water above 1,122 m (3,680 ft) elevation can pass an additional
7,867 m3/s (278,000 ft3/s). This spill volume was needed because flood
flows of >5,660 m3/s (200,000 ft3/s) have been recorded within Glen Can-
yon. The dam was closed in September 1963, after a total project construc-
tion expenditure of $272 million. Through 1987, gross electrical produc-
tion exceeded $700 million (Potter and Drake, 1989~.
Physical Features of the Reservoir
Lake Powell, named in memory of Colorado River explorer and surveyor
J. W. Powell, is impounded by Glen Canyon Dam. Glen Canyon was
named by Powell in 1869 for the cottonwood (Populusfremontii) glens that
dominated the pristine riparian forests on the terraces along the river within
the steep-walled canyon. It is the second-largest reservoir in the United
States, next to Lake Mead located cat 250 km downstream on the Colorado
River.
Full pool in Lake Powell was first reached on June 22, 1980. At full
pool the reservoir yields morphometrics as listed in Table 5-1. Full pool
was exceeded by 3 m on July 4, 1983 (only 2.5 m below the crest of the
dam) as a result of a period of high flows in the Colorado and Green rivers.
Since 1983, the reservoir has filled to capacity every year and has fluctu-
ated only about 8 m annually.
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LIMNOLOGY OF LAKE POWEl1...
TABLE 5-1 Morphometric features of Lake Powell.
Feature
Value
Surface area (km2)
Volume (km3)
Maximum depth (m)
Mean depth (m)
Maximum length (km)
Maximum width ~m)
Shoreline development
Shoreline length (km)
Discharge depth (m)
Maximum operating pool
Drainage area (km2)
Approximate storage ratio (yr)
653
33.3
171
51
300
25
26
3,057
70
1,128
279,000
2
SOURCE: Modified from Potter and Drake, 1989.
77
Air temperatures range in the Lake Powell area from cat 5°C January) to
36°C (August), while the surface water temperatures vary from 6°C (Janu-
ary) to 27°C (August). Average annual precipitation is 14.5 cm, occurring
in about equal increments per month. These data summarize a 25-year record
at Page, Arizona (Potter and Drake, 1989~.
The geology of the reservoir basin is composed of a variety of Jurassic
and Triassic sediments. Outcrops of Navajo and Wingate sandstones and
the varied shales of the Chinle formation are more notable features of the
Glen Canyon. The geology of the Colorado plateau dominates the catchment
of Lake Powell and consists of uplifted Tertiary and cretaceous shale and
sandstone formations with some areas of Tertiary volcanic extrusion. The
Mancos shale formation outcrops over wide areas. The principal feature of
the geology relative to the limnology of Lake Powell and its tributary rivers
is the omnipresence of calcium, sulfate, and bicarbonate as the predominant
dissolution ions within easily eroded substrata (Stanford and Ward, 1986a).
Potter and Drake (1989) provide a more detailed review of area geology.
River Discharge and the Water Budget of Lake Powell
Ninety-six percent of the inflow to the reservoir is derived from the
catchments of the Colorado and San Juan rivers (Figure 5-1), and 60% of
the annual water budget is received in May-July as a result of snowmelt in
the headwaters (Irons et al., 1965; Evans and Paulson, 1983~.
In 1922 the Colorado River Compact apportioned water allocation based
on a virgin (preregulation) flow of 22.2 km3 at Lee's Ferry. However, the
virgin flow is now estimated at 20.72 for the precompact period and 17.52
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78
._~
LEE'S FERRY
COLORADO RIDER ECOLOGY AND DAM MANAGEMENT
DIRTY DEVIL R. ~
t GREEN R.
SAN RAFAEL R. ~ ~
~t~ A OF PoWF
(
i\
, \
L L
SAN JUAN R.
_ ~
~ a'
an,
Utah ~ Colorado
-
Arizona '~°
FIGURE 5-1 Lake Powell and tributaries showing sites where flows and water
quality have been monitored.
SOURCE: Messer et al., 1983.
since 1922 (Holbert, 1982; Upper Colorado River Commission, 1984~. Long-
term flows estimated from tree ring data (Stockton and Jacoby, 1976) sug-
gest that the long-term yield of the basin is 16.7 km3, corroborating the
lower virgin flow estimate. Moreover, the tree ring analysis strongly sug-
gests that the Colorado River flows in the l900s were well above the long-
term (ca. 370-year) average. This is important because the Colorado River
is regulated to the extent that the legal allocation between the upper and
lower basins largely determines the water budget of Lake Powell most years.
The Colorado River Compact divided the Colorado River into upper and
lower basins at the confluence of the Paria and Colorado rivers, which is
located cat 30 km downstream from Glen Canyon Dam. Note that during
the period 1967-1983 only slightly more than the legal allocation to the
lower basin (8.9 km3 = 7.2 million acre-feet) was discharged from Glen
Canyon Dam (Figure 5-2~. However, only 28.8% of the flow at Lee's Ferry
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LIMNOLOGY OF LAKE POWELL...
21
17
Cam
-
o
, 9
13
5
1
it,
79
940 1950 1960 1970 1 980 1990
YEAR
FIGURE 5-2 Annual flows in the Colorado River at Lee's Ferry, located 24 km
downstream from Glen Canyon Dam.
SOURCE: Modified from Bureau of Reclamation, 1989.
is water previously impounded in the largest upstream reservoirs (Flaming
Gorge, Morrow Point, Blue Mesa, and Navajo), owing to consumptive use
and downstream water recruitment (Irons et al., 19653. During future low
water years, inflow to the reservoir probably will be insufficient to meet the
legal allocation downstream and Lake Powell will have to be drafted to
make up the difference. In essence, Lake Powell buffers delivery of water
from the upper basin to the lower basin.
In the decade of the 1980s, flows in the Colorado River system were
generally well above the long-term average (Figure 5-2) and allocation of
water to fulfill legal rights was not a problem. Indeed, flows during the
spring freshet in 1983 were the highest on record, owing to late season
snow accumulation, accelerated snowpack ablation, and high initial reser-
voir levels (Vandivere and Vorster, 1984~. Streamflow forecasts also were
not suitable for the climatic extremes in the basin in 1983 (Rhodes et al.,
1984; Dracup et al., 1985~. Reservoir elevation in Lake Powell exceeded
full pool by 3 m during July 1983, and the dam operators were forced to
bypass flood flows. The next year, 1984, also generated extreme runoff in
the upper basin, and flood flows were again bypassed. Considerable dam-
age to riverine environments downstream occurred, including severe ero-
sion of beach and terrace environments within the Grand Canyon National
Park.
Ground water inflows to Lake Powell apparently are related to aquifers
in the Navajo sandstone, which is associated with the major surficial fea-
tures of the reservoir and its dam (Blanchard, 19861. Quantities of ground-
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80
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
water inflow are not known but are probably insignificant in relation to
fl~vial sources and bank storage.
The Lake Powell water budget is dominated by inflow from the Colorado
and San Juan rivers, although a major volume is included in bank storage
and evaporation (Table 5-2~. Bank storage increases over the life of a reser-
voir (Langbein, 1960~. From 1964 to 1976, an estimated 10.5 km3 went
into bank storage at Lake Powell, for an average of 740,000 m3 yearn.
These storage rates are consistent with the measured porosity of the Navajo
sandstone (Potter and Drake, 1989~. Evaporation was estimated at 6.2 x 108
m3 years or about 180 cm/year, based on measures in 1973-74 (Jacoby et
al., 1977; but see Dawdy, this volume). The difference between riverine
inflow and the losses given in Table 5-2 is the volume of water stored
annually within the reservoir as it is filled and is fairly consistent with the
reservoir's elevation volume trend for the period.
The volume stored annually is determined from runoff delivered by the
rivers to Lake Powell in excess of the release mandate of the Colorado
River Compact for the lower basin. The initial filling of the reservoir was
accomplished a decade sooner than predicted from average flows, as a re-
sult of high flows during the 1980s. This again brings out the point that in
future dry years lower basin allocation could cause Lake Powell to be drafted
well below the volume that may be supplied by runoff in excess of reservoir
storage from the upper basin. Hence, a major problem for the legal pundits
involves determination of which reservoirs, if any, in the system have fill-
ing priority over Lake Powell. For the limnologist it means that prediction
of pool levels in the reservoir and flows downstream require greater model-
TABLE 5-2 Water budget for Lake Powell based on
average data for the period 1964-1975, the period when
the reservoir was filling. Data are km3.
River Inflow
Colorado River
San Juan River
Losses
Bank Storage
Evaporation1
Outflow measured at
Lee's Ferry
Difference
Reservoir storage
10.6408
1.6881
.7404
.6170
10.0818
2.247 1
corrected for precipitation and based on reservoir two-thirds full
SOURCE: Modified from Jacoby et al., 1977.
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LIMNOLOGY OF LAKE POWELL...
81
ing sophistication than currently exists. Without a verified hydrological
model, predicting ecological responses to regulation is problematic.
The presence of Lake Powell has vastly altered the hydrograph of the
river downstream of the dam. Indeed, Lake Powell hydropower operations
effectively control downstream hydrodynamics (Table 5-3~. Except in years
of high-volume inflow to the reservoir (e.g., 1983 and 1984; Figure 5-3),
flow seasonality is absent (e.g., 1982; Figure 5-3), as a result of reservoir
storage. Flows from the dam fluctuate between 85 and 570 m3 (3,000-
20,000 ft3) in a "yo-yo" fashion over short time periods (days) in response
to the economics of hydropower (Figure 5-3~.
TABLE 5-3 Hydrological, thermal, and sediment transport
characteristics of the Colorado River below Glen Canyon Dam based
on the 1941-1977 period of record.
Measurement
Lee's Ferrya
Pre-dam Post-dam
Grand Canyonb
Pre-dam Post-dam
Daily average flow
equaled or exceeded 95%
of the time (m3/s)
Median discharge (m3/s)
Mean annual flood (m3/s)
102 156
209 345
1 13 167
232 362
2434 764 2434 792
Annual maximum stage (m)
Mean 5.04 3.56 6.89 4.79
Standard deviation 0.96 0.17 0.35 0.15
Annual minimum stage (m)
Mean 1.76 1.46 0.46 0.70
Standard deviation 1.40 0.23 0.85 0.45
Annual average
temperature (°C) 10 10 11 12
Range (°C) 2-26 7-10 2-28 5-15
Mean sedunent concen- 1500 7
tration (mg/liter)
Sediment concentration
equaled or exceeded 1%
of the time (mglliter)
1250
21000 700 28000
350
15000
a24 km downstream from Glen Canyon Dam
bl65 km downstream from Glen Canyon Dam
SOURCE: Dolan et al., 1974; Paulson and Baker, 1981; Petts, 1984; U.S. Geological
Survey, unpublished data.
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82
be, 1 00
o
a)
-
c)
a,
.o
D
on
-
LL
C:
a:
I
an
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
80 ._
60 _
40
20
o
Flood
Flood
~ l)J~lI
Flood
~~ -~' I ~ ~
1 1 1 1 1
JFMAMJJASONDJ FMAMJJASONDJFMAMJJASONDJFMAMJJA SONDJF
1982 1983 1984 1985 1986
YEAR
FIGURE 5-3 Inter- and intrayear variation in discharge of the Colorado River at
Lee's Ferry as a consequence of hydropower and flood control operations at Glen
Canyon Dam for the period 1982-1986.
SOURCE: Modified from Potter and Drake, 1989.
Seasonal Stratification
Lake Powell is warm (i.e., the annual heat budget is 40 kcal/cm2), monomictic,
and intensely stratified during most of the year. The intensity of seasonal
stratification is determined by temperature, salinity, suspended solids, depth
of the water column, and extent of riverine advection. Indeed, the reservoir
mixes connectively only during the winter cooling period, when pelagic
temperatures are 10°C (Paulson and Baker, 1983a) and mixing does not
extend to the bottom (Johnson and Merritt, 1979~. Lack of convective
circulation is also partly due to the fact that 53% of the entire shoreline is
vertical cliff (Potter and Pattison, 1976), which reduces wind-driven circu-
lation within the water column of the reservoir. Overflow or interflowing
density currents from the Colorado River occur annually in relation to the
intensity and duration of the freshet. The relatively warmer and less saline
waters from the freshet form a pycnocline between colder and more saline
waters on the bottom of the reservoir. This over- or interflowing wedge of
water from the rivers moves down the reservoir, gradually thinning as it
reaches toward the dam by late summer (Gloss et al., 1980~. Advection,
therefore, mechanically sets the depth and extent of seasonal thermal strati-
fication (Johnson and Merritt, 1979~.
Because of the relatively shallow morphometry and substantial effects of
river inflow, the upper reaches of the reservoir are dominated by advective
mixing. Average retention time for reservoir volume north of Bullfrog Bay
in the San Juan arm is less than 8 months (Potter and Drake, 1989~.
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LIMNOLOGY OF LAKE POWELL...
83
A widespread halocline occurs in Lake Powell. Because of lack of wind
circulation, there is a monimolimnion from the dam to the upper reaches of
the reservoir. Convective circulation penetrates only to about 60 m (John-
son and Merritt, 1979~. However, bottom water remains aerobic (i.e., >3
mg/liter dissolved oxygen), owing primarily to advective circulation gener-
ated by saline underflows from the Colorado and San Juan rivers during low
winter discharge (Johnson and Page, 1981~.
Metalimnetic oxygen depletion (negative heterograde profile) occurs vir-
tually lakewide in the late summer (Stewart and Blinn, 1976; Johnson and
Page, 1981; Sollberger et al., 1989~; anoxia presumably is caused by de-
composition of senescent phytoplankton that accumulates on the chemocline
after settling from the mixolimnion (Hansmann et al., 1974; Johnson and
Page, 1981~. Metalimnetic oxygen stagnation begins in the bay mouths and
temporally develops toward the dam. Convective circulation reduces oxy-
gen depletion in early winter (Johnson and Page, 1981; Sollberger et al.,
1989).
Because of reoxygenation by winter river underflows noted above and
trapping of organic matter in the metalimnion, the monimolimnion remains
aerobic. This limits nutrient mobilization from the sediments, and nutrients
released by mineralization are not returned to the epilimnion due to the lack
of convective circulation (Gloss et al., 1980, 1981~. Calcite precipitation
also may scavenge dissolved substances, particularly phosphate, via adsorp-
tion (Reynolds, 1978~. Moreover, the depth of convective circulation and
halocline formation in Lake Powell is marked by a strong withdrawal cur-
rent at cat 30-50 m (Johnson and Merritt, 1979) which removes epilimnetic
nutrient loads (Gloss et al., 1980~. Thus, there is a persistent nutrient
deficiency in the euphoric zone. Bioproduction in Lake Powell is phospho-
rus limited and dependent on seasonal advective nutrient renewal via the
spring overflow of the river turbidity plume (Gloss, 1977; Gloss et al.,
1980).
Edinger et al. (1984) describe a longitudinal and vertical hydrodynamic
and transport model called LARM. This model allows study of seasonal
circulation, heat, and salinity transport and predicts equilibrium relationship
over an entire reservoir volume. Thus, changes in solute concentrations,
due to precipitation and dissolution, may be evaluated by using LARM.
PARTICULATE AND DISSOLVED SOLIDS DYNAMICS
Long-term data bases that describe the temporal chemistry of the rivers
above and below Lake Powell (i.e., the sites in Figure 5-1) are maintained
by the U.S. Geological Survey in their WATS TORE file. Flow, total dis-
solved solids (TDS) and specific conductance data are evaluated annually to
document trends as related to salinity control projects within the Colorado
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84
COLORADO RIVER ECOLOGY AlID DAM MANAGEMENT
River basin (Bureau of Reclamation, 1989~. These long-term data bases are
readily available in electronic media and have been used extensively in
studies of the biogeochemistry of Lake Powell (e.g., Messer et al., 1983;
and Edinger et al., 1984~.
Suspended Sediment
Stanford and Ward (1986a) characterized the Colorado River as one of
the most erosive in the world. The virgin river was very turbid except at
low flows, and sediment dynamics greatly influenced the riverine ecology,
both within the river channel and in its floodplains and terraces. Most of
the historical and current sediment load is derived from erosion of the soft
sedimentary formations of the Colorado plateau (Irons et al., 1965~.
Construction of mainstem dams has vastly altered the sediment transport
processes of the river. Lake Powell receives 40-140 million tons of sus-
pended sediment annually from its tributaries. Upstream dams on the Gunnison,
San Juan, and Green rivers have not reduced the load (Mayer and Gloss,
1980), again substantiating the influence of the middle reaches of the Colo-
rado, Green, and San Juan rivers on the water and sediment supply to Lake
Powell (Irons et al., 1965~. Retention of fluvial sediments within Lake
Powell reduces the suspended sediment load in the Grand Canyon segment
of the Colorado River by 70-80% (Evans and Paulson, 1983; Table 5-3~.
About 25% of the historic sediment load at Lee's Ferry was derived from
side flows to Lake Powell such as the Dirty Devil and Escalante rivers
(Potter and Drake, 1989~. The Little Colorado River, which joins the Colo-
rado River within the Grand Canyon, and the Paria River can contribute
heavy sediment loads to the river during short-term spates, but the sediment
load of the Colorado River is now virtually controlled by retention in Lake
Powell.
The fluvial sediments entering Lake Powell are dominated by the
montmorillinite clay lattice and have an average in situ bulk density of 1.5
with a mean grain density of 2.65 g/cm3 dry mass, based on analysis of
cores (Potter and Drake, 1989; but see Kennedy [1965] and Mayer t1973]
for clay mineralogy of Colorado River suspended sediments). Mass balance
calculations show that 60 x 109 m3 of sediment is stored on the bottom of
the reservoir annually. Most of the fluvial sediments are presently depos-
ited near the mouths of the Colorado and San Juan rivers. Maximum sedi-
mentation rates were better than 3 m/year through 1974 within the Colorado
River arm and about 2 m/year within the San Juan arm of the reservoir.
Over 50 m of sediment has accumulated near the mouth of the Colorado
River and over 15 m has accumulated on the San Juan River delta as deter-
mined from 1987 sonar profiles presented by Potter and Drake, (1989~.
Bank slumping has contributed additional sediment at various sites on the
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LIMNOLOGY OF LAKE POWELL...
85
reservoir shoreline, particularly in areas of sidehill dunes and steep gradient
soils. Rockfalls from partially inundated cliff walls have also occurred.
Sedimentation rates determined from echo soundings suggest that the reser-
voir basin will fill completely in about 700 years (Potter and Drake, 1989~.
Riverine suspended sediments in the smallest size fraction (e.g., 10-30
,um may be transported far into the pelagic zone of the reservoir). These
fine sediments are nutrient rich, relative to dissolved constituents within the
water column, and fertilize the reservoir. However, the sediment-laden river
waters usually interflow through the reservoir because advectively circu-
lated flood waters are typically colder and more dense than the upper por-
tions of the water column (Gloss et al., 1980, 1981~.
Nitrogen, Phosphorus, and Silica
Because of the patterns of seasonal stratification described above, nutri-
ents such as nitrogen, phosphorus, and silica are most uniformly distributed
within the water column of Lake Powell during the winter and early spring.
Advective circulation, driven by the annual spring freshet, establishes firm
stratification and loads the epilimnion with nutrients. Phytoplankton pro-
duction increases, thereby depleting nitrate, silicate, and soluble reactive
phosphorus within the water column during the summer growing season.
Since the epilimnion is essentially isolated by thermal and haline stratifica-
tion, much of the microbial biomass produced within the epilimnion sinks
toward the bottom. This is corroborated by oxygen depletion in the metalimnion,
where biomass deposited on the pycnocline begins to decompose, creating
the oxygen demand described above. As primary production increases,
calcite precipitation also may increase, which in turn may allow additional
phosphorus loss as a function of adsorption to calcite particles. Moreover,
soluble and particulate nutrients are lost via the withdrawal current from the
dam. Nutrients entrained in the hypolimnion or bottom sediments are not
regenerated within the water column, owing to the lack of convective circu-
lation and an inverse redox gradient (Gloss et al., 1980~.
Thus, the reservoir is a nutrient sink, especially for phosphorus. Flow-
weighted mass balance calculations provide retention coefficients of .74 for
dissolved phosphorus and .96 for total phosphorus (Gloss et al., 1981; Miller
et al., 1983~. Total nitrogen is more conservative; Gloss et al. (1981)
estimated 9% retention. The nitrogen load is cat 47% organic N and 45%
nitrate, and inflow and outflow values of the various forms of nitrogen are
similar. The retention coefficient for silica is estimated at .14, with losses
attributed to uptake by diatoms and subsequent sedimentation (Gloss et al.,
1981~.
Over 95% of the fluvial phosphorus reaching Lake Powell is particulate,
associated with clays in the suspended sediments (Gloss et al., 1981; Evans
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LIMNOLOGY OF LAKE POWELL...
91
Heavy metal contamination within the headwater reaches of the Colorado
River has been a concern for years because of hard rock mine effluents in
many locations (e.g., Gunnison, Dolores, and San Miguel rivers). How
much of this contamination reaches Lake Powell is unknown. Heavy metals
associated with emissions from coal-fired generating plants in the Lake
Powell area have been shown to reduce photosynthesis and respiration in
microcosms containing water from Lake Powell (Medine et al., 1980; Medine
and Porcella, 1980~. However, Potter and Drake (1989) concluded that
mercury and selenium were the only heavy metals reaching Lake Powell in
sufficient concentrations to affect the food web.
Radioactive pollutants enter the reservoir from several sources: (1) natu-
rally associated with basin geology; (2) from waste ponds and heaps located
at various sites (e.g., San Miguel River); and (3) in association with coal-
fired generating plants in the basin (Graf, 1985~. The impact within the food
web is apparently unknown.
LAKE POWELL FOOD WEB
Phytoplankton
Gloss et al. (1980) reported primary productivity in the reservoir of 2-60
mg of i4C m~3 days during the summers of 1975 and 1976. Values varied
an order of magnitude spatially during a given sampling period. Maximum
productivity occurred between 1 and 3m in depth. Hansmann et al. (1974)
reported that in 1971 the highest productivity (1,066 mg of C m~2 days)
occurred in July and the lowest (33 mg of C m~2 days) was measured in
February. Yearly productivity was estimated at 184 g of C m~2. Blinn et al.
(1977a) reported similar values. Because of the variable amounts of sedi-
ments suspended in the water column, productivity varies greatly within the
reservoir in relation to light attenuation (Blinn et al., 1977b). Chlorophyll
concentrations in the water column averaged 5 ,ug/liter near the Colorado
River and 1.5 ,ug/liter lakewide during 1982-1983 (Paulson and Baker, 1983a)
and 1987-1988 (Sollberger et al., 1989~.
Seasonal phytoplankton succession in Lake Powell involves a spring dia-
tom pulse (Fragilaria crotonensis and Asterionella formosa), a diverse summer
community (Dinobryon sertularia and Chloroccocales), a late autumn pulse
of diatoms (Synedra delicatessima var. augustissima) and dinoflagellates,
and no abundant winter forms (Stewart and Blinn, 1976; Blinn et al., 1977b).
The spring pulse yielded chlorophyll values of 1.8 ~g/liter in the down-
stream end of the reservoir (Sollberger et al., 1989~. This vernal diatom
bloom is common in large oligotrophic lakes (e.g., Flathead Lake, Montana)
and does not suggest extreme or abnormal nutrient loading. Indeed, blue-
green blooms do not occur in Lake Powell as commonly occur in large
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92
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
upstream impoundments (e.g., Flaming Gorge Reservoir on the Green River)
(Miller et al., 1983~. This is undoubtedly due to lower concentrations of
bioavailable nutrients in Lake Powell and the fact that thermal and haline
stratification promotes nutrient loss via withdrawal and hypolimnetic en-
trainment. Relatively high salinity does not seem to play much of a role,
although indigenous Lake Powell diatoms are apparently more tolerant of
salinity additions than are standard bioassay test algae (Cleave et al., 1981~.
On the basis of the general nutrient regime and importance of allochthonous
sediments, we suspect that picoplankton (i.e., cells <10 Em in size) are
probably very important in Lake Powell, but the size distribution of the
plankton has apparently not been investigated.
Zoop lank ton
Stone and Rathbun (1969) reported that Daphnia, Cyclops, and Diaptomus
were dominant zooplankters with densities that varied between 15 and 100
individuals per liter in the upper 30 m of the water column. Densities were
higher in the summer and at the upper end of the reservoir. At the down-
stream end, samples rarely exceeded 20 per liter and did not vary much
seasonally.
In a more detailed study conducted in 1987-1988, Sollberger et al. (1989)
found 36 species (8 Copepoda, 13 Cladocera, and 15 Rotifera); Cyclops
bic uspidatus, Mesocyc lops eden, and Diaptomus ash landi were the most
common copepods, whereas Bosmina longi-rostris, Daphnia galeata and D.
pulex, Collotheca sp., Polyarthra sp., and Syncheata sp. were the most
common cladocerans and rotifers. Copepods were the most abundant group.
Densities of total zooplankton in the water column were normally 20 per
liter and never exceeded 50 per liter, with the higher concentrations occur-
ring in the upper 20 m of the water column during April (average densities
ranged from 3 to 26 per liter within the upper 40 m). These data are
surprisingly similar to the counts made two decades earlier by Stone and
Rathbun (1969), suggesting little if any decline in productivity. Density of
zooplankton was inversely related to chlorophyll content. Densities de-
clined through the summer, perhaps as a result of predation. No significant
vertical migration was observed, and zooplankters primarily inhabited the
epilimnion. The lipid-ovary index was generally <0.5 for the Daphnia, and
egg ratios were <0.2, indicating that populations were food limited most of
the year.
Benthos
Artificial substrata (plastic trees) placed in the littoral zone of Lake Powell
were colonized by 69 diatom taxa, 6 of which made up 65-94% of the
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LIMNOLOGY OF LAKE POWER...
93
periphyton community numerically. Maximum periphyton occurred in spring
and fall. Macroinvertebrate communities colonizing the substrata were dominated
by chironomids, with Physa, jellyfish (Craspedacusta sowerbyi), water mites,
and damselflies occasionally present (Potter and Louderbough, 1977~.
Potter and Pattison (1976) used a bilge pump and scuba equipment to
collect natural benthic biofilms and found an average of 2.14 g/m2 with an
average chlorophyll content of 352 mg/m2, or about ten times greater than
average concentrations in the water column. In doing this work they no-
ticed that wood rat (Neotoma) middens, submerged upon closure of the
dam, could be identified by plumes of benthic periphyton. The benthic
algae was apparently stimulated by nutrients leached from the middens.
Since so much of the shoreline is near vertical cliffs (54%), the high
water line is marked by epilithic biofilms. This biofilm is oxidized during
the drawdown period, leaving a white bathtub ring that grades into charac-
teristic black streaks on red sandstones. The latter is caused by cementing
of clays with oxides and hydroxides resulting from the oxidation of miner-
als and organics by mixotropic bacteria (Dorn and Oberlander, 1982~.
Fishes
Fisheries in Lake Powell are derived from a hodgepodge of native and
introduced species. The introduced species predominate. Indeed, the status
of the endemic big river fishes (e.g., squawfish, humpback chub, and razor-
back sucker; Stanford and Ward, 1986c; Carlson and Muth, 1989) that in-
habited Glen Canyon prior to impoundment is unknown. Persons and Bulkley
(1982) reported that Dorosoma petenense (threadfin shad) were primary
food items in guts of Merone saxatalis (striped bass) that spawned in the
mixing zone of the Colorado River. They observed that none of these
reservoir fish moved very [ar upstream, and no evidence of predation on
endemic fishes was found. Dynamics of the pelagic shad and striped bass
populations appear to be the most economically significant aspect of the
Lake Powell fisheries. Other bass (Micropterus salmoides, M. dolomieui),
channel cat (Icralurus punctatus), northern pike (Esox lucius), walleye
(Stizostedion verreum), crappie (Pomoxis nigromaculatus, P. annularis), and
sunfishes (Lepomis cyanellus, L. gulosus, L. macrochirus, L. microlophus)
are common, especially near shore. Rainbow trout (Onocorhynchus mykiss)
and brown trout (Salmo trutta) occupy cooler waters in the lower end of the
reservo~r.
Quantitative food web relationship for the fishes relative to reservoirwide
plankton production have not been established. The trophic status of Lake
Powell appears to be oligotrophic, on the basis of primary production, aver-
age chlorophyll content, and the depressed nutritional status of the zoop-
lankton. Thus, Lake Powell fisheries should not be expected to be very
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94
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
productive, and populations may fluctuate as bioproduction of the plankton
waxes and wanes in response to riverine nutrient loading and drafting for
hydropower and downstream water demands. Fluctuations may also mani-
fest as a consequence of angler harvest, disease, poor recruitment (e.g.,
related to reservoir operations), and/or introduction of new species (e.g.,
rainbow smelt [Osmerus mordant, a pelagic planktivore, may be introduced
in an attempt to supplement forage for striped bass and other piscivores;
Courtenay and Robins, 1989; Gustaveson et al., 1990~.
Indeed, the food web currently appears to be destabilized. Densities of
threadfin shad have decreased from >600 adult fish per standardized trawl
in 1977-1978 to 0 in 1989. Concomitantly, the mean size of striped bass
declined from 620 mm (24 inches) to 348 (14 inches); the mass/length rates
declined from 1.6 to 1.0. Formerly abundant walleye and largemouth bass
populations have also declined (Gustaveson et al., 1990~. Destabilization of
the food web appears to have occurred from the top down, since the fertility
of the reservoir seems to have changed very little and zooplankton crops are
about the same today as in 1969 (see above). The striped bass cohorts that
were dominated by large-bodied individuals in the late 1970s probably were
very productive, and subsequent high recruitment of juveniles may have
gradually reduced the shad forage. Moreover, juvenile striped bass are able
to forage in the warm upper layers of the reservoir (>10 m) where shad
densities are greatest; adult bass are more stenothermic and habitually re-
side in deeper waters (Larry Paulson, personal communication). Rainbow
smelt prefer colder waters and could be good forage for the larger bass
cohorts. However, the net growth potential of the Lake Powell food web is
limited because of its oligotrophic status, and productivity can be packaged
only in so many ways. The bass-shad relation could be long-term cyclic
and/or strongly influenced by angler harvest. Introduction of another planktivore
may only further destabilize the food web and produce unanticipated nega-
tive effects (see Moyle et al., 1986~. Moreover, smelt may rapidly disperse
above and below the reservoir, perhaps compromising recovery of the en-
dangered native fishes and creating a plethora of other problems (Courtenay
and Robins, 1989~.
Riparian Systems
The flora of the xeric uplands of the Lake Powell area is characterized by
patches of pinonjuniper (fin us edulia-Juniperus spp.) and a wide variety
of desert shrubs, such as blackbrush (Coleogyne ramosissima), Mormon tea
(Ephedra spp.), sagebrush (Artimesia spp.), shrub oak (Quercus spp.), and
Yucca galeata. These trees and bushes occur commonly on the rim and
flanks of Glen Canyon and, prior to impoundment, graded into terrace flora
that included deep-rooted shrubs (such as four-winged saltbrush [A triplex
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LIMNOLOGY OF LAKE POWELL...
95
canescens], greasewood [Sarcobatus spp.], rabbitbrush [Chrysothammus spp.]
and squawbush [Rhus trilolata]) which formed irregular patches. The ripar-
ian flora was mainly large galleries of Fremont cottonwood (Populus fremontiO
interspersed with patches of willows (Salur spp.), hackberry (Celris reticulata),
gambel oak (Quercus gambeli), saltgrass (Distichilis spp.), slender dropseed
(Sporobolus spp.), and clumps of tall reed (Phragmites spy.. Many species
of birds and mammals seasonally inhabited these plant communities (Stanford
and Ward, 1986a).
Potter and Drake (1989) provided an excellent summary of the shoreline
vegetation and associated changes as impoundment ensued, based on origi-
nal work by Potter and Pattison (1977~. The main conclusion is that most
of the preimpoundment riparian forests and other streamside plants are gone
and availability of riparian habitat is limited within the reservoir landscape.
Only 3% (ca. 1,000 hectares) of the shoreline is sandy and aggraded. These
areas have been uniformly invaded by exotic saltcedar (Tamarisk chinens~s).
The few cottonwood seedlings that managed to root in a few places were
largely eliminated by dense and fast-growing saltcedar stands. Saltcedar
remain very viable throughout the upper drawdown zone of the reservoir
and are very tolerant of flooding. The lower drawdown zone is colonized
by Russian thistle (Salsola kali).
INFLUENCE OF LAKE POWELL ON
ENVIRONMENTS DOWNSTREAM
Lake Powell as a Heat and Materials Sink
River temperatures at Lee's Ferry in summer are on the average 11°C
colder than prior to regulation because the dam discharges water from the
metalimnion of the reservoir. Winter temperatures in the post-dam river
average 8°C warmer than pre-dam conditions (Table 5-3~. Consequently,
there is little or no seasonality to the river temperatures until insolation and
side flows begin to reestablish the riverine heat budget within the Grand
Canyon.
Materials balance calculations show that most of the sediments and most
of the phosphorus and other metals entering the reservoir are retained in the
basin either in the profundal waters and sediments or in bank storage. Thus,
the reservoir sequesters the solids load of the Colorado River. On the other
hand, soluble phosphorus, nitrate, and silicate are depleted within the epil-
imnion by the withdrawal current (see above). Therefore, dam tailwaters
are continually transparent and loaded with labile nutrients. These condi-
tions persist downstream into the Grand Canyon and greatly promote
bioproduction within the riverine food web, as evidenced by presence of
dense mats of green algae (Cladophora) and large biomasses of inverte-
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96
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
brates (Gammarus) and fish (Stanford and Ward, 1986c; Blinn and Cole,
this volume).
Controversy persists concerning the effects that Lake Powell and other
Colorado River storage projects have on salinity. Paulson and Baker (1983a,b)
suggest that reservoir operations can control salinity; Messer et al. (1983)
and Edinger et al. (1984) report that the reservoirs have little or no effect
(see above). We note that the error terms on the mass balance estimates are
often higher than the estimated TDS retention (in excess of bank storage).
The issue may be academic except for the fact that salinity at Lee's Ferry is
indeed declining (Figure 5-5~. Is the decline due to salinity control mea-
sures in the headwaters, salinity losses in the reservoirs, or more mesic
conditions in the last decade? The question is relevant because considerable
money continues to be spent on salinity control projects in the Colorado
River basin (Bureau of Reclamation, 1987) when the predominant ions con-
tributing salinity are sulfate and bicarbonate salts that have little effect on
agricultural or potable uses. Indeed, sulfate constitutes one-half of the TDS
in the Colorado River (Paulson and Baker, 1983b). Mancos and other shale
formations, which are widespread on the Colorado plateau (Schumm and
Gregory, 1986), are the sources of most of the water, sediments, and salts
entering Lake Powell. Harmful NaC1 is localized in the Paradox Valley and
Glenwood and Dotsero springs areas in the headwater reaches (Paulson and
Baker, 1983b). Resolution of the salinity question probably requires con-
tinued monitoring of conditions in the rivers above and below Lake Powell.
Retention of >80% of the riverine nutrient load in Lake Powell has greatly
decreased the fertility and bioproduction of Lake Mead; shad-striped bass
fisheries have also declined, and reduced fertility has been inferred as the
cause (Paulson and Baker, 1981; Stanford and Ward, 1986c). However, we
note that a similar destabilization of the food web occurred in Lake Powell
with little or no change in overall trophic status. Moreover, Lake Powell is
oligotrophic despite the fact that the reservoir sequesters the riverine solids
load. Attempts to artificially fertilize Lake Mead to increase bioproduction
and recover the fishery did not provide sustained results; rather, the artifi-
cial fertility pulses quickly attenuated and also desynchronized cohorts of
predator and prey species (as described above). These observations support
the idea that the producer components of lacustrine food webs are not af-
fected much by cyclic or transient changes in the higher consumer popula-
tions (i.e., trophic status and food webs generally are controlled bottom up
by nutrient supply). We suggest that greater attention be given to cohort-
specific growth patterns and the effects of angler harvest in an attempt to
better explain shad-bass dynamics in Lake Powell and Lake Mead. How-
ever, it is clear that the presence of Lake Powell has vastly altered the
chemistry and bioproduction of both the Colorado River and Lake Mead.
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LIMNOLOGY OF LAKE POWELL...
97
Paulson (1983) also showed that cold water from Lake Powell has re-
duced the net heat budget of Lake Mead, causing less evaporation and an
average salinity decrease of at least 9 mg/liter, and suggested that salinity
control would be enhanced if water were discharged from the surface rather
than the hypolimnion of Lake Mead. This would also retain nutrients and
perhaps stimulate productivity, in turn which would encourage calcite pre-
cipitation, providing a secondary control mechanism.
Clearly, management of water quality or fisheries must take into account
the effects of upstream regulation and management actions in addition to
reservoir-specific considerations.
Stream Regulation and the Manifestation of Serial Discontinuity
Production of algae, mainly Cladophora glomerata, is maintained by
conditions of constancy (i.e., transparent summer-cool, winter-warm waters,
stable substratum, and nutrient regime) in the tailwaters. Large populations
of invertebrates, especially amphipods (Gammarus lacustris) support a very
productive trout fishery. Similar biogeochemistry exists in headwater seg-
ments upstream. Thus, Lake Powell resets river conditions to reflect habi-
tats that exist 400+ km upstream (two to three stream orders). This discon-
tinuity (sensu Ward and Stanford, 1983) is gradually ameliorated in the
Grand Canyon segment and then reset again by Lake Mead (Stanford and
Ward, 1986b).
AN ECOSYSTEM APPROACH TO MANAGEMENT
The limnology of Lake Powell is influenced by hydrodynamics of the
rivers that fill the reservoir. Moreover, the hydrodynamics of the reservoir,
as related to use of the dam to store flood waters and generate hydropower,
control the biophysiology of the Colorado River downstream from Lake
Powell through the Grand Canyon. The limnology of Lake Mead is also
coupled to Lake Powell. Thus, it seems reasonable to expand the idea of
the Glen Canyon area as an ecosystem (Marzolf et al., 1987) to include the
tributaries of Lake Powell, the reservoir itself, the Colorado River from
Glen Canyon Dam to Lake Mead, and Lake Mead. However, even bound-
ing the ecosystem in this manner is probably flawed, since the limnology of
the upper basin has a major influence on the quality and quantity of water
reaching Lake Powell and effluents from Lake Mead clearly influence downstream
environments. We conclude that (1) an ecosystem perspective is requisite
because of the biophysical connectivity of the reservoir and rivers and (2)
the ecosystem boundaries must be determined by the nature of the manage-
ment or scientific question.
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98
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
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Representative terms from entire chapter:
colorado river
