Lake and Reservoir Management 23:69-82, 2007
© Copyright by the North American Lake Management Society 2007
Long-term Increases in Oxygen Depletion in the
Bottom Waters of Boulder Basin, Lake Mead, Nevada-
Arizona, USA
James F. LaBounty and Noel M. Burns*
Southern Nevada Water Authority
1900 East Flamingo Road, Suite 255
Las Vegas, Nevada 89119
(702) 822-3357, jim.labounty@SNWA.com
Lakes Consulting
*
42 Seabreeze Rd.
Devonport, New Zealand 0624
(09) 445-7561, lakescon@xtra.co.nz
Abstract
LaBounty, J.F. and Burns, N.M. 2007. Long-term increases in oxygen depletion in the bottom waters of Boulder
Basin, Lake Mead, Nevada-Arizona, USA. Lake Reserv. Manage. 23:69-82.
Long-term changes in the hypolimnetic volumetric oxygen demand (HVOD) of Boulder Basin, Lake Mead were
determined from dissolved oxygen profiles collected from 1991 to 2007. HVOD is the rate at which oxygen in a deep
layer in contact with the sediments is depleted during the period of thermal and/or chemical stratification. Generally,
the rate at which oxygen is depleted is correlated to the amount of organic debris in the hypolimnion and sediments.
The sediment oxygen demand reflects historical organic loading, while HVOD is a measure of productivity because
of the organic particles settling from above. The lower hypolimnion in Boulder Basin remains relatively stable
during the stratification period, enabling the calculation of HVOD in the near-bottom water layer. Small increases
and/or decreases that occur in temperature and dissolved oxygen concentrations are detectable. Boulder Basin fully
destratifies every other year on average, but mixes only partially in the spring (before May) of the remaining years.
The HVOD rates after partial and complete destratification have been assessed separately for 1995-2005. The an-
nual HVOD rate is generally lower the year after partial destratification than after complete destratification due to
greater downward transport of oxygen into the hypolimnion. The HVOD of Boulder Basin is variable depending on
loading of nutrients and water into the Basin. The rate dropped significantly following commencement of advanced
wastewater treatment practices in 1994. The rates then increased 1996-2006 at a rate of approximately 0.75 mg
DO/m3/day per year, or about 7% annually. During those years the inputs of nutrients steadily increased. Rates have
been dropping from 2005 to present (2007) following further reduction of phosphorus input. A multiple regression
analysis revealed that HVOD is significantly positive related to the total phosphorus concentration in Las Vegas
Bay, but significantly negative to inflows of Colorado River water. That means HVOD was highest when reservoir
water was nutrient-rich and flow rates were low. HVOD should be considered a major tool for monitoring trophic
state changes in Boulder Basin.
Key Words: applied limnology, dissolved oxygen, HVOD, hypolimnion, Lake Mead, phosphorus, reservoir ecology,
temperature
A fraction of the organic material produced by primary pro- 1979, Wetzel 2001, Beutel 2003). Development of the theory
duction in the epilimnion of lakes settles into the hypolimnion of oxygen deficits as a measure of lake health has a long his-
where it decomposes and causes a hypolimnetic oxygen tory since the fundamental work by Birge and Juday (1911),
deficit. Changes in the hypolimnetic oxygen depletion rates Thienemann (1928), and Juday and Birge (1932). The relative
over long periods can be indicative of overall changes of areal deficit (AHOD) was introduced by Strøm (1931) and
productivity and lake trophic level (Lasenby 1975, Walker modified by Hutchinson (1938, 1957), and was recently used
69

LaBounty and Burns
by Matthews and Effler (2006). Because the AHOD is the sum
of the oxygen consumed under unit area of hypolimnion, it Virgin River
Moapa River
is sensitive to changes in hypolimnion thickness. In contrast,
the volumetric oxygen deficit, the rate of disappearance of
oxygen from unit volume of the hypolimnion, or hypolimnetic
volumetric oxygen demand (HVOD), is less dependent on
hypolimnion thickness. The HVOD is an indirect measure
of epilimnetic primary production, provided no significant Overton Arm Grand Wash
primary production is occurring in the hypolimnion (Burns Colorado River
Iceberg Canyon
et al. 2005). HVOD rates can also be modified by oxygen
Boulder Canyon
transported downward into the hypolimnion. Significant and “The Narrows”
Las Vegas Wash
variable amounts of DO transferred from the thermocline Virgin Canyon
Virgin Basin
into the hypolimnion must be determined if HVOD rates are Bo u ld e r Ba s i n Gregg Basin
Temple Basin
to be meaningful indicators of change in trophic level. The Pearce Ferry
occurrence of these processes has been documented in Lake Hoover Dam (Black Canyon)
Colorado River
Erie (Rosa and Burns 1987, Burns et al. 2005). HVOD has Figure 1.-Location map of Lake Mead, Arizona-Nevada, with
been used repeatedly to investigate changes in the health of an insert of a map of the Colorado River Watershed in the
southwestern U.S.
lakes and reservoirs (Burns 1995, Burns et al. 2005, Charlton
1980, Cornett and Rigler 1979, Hieskary and Wilson 2005,
Lasenby 1975, Vincent et al. 1984).
The primary focus of this investigation was to evaluate long- Virgin, Temple, and Gregg. Between these basins are 4 nar-
term trends of HVOD, phosphorus concentrations and inflows row canyons: Black, Boulder, Virgin, and Iceberg (Fig. 1).
in Boulder Basin, and determine the causes of variability. The
most complete sets of annual data are from May 2000 to 2007. Retention time in the reservoir ranges between approximately
A previous analysis of data from 2000 to 2004 was reported 1 to 3 years, depending on release and inflow patterns as
in LaBounty and Burns (2005). The oxygen data collected well as reservoir volume at any one time. The Colorado
during that time have now been combined with additional River contributes about 97% of the annual inflow to Lake
data from 1991 to 2007 to calculate 16 annual HVOD rates. Mead; the Virgin and Muddy rivers and Las Vegas Wash
Drastic changes in annual HVOD rates required arranging the provide the remainder. Inflow to Lake Mead is controlled
rates into three groups. Rates from 1995 to 2006 are further by the amount of water released from Glen Canyon Dam.
used to elucidate potential consequences when various sce- The minimum release from Glen Canyon dam is 10.2 × 109
narios of environmental conditions occur. Downward mixing m3 yr-1, the 10-year average is <18.5 × 109 m3 yr-1. Annual
of oxygen into the hypolimnion was estimated for each of the inflow via Las Vegas Wash is currently about 1.9 × 108 m3,
eleven stratified seasons from 1995 to 2005, and the effects about 1.5% of total inflow to Lake Mead. Discharge from
on the observed HVOD rates determined. Hoover Dam is hypolimnetic and occurs from 2 intakes, one
at elevation 272.8 m and the other at 318.5 m a.s.l. Annual
discharge is approximately 9 × 109 m3. During the time of this
Materials and Methods investigation the 2 intakes for the Southern Nevada Water
System (SNWS) were located at 304.8 m and 320 m a.s.l
Description of Study Area until mid-2004, when they were combined at 304.8 m a.s.l.
Annual withdrawal through the SNWS in Boulder Basin is
Lake Mead is a large mainstream Colorado River reservoir in
currently about 5.5 × 108 m3.
the Mohave Desert, Arizona-Nevada (Fig. 1). Its lower end
is 15 km east of Las Vegas, Nevada. Lake Mead, formed in
LaBounty and Horn (1997), LaBounty (2005), LaBounty and
1935 following construction of Hoover Dam, is the largest
Burns (2005) and Holdren et al. (2006) describe the hydro-
reservoir in the United States by volume (36.7 × 109 m3, 2.975
dynamics and relationships between various limnological
× 107 ac-ft)1, and is second only to Lake Powell in terms of
variables of Boulder Basin. As with other reservoirs, dam
surface area (660 km2; Lara and Sanders 1970). At full pool
operations exert a great influence on the water quality and
(reservoir elevation 374 m a.s.l.), Lake Mead extends 106
ecology of the system (Thornton 1990). The hydrodynamics
km from Black Canyon (Hoover Dam) to Pearce Ferry. Its
of such large reservoirs are complex and require analysis of
greatest width is 15 km, and the highly irregular shoreline is
large data sets to understand them. Each basin within Lake
885 km in length. Lake Mead has 4 large sub-basins: Boulder,
Mead is ecologically unique and therefore responds differ-
ently to the inflow-outflow regime. Furthermore, the different
sources of water entering Lake Mead, as in other reservoirs,
1 ac-ft = 3.259 × 105 gal
1
70

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
Pre- and Post 2000 sample locations
Post 2000 sample locations
T emper at u re ( ° C )
Las Vegas Wash
12 14 16 18 20 22
0 DO m g/L
Epilimnion
DO % /10
20 Temperature
Las
Flow
Wash Veg
low
a sB
Metalimnion
rF
40
ay
ve
Depth (m)
Ri
o
ad
60
lor
Co
80
Classic
Hypolimnion
100
120
2 3 4 5 6 7 8 9
DO mg/L -- DO %Sat/10 Hypolimnion
used in this
Hoover Dam
study
Figure 2.-Map of Boulder Basin, Lake Mead, Arizona-Nevada. The Figure 3.-Temperature and dissolved oxygen profiles from
sampling locations for this investigation are depicted. sampling location CR346.4 on 20 October 2005.
often retain their identity and influence for substantial dis- Profile data used for the reported analyses were collected by
tances into the reservoir and do not necessarily mix com- 3 agencies: the City of Las Vegas, Bureau of Reclamation,
pletely with the rest of the water column (Ford 1990). This and the Southern Nevada Water System. HVOD analysis
spatial heterogeneity can lead to significant underestimates was performed using data from the hypolimnetic portion of
of water retention time and uncertainties in the conveyance 8 deep-water sampling locations within the Colorado River
and fate of materials transported into the reservoir. thalweg of Boulder Basin from the Narrows to Hoover Dam
(Fig. 2). Data from 1991 through April 2000 are from 2
Boulder Basin (Fig. 2) is the most downstream basin and deep-water sampling sites, CR346.4 and CR342.25. These
collects the combined flows from the reservoir’s 2 main locations were sampled twice monthly from March through
arms. Additionally, it receives all drainage from the Las November and at least monthly from November through
Vegas Valley via Las Vegas Wash into Las Vegas Bay. April. Data collected after May 2000 through early 2007 are
Boulder Basin is 15-km wide from Boulder Canyon to from all stations shown in Fig. 2. Each location is sampled at
Hoover Dam (Black Canyon), and the distance from the least once per month; many are sampled once or twice weekly
confluence of Las Vegas Wash to Hoover Dam is approxi- by one or more of the participating agencies.
mately 16 km. The historical Colorado River channel lies
along the eastern side of the basin. Lake Mead is mostly
Field and Analytical Methods, and Data
mesotrophic and monomictic (LaBounty and Burns 2005).
Analysis
Boulder Basin destratifies completely about every other year.
Stratification lasts for approximately 265 days from 1 May
Each profile was sampled in a similar manner. Data were
to 15 January during years when complete destratification
collected from the following depths: surface-30 m every 1 m;
occurs. During the stable summer stratification period the
30-60 m every 2 m; 60-100 m every 5 m; 100-5 m from the
epilimnion, metalimnion and hypolimnion are about 15, 20,
bottom every 10 m; and from 5 m-bottom at least every 1 m
and up to 120 m thick respectively, depending on the depth
(Fig. 3). Each profile includes temperature and DO. Profile
of the reservoir and particular location. The hypolimnion
data were collected using profiling sondes manufactured by
is the thickest and most stable layer, with an average water
Hydrolab Corporation (1995-early 2007) and Eureka Corpo-
temperature of 12°C. The deepest portion of Boulder Basin
ration (mid-2005-early 2007). Repeatability of DO measure-
does not completely destratify each year, leaving a remnant
ments is within 0.2% and instrument accuracy = 0.1 mg/L for
portion of the hypolimnion >20 m above the bottom in about
readings <8 mg/L, 0.2 for readings >8 mg/L. Air calibration
40% of the years (LaBounty and Burns 2005, Holdren et al.
is electronic, utilizing the barometric reading at the beginning
2006). Ecological dynamics until early 2007 are described
of the survey. Quality control was accomplished by annual
in this paper. Significant populations of the quagga mussel
comparison of up to 11 probes from participating data col-
(Dreissena bugensis) were discovered throughout Boulder
lection groups at the same time and place.
Basin in January 2007. As populations of this invasive species
explode, the ecological dynamics of Boulder Basin could be
drastically altered.
71

LaBounty and Burns
Water samples were collected weekly for analysis of nutrients time (days), and then these data were fitted to a straight line
including total phosphorus (TP). Samples were collected us- by linear regression analysis. This annual rate of DO loss
ing a Van Dorn sampler at all depths except from the surface, (HVOD) is expressed in mg/m3/day. Regressions were cal-
where grab samples were collected. Two samples were taken culated at the observed water temperatures and then corrected
within the epilimnion when it was ≤15 m thick; 3-4 samples to a standard water temperature of 12°C for all years. This
within the thermocline, including one at peak conductivity; method was chosen because the HVOD rate is presumed to
and up to 4 samples within the hypolimnion. Analyses for TP double for every 10°C rise in temperature, as rates do in many
and orthophosphate were by the USBR laboratories in Den- biochemical reactions. Standardization is necessary for the
ver, Colorado and Boulder City, Nevada, the Clark County comparison of data from different years because they have
Water Reclamation District Water Quality Laboratory, Las different average temperatures. The temperature of 12°C was
Vegas, Nevada, and Montgomery Watson Harza Laborato- chosen as the standard temperature because it was close to
ries, Monrovia, California. Standard Method SM4500P-E, the average temperature observed for all years. The annual
colorimetric analysis was used for most analyses. Detection HVOD rates were plotted separately for 1995-2006, and
limits were 10 µg P/L for TP. All values that were nondetect- linear regressions were obtained for each plot for 2 types of
able were halved for calculations of averages. years: Type 1 year follows complete destratification; Type
2 follows partial destratification (as described in the follow-
Processing of data, statistical analysis and development ing section). The slopes of the regression lines gave the rate
of graphical presentations were completed using Excel, of change of HVOD in Boulder Basin in mg DO/m3/day
PowerPoint (www.microsoft.com), and LakeWatch (www. per year. The strength of relationships between HVOD and
earthsoft.com; www.lakewatch.net). other reservoir characteristics was determined by simple and
multiple regression analysis.
Hypolimnetic Volumetric Oxygen Depletion
(HVOD) Calculations Results
The HVOD rates for Boulder Basin were determined using
Vertical Mixing
the HVOD module in the LakeWatch software. To determine
While temperature and DO of the epilimnetic layer are sea-
the most adequate method, HVOD analysis of the bottom
sonally variable, within the hypolimnion they change slowly
layer was attempted using 3 methods. In the first, the hypo-
throughout the stratified period. During fall the epilimnion
limnion layer was selected from the normal lower inflexion
thickens due to seasonal cooling. From May to early October
point on the temperature/depth profile. In the second and third
the epilimnion layer of Boulder Basin is generally 11-13
methods, the bottom layer depths were selected as being 5 m
m. In early October cooling begins to thicken this layer.
and 20 m, respectively, above the bottom. The first method
For example, on 20 October 2005 the epilimnion was 25 m
was rejected because the hypolimnion water mass under
(Fig. 3). Despite cooling temperatures in fall 2005, neither
analysis changes as the selected hypolimnion depth deepens
the water temperature nor the DO profiles indicated strong
as the season progresses. The bottom 5-m layer did not yield
vertical mixing of the hypolimnion. A typical metalimnetic
enough observations to give stable values. Consequently,
DO minimum was prominent, while the concentrations within
the bottom 20-m layer was chosen for analysis because
the hypolimnion steadily decreased with depth until there
it gave stable values from an unchanging water mass and
was a sudden drop of >2 mg/L within 5 m of the bottom.
contained water with the lowest DO concentrations in the
Below 90 m, temperature cools with depth into the hypo-
water column. These values could thus show how low the
limnion by approximately 0.1°C per 20 m. This temperature
oxygen concentrations became at the end of each stratified
gradient is maintained from May to December indicating a
season. The average DO content for the bottom layer was
very stable hypolimnion. At the same time the bottom 20
calculated for each sampling station on each sampling date.
m water temperatures increased by about 0.3°C throughout
In the remainder of this study, the term hypolimnion refers
the stratified season, indicating a small, steady downward
to the 20-m layer above the bottom (Fig. 3).
transfer of heat (Fig. 4).
HVOD rates were determined according to the stratification
There was a typical annual pattern of temperature and DO
conditions prevailing each year. The numbers of days the
within the bottom 20 m of the hypolimnion (Fig. 4) indicating
reservoir remained stratified each year was determined by
4 phases of vertical mixing:
examining water temperature and DO from the initiation of
stratification (usually 1 May) to the point of its interruption
Phase 1. During the first 220 days of stratification (normally
(usually 15 January, or 265 days later). The stratification
1 May to about 6 December) the average water temperature
period lasted from 190 to 280 days. The annual oxygen deple-
of the bottom 20 m of the hypolimnion gradually but steadily
tion rate was calculated by plotting the average hypolimnetic
warmed from 11.9°C to 12.2°C, or 0.3°C/220 days. At the
DO concentration sampled at each station as a function of
72

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
same time the average DO concentration steadily declined
Dissolved Oxygen (mg/L) Temp ºC 2
9
12.4 from 5.2 mg/L to 2.1 mg/L, or -3.1 mg/L/220 days.
DO mg/L
8
Temperature ºC
12.3
7
4 1
Phase 2. From day 220 to day 300 (about 6 December to 24
6
12.2
3
5
February), warming of the bottom 20 m of the hypolimnion
12.1
4
accelerated, and partial vertical mixing occurred as indicated
3 12
by the reoxygenation from 2.1 to 8.5 mg/L in the early part
2
0 40 80 120 160 2 00 240 28 0 3 20 36 0
of Phase 3. Water temperature increased nearly 0.3°C dur-
Ma y 1 April 30
Dec 6 Feb 24
Number of Days from May 1 ing this phase (12.2°C to about 12.5°C). The surface water
temperature at the end of February 2006 was 13.2°C, so when
1 Slow warming – Weak vertical mixing with slight downward heat and DO transfer
the surface water mixed downward there was relatively little
2 Faster warming – Partial mixing; Some DO transfer
increase in temperature because the surface water tempera-
3 Intrusion of Colorado River water; Reoxygenation to 80% saturation
ture was close to that of the deep water, while at the same
4
time there was considerable downward transfer of DO by
Stratification strengthens-Oxygen uptake recommences
vertical mixing. However, complete vertical mixing was not
Figure 4.-Average daily water temperature and dissolved oxygen
achieved because a bottom water temperature of 13.2°C was
(DO) concentrations from the bottom 20 m of the hypolimnion, 1
May 2005 to 30 April 2006. not obtained. Complete vertical mixing on this date proceeded
only to a depth of 46 m.
Phase 3. After day 300 (about 24 February) intrusion of low
De st ra tif i ca t ion
temperature, low conductance, higher DO Colorado River
Some reoxygenation water from Boulder Canyon was observed, lowering the water
temperature suddenly from 12.4 to 12.1°C.
Phase 4. After 1 May (day 1) the cycle began as stratifica-
Dissolved Oxygen (mg/L)
DO mg/L
tion strengthens. Oxygen uptake in the bottom 20 m of the
T e m p er a tu r e ° C
hypolimnion occurred.
9 12.4
Temperature °C
8
7 12
6
11.6
Fully Destratified (Type 1) Versus Partially
5
4
11.2
Destratified Years (Type 2)
3
1 00 200 300 100 200 300
Ma y 1
May 1 Jan 1 M ay 1 Jan 1
Lake Mead is a monomictic lake with turnover (destratifica-
2002 2 0 03 20 0 4
tion) occurring generally from 1 March-1 May (LaBounty
Figure 5.-Average daily temperature and dissolved oxygen (DO)
and Burns 2005). Since stratification is usually complete by
concentration in the bottom 20 m of the hypolimnion of a partially
1 May, the “lake year” for every period of 365-days is de-
destratified year (Type 2) followed by a fully stratified year (Type
fined as extending from 1 May to 30 April, and thus spans 2
1), 1 May 2002 to 1 May 2004.
calendar years. Lake years are named from the second of the
2 years spanned. The Type 1 group are those years follow-
Average Monthly Temperature ing the complete destratification of the reservoir (lake years
1997, 1999, 2001, 2002, 2004, and 2006). The Type 2 group
12.5
Temperature (° C)
includes those years following a year when the reservoir did
12
not fully destratify (lake years 1995, 1996, 1998, 2000, 2003
11.5
and 2005). The pair of years plotted in Fig. 5 (May 2002
through April 2004) are Type 1 and 2, respectively. The
11
Type 2 group of years have higher hypolimnion temperatures
10.5
2007
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
and usually lower DO concentrations than the Type 1 years
Average Monthly Dissolved Oxygen
Dissolved Oxygen (mg/L)
12
(Figs. 6 and 7).
10
8
Study of numerous profiles of temperature, DO and specific
6
conductivity show that Boulder Basin has normal vertical
4
temperature and DO structure for most of the year, with the
Following Partial Following Full
Destratification Destratified
2
thermocline deepening after mid-November. In some years
Y e ar 2 ty p e Y e ar 1 t y p e
0
2007
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
the downward mixing progresses only to a depth of 70 m
to give a partially destratified year (Type 2 year). In other
Figure 6.-Average daily water temperature and dissolved oxygen
(DO) concentration of the bottom 20 m of Boulder Basin, 1995- years, downward mixing proceeds to the lake bottom to give
2007.
73

LaBounty and Burns
a fully destratified year (Type 1 year). But Boulder Basin 2005 – 2006 (Type 2) DO Depletion Rate
12.2
has a unique mixing pattern in one respect. In a normal lake,
Temperature C
DO (mg/L)
5
the temperature at the bottom of the lake at the end of the
isothermal period is the lowest temperature observed in the 12.1
4
water column that year. However, in Boulder Basin, when
3 12
the surface water begins to warm after the isothermal period, DO Depletion
0
the hypolimnion from a depth of about 95 m can continue Regression
0 30 60 90 120 150 180 210 T em p er atu r e
Nov 15
M ay 1 Jul 6 Sept 10
to get colder. This is attributed to a large pool of cool water Regression
2004 – 2005 (Type 1) DO Depletion Rate
from the Colorado River slowly moving into Boulder Basin 11.8
8
11.7
Temperature C
into the bottom of the deepest part of the Basin and usually
DO (mg/L)
7
11.6
results in the deeper hypolimnion having a temperature 11.5
6
of ≤12°C. The process of an intrusion of Colorado River 11.4
5
water into Boulder Basin takes 2 months to arrive from the 11.3
11.2
4
Colorado River inflow (Dr. Imad Hannoun, Flow Science, 0 30 60 90 120 150 180 210 240
Harrisburg, VA., pers. comm.). The intrusion (interflow) Jan 21
May 1 Jul 6 Sept 10 Nov 15
of Colorado River water forming the deeper hypolimnion Figure 7.-Dissolved oxygen depletion (HVOD) rate (regression
is usually complete by 1 May and sets up a Type 1 year. = -0.01375 mg DO/L/day; p < 0.0001) and temperature increase
During the remainder of the year, until the end of Novem- (regression = +0.00112°C/day; p < 0.0001) for (top panel) the first
210 days of 2005 (beginning 1 May 2005), and (bottom panel)
ber when the Colorado River interflow is found within the
the first 265 days 2004 (beginning 1 May 2004) dissolved oxygen
upper portion of the hypolimnion or in the metalimnion, depletion (HVOD) rate (regression = -0.01471 mg DO/L/day; p <
the temperature in the bottom hypolimnion layer increases 0.0001) and temperature increase (regression = +0.00112°C/day;
slowly as the DO concentrations decrease (Fig. 4 and 5). A p < 0.0001).
Type 1 year is usually followed by a Type 2 year with some
downward mixing and/or some Colorado River water being
added to the deep hypolimnion between February and May,
menced with a Colorado River intrusion of 11.1°C and 980
with DO concentration increasing but with little change in
µS/cm water on 12 April 2004.
temperature (Fig. 6). At the end of the Type 2 year the hy-
polimnion temperature is usually above 12°C enabling full
Average monthly temperatures of the bottom 20 m of the
vertical mixing (Holdren et al. 2006), illustrated by the data
hypolimnion ranged between 11 and 12.6°C from 1995 to
on a biennial period (Fig 5).
early 2007 (Fig. 6). Holdren et al. (2006) stated that mixing
occurs in Boulder Basin when early spring hypolimnetic
The water in Boulder Basin has a higher conductance than
temperatures are above 12°C; mixing does not occur below
the water from the Colorado River because of evaporation
12°C. When destratification is not complete between strati-
and the higher conductivity water from Las Vegas Wash and
fied periods (black arrows, Fig. 6), the water temperature
the Virgin and Muddy Rivers. The first year (1 May 2002-30
continues to rise until destratification does occur (red arrows,
April 2003) of a pair of years (Fig. 5) follows a complete
Fig. 6). Full mixing that results in bottom waters with the
destratification during February-April 2002. At Station
same temperatures as the surface waters occurs only after
CR346.4 on 12 February 2002, the water was isothermal at
the temperature difference between the surface and bottom
11.9°C with a conductance of 935 µS/cm, but by March the
waters is <1°C. Once destratification occurs, its onset is sud-
Colorado River intrusion water had changed the tempera-
den and lasts 30-60 days. The pattern for DO concentrations
ture to 11.1°C with a conductance of 919 µS/cm to set up a
reflects that of temperature. In years when destratification was
Type 1 year. The second year (1 May 2003-30 April 2004)
not complete (black arrows), DO concentrations remained
followed partial destratification. Stratification weakened
generally below 6 mg/L. Upon destratification, however,
between January and April 2003 but remained present. It
reoxygenation to saturation near 80-85% was achieved (red
was weakest on 4 March 2003 with a surface temperature
arrows). A similar pattern, but the reverse of that of tem-
of 12.8°C and a bottom temperature of 11.6°C. Average
perature, existed for average DO concentration from 1 May
bottom water temperature began at about 11.1°C on 1 May
2002 to 30 April 2004 (Fig. 5). Average DO concentrations
2002 and was 11.8°C on 30 April 2003, a rise of 0.7°C in
were 8.2 mg/L on 1 May 2002 and 5.1 mg/L 265 days later
365 days. From 1 May 2003 to about 1 February 2003 (275
in mid-March 2003, a loss of 3.1 mg/L DO in the bottom 20
days) the temperature rose from 11.8°C to about 12.4°C, or
m of the hypolimnion. DO depletion continued after some
0.6°C. The temperature rose at a rate 12.1% greater in the
oxygenation in late April and early May 2003. The average
second year. After 1 February 2004 (within the second year)
DO concentration was 5.8 mg/L in May 2003 and ended at
destratification, or complete mixing, occurred with the water
3.6 mg/L in mid-January 2004, about 245 days later, a loss
column isothermal at 12.2°C and conductance between 998
and 1000 µS/cm on 13 February 2004. A Type 1 year com-
74

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
of 2.2 mg/L. The depletion rate for DO was 23.2% greater
Table 1.-Annual dissolved oxygen depletion rates for 16 years,
1991–2006, Boulder Basin, Lake Mead, Nevada-Arizona. during the first year than the second.
DO on Ending Annual
Lake Type May 1 DO Rate2
Annual HVOD Rates
Year1 of Year (mg/L) (mg/L) (mg/m3/day)
The flow into Las Vegas Bay of Boulder Basin consists
1991 1 8.8 5.6 18.1
mostly of treated wastewater from the Las Vegas Valley.
1992 1 8.4 4.9 17.2
Since 1994, several improvements to waste water treatment
1993 1 9.0 5.1 15.1
(i.e., tertiary treatment of phosphorus and nitrogen) were
1994 1 8.8 5.1 17.4
made, affecting Las Vegas Bay. As a result of this and other
1995 2 5.2 3.6 8.1
1996 2 5.5 4.1 7.5 external factors (such as above vs. below average inflow
1997 1 8.4 6.3 9.3 to Lake Mead), certain patterns form individual groups of
1998 2 8.2 6 9.6 years, each having one or more major events that cause them
1999 1 8.1 5.7 10.7
to be considered a separate group no matter which years are
2000 2 7.1 4.8 11
included in other groups. Accordingly, annual depletion rates
2001 1 8.2 5.2 11.8
for lake years 1991-2006 are in 3 distinct groups, each with
2002 1 8.2 5.1 14.1
a major event related to changes in treatment of nutrients
2003 2 5.8 3.6 11.7
released into the Las Vegas Wash and thus into Las Vegas
2004 1 7.8 4.2 14.7
Bay (Table 1; Fig. 8).
2005 2 5.1 2.7 13.7
2006 1 8.8 4.8 11.6
Group 1, 1991-1996, includes the initiation of advanced
Average 1 8.5 5.2 14.0
wastewater treatment in 1994. The early HVOD rates were
Std Dev 1 ±0.4 ±0.5 ±2.9
the highest of the 15 years (15.1-18.1 mg/m3/day)2. Group
Average 2 6.2 4.1 10.3
2, 1996-2004, begins with the lowest annual rates (7.5-8.1
Std Dev 2 ±1.2 ±1.1 ±2.3
mg/m3/day), and ends with an annual rate of 14.7 mg/m3/day.
May of named year to April of the next year
1
This group includes the commencement of tertiary treatment
of nutrients in discharged wastewater as well as a period of
The HVOD at standard temperature (12°C) regression presented in this
2
table abnormally high inflow to Lake Mead (1995-1999). While
average outflow from Glen Canyon Reservoir was always
<335 m3/day in 1991-1994 and 2000-2006, average outflow
in 1995-1999 was >400 m3/day (peaking at about 600 m3/day
in 1997). There was ever-increasing organic loading during
Enhanced
this time period due to the population surge in the watershed
Advanced W int e r Treatment
Treatment Treatment f or P
leading to steadily increasing flows of treated wastewater
Average Glen Canyon
Begins Begins Begins
Outflow > 400 m3/day
via Las Vegas Wash. Group 3, 2005-2007, shows declining
annual rates (14.7-11.6 mg/m3/day). Advanced treatment
HVOD
Regression
of phosphorus in the discharged wastewater effluent is not
18
HVOD at Std Temp
Regression
DO Depletion Rate (mg/m3/day)
officially regulated from October through March. However,
16
beginning in winter 2002 treatment of phosphorus in the
14
Group 2
wastewater outflows from all facilities occurred much more
Group 1
12
frequently from October to April. Additionally, in 2005
10
enhanced treatment for phosphorus occurred that further
Group 3
reduced the total load of organics to Boulder Basin. All
8
these efforts reduced the current TP load by 97-98% from
1991 1993 1995 1997 1 99 9 2001 2003 2005
levels from the 1970s and 1980s (Dr. Doug Drury, Clark
County Water Reclamation District, Las Vegas, Nev., pers.
Figure 8.-Rate of HVOD change in the bottom 20 m of the
comm.).
hypolimnion of Boulder Basin, 1991-2006. Group 1 (1991-1996):
high HVOD rates early (during period of high organic loading),
dramatically lower rates later (following implementation of
Annual HVOD Rate for Years of Type 1 and 2
advanced wastewater treatment technology); Group 2 (1996-
2004): progressively increasing HVOD rates due to rapid
Data for 1995-2005 were selected for analysis and clas-
population expansion within the watershed leading to continuous
sification into Type 1 and 2 years, because even though the
increase in organic loading; Group 3 (2004-2006): beginning of a
trend of decreasing HVOD rates due to decreased organic loading.
HVOD rates at standard temperature of 12°C
2
75

LaBounty and Burns
wastewater treatment remained the same, loading increased Type 1: Years following complete destratification
DO Depletion Rate (mg/m3/day)
with population surges. Dissolved oxygen depletion pro- 14
gresses at a steady rate, but there is a measurable difference 12
between Type 1 and Type 2 years (Fig. 7). Disruption of the
10
upward temperature trend and the downward DO depletion HVOD
Regression
trend happens sometime after day 180 (Fig. 4, a temporal 8 HVOD at Std Temp
Regression
extension of the top panel of Fig. 7). In 2005, the pattern 1997 1998 1999 2 0 00 2 001 20 02 20 03 2 00 4
began falling apart after day 210 (6 December 2005). After Type 2: Years following partial destratification
DO Depletion Rate (mg/m3/day)
complete destratification, the stable period lasted at least 265 14
days to 21 January.
12
Dissolved oxygen concentrations at the beginning and end 10 HVOD
Regression
of annual stratification were different for the two categories 8 HVOD at Std Temp
of years (Table 1). Each Type 1 year started and finished the Regression
stratified season with higher DO concentrations than Type 1 99 5 19 9 6 19 9 7 1 9 98 19 99 2 000 20 01 2002 2 00 3 20 04 2 0 05
2 years. The HVOD rates for the Type 1 group (14.0 ± 2.9 Figure 9.-Rate of HVOD change. Top panel: Type 1 Category
mg/m3/day) are higher than those of Type 2 (10.3 ± 2.3 years; years that followed complete destratification (regressions:
HVOD = +0.803 mg/m3/day per year, p < 0.01; HVOD at 12°C
mg/m3/day). The annual DO depletion rates for 1995-2005
= +0.906 mg/m3/day per year, p < 0.05). Bottom panel: Type
were plotted separately for Type 1 and Type 2 years (Fig.
2 category years; years that followed a short period of partial
9). Regressions were calculated and corrected to HVOD destratification (regressions: HVOD = +0.581 mg/m3/day per year,
at 12.0°C and illustrate increasing trends since 1995 for p < 0.01; HVOD at 12°C = +0.586 mg/m3/day per year, p < 0.01).
both types of years. The annual rates, corrected to 12.0°C,
increased by 0.91 mg/m3/day per year for the Type 1 years
Average Daily Outflow, Glen Canyon Dam
and 0.59 mg/m3/day per year for the Type 2 years. Each of
these trends is statistically significant (p < 0.01). 650
600
HVOD Change Based on Type 1 and Type 2 1 9 95 , 96 , 9 7 , 9 8 , & 9 9
550
Flows > average,
Years reservoir rose 11 m
500
m3/day
The mixing period of larger lakes is usually annual if the 450
lake is monomictic. Finding a biennial mixing period was 400
unexpected, but even more unexpected was the discovery 350
of different HVOD rates for the different types of years. A
300
number of scenarios were investigated to determine the cause
for the different HVOD rates. The first possibility was that 250
1991 1993 1995 1997 1999 2001 2003 2005 2007
different mixing regimes resulted in different organic carbon
Figure 10.-Average daily outflow from Glen Canyon Dam (Lake
concentrations in the hypolimnia of the 2 different types of
Powell), 1991-2006. Outflow from Glen Canyon Dam travels
years, thus causing different depletion rates; this was found through the Grand Canyon into Lake Mead unimpeded. Circled
not to be the case, however; organic carbon concentrations values are years when average flow from Glen Canyon Dam was
were similar in the pairs of years (Type 1 years 2.94 ± 0.31 >average.
mgTOC/m3; Type 2 years 3.00 ± 0.30 mgTOC/m3).
Varying inflow rates and water temperature from Lake Pow-
(Fig. 6; 1997, 1999) and 3 Type 2 years (Fig. 6; 1995, 1996,
ell (Glen Canyon Dam outflow, Fig. 10) were investigated
1998), which means the distribution of year types was similar
as possible causes of the different HVOD rates. The inflow
to the low flow period.
from Lake Powell via Boulder Canyon dominates the water
mass in Boulder Basin, and the volume and temperature of
The possibility of variable downward transport of oxygen was
this inflow is quite variable. Both the volume/inflow rate
next investigated. The full and partially destratified years tend
and water temperature depend on the outflow from Glen
to occur in pairs (Fig. 6). However, as Holdren et al. (2006)
Canyon Dam (Lake Powell). In general but not always, the
pointed out, and as we have shown, winter destratification
temperature of Lake Powell outflow depends on the depth of
was not completed for the 3-year period 1994-1996. Inflow
Lake Powell. At lower lake levels, outflow temperatures are
of colder water into the hypolimnion as an interflow from
higher because water is removed from a shallower layer. The
the Colorado River occurred in those 3 years but also in the
magnitude of the flow did not affect the type of year because
other years when destratification occurred. The resistance
in the 1995-1999 high flow period there were 2 Type 1 years
76

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
to vertical destratification is strongest in the first year of a a small rise in temperature of the water column. The transfer
pair because the water temperature of the hypolimnion is 1-2 of DO into the bottom 20 m of Boulder Basin by eddy dif-
degrees cooler than in the second year of the pair. When the fusion was estimated by an equation successfully developed
water temperature rises during the second year of the pair, and applied in Lake Erie (Rosa and Burns 1987):
resistance to full vertical mixing weakens and full vertical
∆O = (Ot - Oh)/(Tt - Th) × ∆T eq. (1)
mixing can occur.
where ∆O = increase in DO concentrations if DO was con-
From 1995 to 2005, the average HVOD rate for Type 1
served; ∆T = increase rate in hypolimnion temperature; Ot =
years was 2.0 mg/m3/day higher than Type 2 years (Table 2).
average thermocline DO concentration for stratified period;
Further, the rate of increase in the HVOD rate was less for
Oh = average hypolimnion DO concentration for stratified
Type 2 years. We investigated the cause of the differences by
period; Tt = average thermocline temperature for stratified
determining whether there is a difference in the downward
period; and Th = average hypolimnion temperature for strati-
transfer of heat and DO between the two types of years.
fied period. The temperature increase rates in the bottom 20
Downward transfer of heat occurs by conduction from m are small but can explain substantial transfer of DO into
warmer, upper waters to cooler, deeper waters. Similarly, the hypolimnion (Table 2).
oxygen can move downward by molecular diffusion to the
The temperature increase rate is similar for both Type 1 and
deeper waters containing less oxygen, 2 independent mecha-
Type 2 (Table 3), but because of the greater Ot - Oh gradi-
nisms. The conduction of heat through water is calculated by
ent in Type 2 years, eddy diffusion is able to transfer more
using the thermal conductivity coefficient of water (0.0014
oxygen downward than in Type 1 years. The extra oxygen
cal/cm/sec) and the vertical temperature gradient, which at
added to the hypolimnion in Type 2 years significantly lowers
the depth of 20 m above the bottom is approximately 0.6 ×
the observed HVOD rate in those years. Also, the true (as
10-4°C/cm in Boulder Basin. This produces a warming in the
adjusted) oxygen uptake rates are significantly higher than
20-m thick hypolimnion of approximately 0.0017°C from 8
the observed HVOD rates. The rates of HVOD change are
May-28 December of this particular year. In 2000, the bot-
0.59 mg/m3/day per year for Type 2 years and 0.91 mg/m3/day
tom waters at the central sampling site (CR346.4) warmed
per year for Type 1 years. The increased downward diffusion
by 0.3°C during this period. Thus the downward transfer of
also explains why Type 2 HVOD rates are increasing more
heat by conduction is of little importance in the warming of
slowly than Type 1 rates. As hypolimnetic DO concentrations
the hypolimnion.
become lower, the DO concentration gradient between the
The molecular diffusion of oxygen through the surface of the hypolimnion and the water above it becomes greater; there-
hypolimnion can be estimated by using the diffusion coef- fore, eddy diffusion transports more oxygen downward.
ficient of oxygen in water of 2 × 10-9 m2/sec (Pilson 1998) and
Factors other than hypolimnetic temperature prevent or
the average DO concentration gradient at the hypolimnion
enhance complete mixing after the second year of a pair of
boundary of 0.20 g/m4. This yields a downward flux of DO
years: winter climatic factors such as ambient air tempera-
of 3.5 × 10-3 mg/m2/day, which would increase the concen-
ture, wind speed and duration; and as discussed, erratic high
tration of DO in a 20-m thick hypolimnion by 0.17 × 10-3
and low flow patterns to and from Lake Mead (Holdren et
mg/m3/day. This is much smaller than the observed HVOD
al. 2006). These factors altered the pattern of full biennial
rates ranging from 8.1 to 14.7 mg/m3/day. Molecular diffu-
restratification, caused complete mixing in 2 subsequent years
sion of oxygen would be of little importance in explaining
in 2000 and 2001 (Fig. 4), and prevented complete mixing
the difference of 2.0 mg/m3/day between the Type 1 and
for 3 other years (1994-1996).
Type 2 HVOD rates.
During the stratified season, heat and DO mostly move
Causes of Change in HVOD
downward in lakes and reservoirs by a vertical mixing process
known as vertical eddy diffusion, a process found to be sig- The period of relatively low HVOD rates from 1995 to 2000
nificant in Onondaga Lake, New York (Mathews and Effler occurred just after the commencement of advanced treatment
2006). In eddy diffusion, heat and DO are moved downward of the wastewater entering the Las Vegas Wash (Fig. 8),
in the same water mass; thus, the increase of temperature which indicates that the supply of phosphorus to Boulder
in the hypolimnion is an indication of the quantity of DO Basin probably has an effect on the HVOD rates. However,
transferred downward in the water. However, the increase this period of low HVOD rates also coincides partially (1995-
in DO concentrations will not be observed if oxygen is being 1999) with the period of high flows from the Glen Canyon
consumed via the degradation of organic matter at a greater Dam, which also tend to lower the HVOD rates (Fig. 10 and
rate than its downward displacement. If the eddy diffusion 11). Thus both the Glen Canyon Dam outflow and TP con-
is not extensive and happens to a similar degree from the centrations in Las Vegas Bay were investigated as affecting
thermocline downward, it is not easily discernable except for
77

Table 2.-Estimate of downward transport of dissolved oxygen (DO) into the hypolimnion of Boulder Basin 1995-2005 by eddy diffusion.
Av Av Av Av Temp Downward
Thermo Hypo Thermo Hypo Tt - Th increase Downward HVOD HVOD DO flux as
Year Type DO DO Ot - Oh Temp Temp Temp rate DO flux Obs. Adj. % of HVOD
and Year Adj.
(g/m3) (g/m3) (g/m3) (°C) (°C) (°C) (°C/day) (g/m3/day) (g/m3/day) (g/m3/day)
Type 1 – Years After Full Destratification
LaBounty and Burns
1997 7.39 7.03 0.36 12.091 12.012 0.079 0.0009 0.004 0.009 0.013 29
1999 7.40 7.03 0.36 12.062 11.987 0.075 0.0011 0.005 0.011 0.016 33
2001 7.13 6.81 0.32 11.876 11.827 0.049 0.0012 0.008 0.012 0.020 39
2002 7.25 6.69 0.55 11.405 11.328 0.077 0.0015 0.011 0.014 0.025 43
2004 6.76 6.09 0.67 11.463 11.372 0.091 0.0018 0.014 0.015 0.028 48
Averages 7.18 6.73 0.45 11.779 11.705 0.074 0.0013 0.008 0.012 0.020 40
Type 2 - Years After Partial Destratification
1995 5.21 4.74 0.47 12.003 11.894 0.109 0.0017 0.007 0.008 0.015 47
1996 5.62 4.87 0.75 12.653 12.545 0.108 0.0011 0.007 0.007 0.014 52
1998 7.53 7.09 0.44 12.252 12.220 0.032 0.0016 0.022 0.010 0.032 70
2000 7.01 6.28 0.73 12.390 12.341 0.049 0.0013 0.019 0.011 0.030 63
2003 5.71 4.73 0.99 12.132 12.007 0.125 0.0015 0.012 0.012 0.024 50
2005 5.31 3.95 1.37 12.202 12.079 0.123 0.0016 0.017 0.014 0.031 56
Averages 6.07 5.27 0.79 12.272 12.181 0.091 0.0014 0.014 0.010 0.024 58
78

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
Table 3.-Annual dissolved oxygen (DO) depletion rates and HVOD versus TP—Years of Low Inflow Only
calculated years to anoxia for 11 years, 1995-2005, Boulder Basin,
80
Lake Mead, Nevada-Arizona. * indicates those lake years that
Average Annual TP (µg/m3)
began fully destratified (Type 1).
60
Year (May to Apr Annual Rate Years to 40
of Named Year) (mg/m3/day) Anoxia
20
1995 8.1 20 HVOD vs. TP
1996 6.8 24 Linear (HVOD vs. TP)
0
10 12 14 16 18 20
1997* 9.3 25
HVOD (mg/m3/day)
1998 9.6 24
1999* 10.7 22 Figure 12.-HVOD vs. TP. Average annual TP in Las Vegas Bay
2000 11 27 for the years when the flow from Glen Canyon Dam <335m3/day
2001* 11.8 21 (regression: y = 8.562 + 86.3, R2 = 0.80, p < 0.001).
2002* 14.1 19
2003 11.7 17
2004* 14.7 16
2005 13.7 9
Indeed, the multiple regression of HVOD on TP-City of Las
Vegas and Glenn Canyon Flow is highly significant:
HVOD = 18.2 + 0.082TP − 0.023Flow (R2 = 0.68);
partial-p (TP) = 0.005, partial-p (Flow) = 0.003; over-
HVOD vs. Glen Canyon Outflow
all-p <0.001
21
HVOD vs. Outflow
1995, 96, 97, 98, & 99
19
HVOD (mg/m3/day)
Flows � average, Linear (HVOD vs. Outflow)
This relationship provides evidence that TP in Las Vegas
reservoir rose 11 m
17
Bay has a positive and Flow a negative significant effect
15
13
on HVOD.
11
9
During the low-flow years average daily flow was nearly
7
constant (Fig. 10), so that the regression of HVOD on TP
5
275 325 37 5 425 475 525 57 5 625
concentrations in Las Vegas Bay was not influenced by any
Average Daily Flow (m3/s)
changes in the Glen Canyon outflow. This relationship is
HVOD vs. TP in Las Vegas Bay
21
highly significant (n = 11, R2 = 0.80, p < 0.001; Fig. 12) and
19
HVOD (mg/m3/day)
supports a strong positive relationship between HVOD and
17
TP in Las Vegas Bay. Similar relationships were observed
15
in a Snake River reservoir, where high TP concentration
1995, 96, 97, & 98
13
Flows � average,
increased anoxia and hypoxia, but flow diminished it (Nürn-
11 reservoir rose 11 m
9
berg 2002).
HVOD vs. TP CLV
7
Linear (HVOD vs. CLV TP)
5
0 10 20 30 40 50 60 70 80 90
Average Annual TP (µg/m3)
Future Trends
Figure 11.-HVOD vs. flow and TP. Top panel: HVOD vs. average
HVOD rates increased from 1995 through 2005. If this pat-
daily flow from Glen Canyon Dam, 1991-2006 (regression: y
tern continued, the rates would eventually be large enough to
= -0.025 + 21.87, R2 = 0.405, p < 0.05). Circled years indicate
result in the hypolimnion of Boulder Basin becoming anoxic.
average daily flows >400 m3/day). Average daily flow for the other
years <335 m3/day. Bottom panel: HVOD vs. TP in Las Vegas Fortunately, changes have occurred that have interrupted this
Bay, 1991-2006 (regression: y = 0.093 + 9.5, R2 = 0.360, p < 0.01).
trend (decreased phosphorus loading). Average daily flows
Circled values are years when average daily flows were >400
from Lake Powell (Glen Canyon Dam) >400 m3/day in 1995
m3/day. Average daily flow for the other years <335 m3/day.
decreased the oxygen depletion rate for that year. Decrease
of organic loading (= decreased phosphorus loading) and
increase of inflows combine to decrease the oxygen depletion
the HVOD in Boulder Basin. Both these variables have an rate of Boulder Basin.
effect on HVOD (Fig. 11). HVOD is positively correlated
When the 11-year data set from 1995 through 2005 is consid-
with TP in Las Vegas Bay (Fig. 11, bottom: r2 = 0.36; p =
ered alone, the conclusion is that Boulder Basin was changing
0.014) and negatively correlated with Glen Canyon Outflow
to become anoxic in the lower portion of the hypolimnion.
(Fig. 11, top: r2 = 0.40; p = 0.08). These simple regressions
The analysis of the years to anoxia assumes that phospho-
are barely significant, because both variables are changing
rus loadings were increasing as total discharge of treated
simultaneously so that a multiple regression is more adequate.
79

LaBounty and Burns
wastewater effluent increased, and that average daily inflow explodes as it has in the Great Lakes, it could cause changes
from Glen Canyon is not >400 m3/day. Based on calculations in the HVOD regime of Boulder Basin. Nevertheless, if the
(Appendix A), the number of years from 2005 to when anoxic trend to decreasing HVOD rates shown by the Group 3 years
conditions might be expected to occur is predicted separately (2004-2006) continues, the progress shown by the Group 2
for the 2 different stratification scenarios for Type 1 and 2 years (1996-2004; Fig. 8) toward the onset of anoxic condi-
years. Using the most recently collected data from Lake Years tions could be averted.
2004-2006 (Fig. 7), calculations show that anoxia of Boulder
This study demonstrates that hypolimnetic oxygen deple-
Basin would be expected by day 265 within about 18 years
tion rates can provide a useful tool for assessing long-term
for years following complete destratification (Type 1 years)
changes in the metabolism of stratified lakes and reservoirs.
and within 16 years for Type 2 years.
However, observed depletion rates must be adjusted to re-
The large loading of organic material into Las Vegas Bay flect system-specific characteristics, such as hypolimnetic
prior to 1994 (1,300 kgP/day in 1978; Dr. Doug Drury, temperature, the downward flux of oxygen to the hypolim-
Clark County Water Reclamation District, pers. comm.) nion and the volume of inflows. Vertical mixing inputs of
explains the high HVOD rates in those early years. The oxygen were found to be important in this study, probably
>400 m3/day average flows from Glen Canyon Dam from accounting for 29-70% of the oxygen consumed annually in
1995 through 1999, along with the incorporation of tertiary the hypolimnion and increasing as the DO concentrations
treatment process in 1994, caused rapid decline of HVOD become lower. Failure to account for this source of oxygen
in 1995. The increasing rate of organic contribution to the can result in underestimation of oxygen depletion rates and
hypolimnion of Boulder Basin from 1994 to 2004 due to a inaccurate representation of long-term trends.
greatly expanding population resulted in increasing organic
and nutrient loading, which caused accelerated annual rate
Summary
of hypolimnetic oxygen depletion between 1996 and 2004.
From 1994 through 2005 TP increased in the hypolimnion The pattern of thermal stratification of Boulder Basin is
at a rate of 1.6 mg P/m3/yr, indicating a probable increase dependent on several climatic and hydrologic factors such
in organic matter in the hypolimnion. In this regard, Secchi as the annual flow volumes from the Glen Canyon Dam and
depth showed a decrease and total organic carbon showed the temperature of the inflow that varies from year to year.
an increase in Boulder Basin from 2000 to 2004 (LaBounty Temperatures of water entering the upper portion Lake
and Burns 2005). The years to anoxia (Table 3), calculated Mead from the Colorado River are normally 9-11°C dur-
from actual start-of-season DO values versus those calculated ing winter months (Holdren et al. 2006), while the surface
using average start-of-season DO concentrations (Appendix water of Boulder Basin remains ≥12°C. This temperature
A), more accurately display the trends during this time period. difference provides more than enough stability to resist
The data can be generally interpreted to mean that from 1995 vertical mixing. However, the timing and magnitude of the
to 2000, the time to anoxia was 20-27 years; but from 2000 to inflow and the average water temperature of the inflow from
2005, the time to anoxia diminished steadily, largely because the Colorado River determine whether Boulder Basin will
of decreasing start-of-season DO concentrations (Table 1). destratify completely.
The potential occurrence of a series of ≥10 years when in- Complete destratification most likely occurs when the aver-
flows are below normal and organic loading is not decreased age water temperature of the lower portion of the hypolim-
is always possible for Lake Mead based on hydrology and nion is >12.5°C and ambient conditions in January cool the
water use patterns in the Colorado River Basin. If drought epilimnion to an average of <12.5°C. Additionally, arrival
conditions were to persist along with unchecked organic of the cooler Colorado River underflow is delayed until at
loading, conditions could lead to the onset of years when least mid-February. This is usually a short period of time (<1
anoxia begins prior to destratification. Anoxia in a lake is month). Once the cooler Colorado River underflow arrives,
deleterious for the lake because it results in a large increase stratification instantly begins. New water from the Colo-
in the internal loading of phosphorus, ammonia, iron, and rado River forms a layer of colder water at the bottom and
manganese from the sediments to the overlying water (Burns becomes the deep hypolimnion for the next 2 years. Under
and Ross 1972, Nürnberg 2004). Usually the increased inter- normal or below average inflow from the Colorado River,
nal load of phosphorus enters the life cycle of the lake and stratification generally sets up for 2 years in Boulder Basin
can cause even larger releases of phosphorus in subsequent with a small degree of vertical mixing at the end of the first
years. This is a particularly undesirable situation for Boulder year. The onset of stratification is so strong that 2 years of
Basin because of the strong phosphorus limitation to algal warming the bottom waters are usually required to enable
growth in the Basin. This situation may be exacerbated by complete destratification.
the presence of quagga mussels (discovered throughout
Boulder Basin in early January 2007). If the population
80

Long-term Increases in Oxygen Depletion in the Bottom
Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA
Complete destratification does not occur in years when for this study: U.S. Bureau of Reclamation, Clark County
inflow volumes are greatly below normal, and/or water Water Reclamation District (CCWRD), and the City of Las
temperature of the Colorado River inflow during fall and Vegas Water Pollution Control Facility. We especially thank
winter are >12°C, and/or Colorado River interflow arrives Dr. Doug Drury, CCWRD, for retrieving some of the oldest
in Boulder Basin before the average water temperature of data and providing guidance on the water treatment history.
the epilimnion >12.8°C. We acknowledge the anonymous reviewers of this manu-
script. The authors greatly appreciate the editorial efforts
This leads to 2 types of stratification regimes that affect an- of Dr. Gertrud Nürnberg, Freshwater Research, Baysville,
nual HVOD rates; those following complete destratification Ontario. Dr. Nürnberg handled reviews and she provided
and those following partial destratification. The hypolimnetic her own professional advice. Her input was essential to this
DO concentrations in the second year of a pair of years are endeavor.
lower that those of the first year of a pair. This difference
enables vertical eddy diffusion to transport more oxygen
References
downward in the Type 2 years, leading to lower observed
HVOD rates in these years. When eddy diffusion is taken
Birge, E.A. and C. Juday. 1911. The inland lakes of Wisconsin: The
into consideration, both Type 1 and Type 2 years probably
dissolved gases and their biological significance. Bull. Wisc.
have similar HVOD rates. The downward diffusion of oxy- Geol. Nat Hist. Surv. Number 22. 259 p.
gen increases as the hypolimnetic oxygen concentrations Beutel, M.W. 2003. Hypolimnetic anoxia and sediment oxygen
decrease; however, this does not prevent the progress toward demand in California drinking water reservoirs. Lake Reserv.
anoxic conditions, unless the loading of organic carbon to Manage. 19:208-221.
the hypolimnion decreases. Burns, N.M. 1995. Using hypolimnetic dissolved oxygen depletion
rates for monitoring lakes. N.Z. J. Mar. Freshw. Res. 29:1-
The HVOD rates for the years following full destratification 11.
Burns, N.M. and C. Ross. 1972. Oxygen-Nutrient relationships
(Type 1 years) averaged 12.1 mg DO/m3/day from 1995 to
within the Central Basin of Lake Erie. In Project Hypo. Canada
2005 and increased annually at the rate of 0.91 mg DO/m3/day
Centre for Inland Waters, Paper No. 6; USEPA Technical
per year. The HVOD rates for these years following partial
Report, TS-05-71-208-24:85-117.
destratification (Type 2 years) averaged 10.1 mg DO/m3/day
Burns, N.M., D.C. Rockwell, P.E. Bertram, D.D. Dolan and J.J.
and increased at the rate of 0.59 mg DO m3/day per year. The Ciborowski. 2005. Trends in temperature, Secchi depth and
increase in the HVOD rates resulted from increased organic dissolved oxygen depletion rates in the Central Basin of Lake
and nutrient inputs. These results demonstrate quite clearly Erie, 1983-2002. J. Gt. Lakes Res. 31:35-49.
that higher TP concentrations in Las Vegas Bay lead to Charlton, M.N. 1980. Hypolimnion oxygen consumption in lakes:
higher HVOD rates. A positive relationship between HVOD Discussion of productivity and morphometry effects. Can. J.
Fish. Aquat. Sci. 37:1531-1539.
and the previous year’s phosphorus loading has also been
Cornett, R.J, and F.H. Rigler. 1979. Hypolimnion oxygen deficits:
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Appendix A
The “years to anoxia” can be estimated for each year by using the starting DO concentration for that year, its HVOD rate
and the appropriate increase in HVOD rate from Fig. 9.
1) Type 1 example (i.e., following full destratification); Lake Year 2004-2005;
a) Average start of summer concentration = 8.1 mg/L (see Table 1)
What rate is required for anoxia to set in after 265 days?
b)
c) Rate to cause anoxia = 8.1/265 = 0.031 mg/L/day = 31.0 mg/m3/day
d) 2004-2005 HVOD rate = 0.0147 mg/L/day = 14.7 mg/m3/day (see Fig 7: bottom panel)
e) HVOD rate increase to reach anoxic HVOD rate = 31.0 - 14.7 = 16.3 mg/m3/day
f) HVOD increase rate = 0.9062 mg/m3/day per year (see Fig. 8: top panel)
g) Number of years to anoxia = 16.3 mg/m3/day ÷ 0.9062 mg/m3/yr
= 18 years
h)
2) Type 2 example (i.e., following partial destratification); Lake Year 2005-2006.
a) Average start of summer concentration = 6.2 mg/L (see Table 1)
What rate is required for anoxia to set in after 265 days?
b)
c) Rate to cause anoxia = 6.2/265 = 0.0233 mg/L/day = 23.3 mg/m3/day
d) 2005-2006 HVOD rate = 0.0138 mg/L/day = 13.8 mg/m3/day (see Fig. 7: top panel)
e) HVOD rate increase to reach anoxia at day 265 = 23.3 - 13.8 = 9.5 mg/m3/day
f) HVOD increase rate = 0.5863 mg/m3/day per year (see Fig. 9: top panel)
g) Number of years to anoxia = 9.5 mg/m3/day ÷ 0.5863 mg/m3/yr
= 16.2 years
h)
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