Lake Powell Water Quality Update - July 12, 2005
Bill Vernieu, Grand Canyon Monitoring and Research Center
August 19, 2005
Synopsis
Warm summer weather has warmed the surface of Lake Powell to around 26°C (79°F) and mixed the epilimnion, or surface layer of the reservoir, to a depth of about 5 m. Since the reservoir-wide survey of June 4-9, 2005, the forefront of the inflow plume from this season's snowmelt runoff has appeared in the forebay, at depths between 10 m and 32 m.
The penstocks are withdrawing water from the metalimnion of Lake Powell, a transition layer between warm surface water and cooler hypolimnetic water, at a depth of 42 m. A mean daily temperature of 13.6 °C (56.5 °F) was recorded belwo Glen Canyon Dam on July 11, 2005. Because of this metalimnetic withdrawal, this is the warmest release from Glen Canyon Dam since 1971, with minor exceptions during spillway releases in the 1980s. Temperatures above 16°C are now being recorded at the mouth of the Little Colorado River.
As the inflow plume continues to move through the reservoir, it is expected to increase in thickness and will influence Glen Canyon Dam releases to a greater degree. Release temperatures are expected to increase further through the summer, possibly reaching 17-18 °C by late September. Entrainment of this inflow plume in dam releases may also cause a reduction in dissolved oxygen concentrations in late summer 2005.
The effects of last winter's underflow density current ceased in March 2005, as inflow began flowing into higher layers of the reservoir. Since that time, conditions in the hypolimnion, or bottom layer of the reservoir, have remained stable, except for a reduction in dissolved oxygen concentration of approximately 1 mg/L since June 2005.
Unregulated snowmelt runoff through the months of April through July 2005 was 8.813 MAF (111% of average), as reported by the Colorado Basin River Forecast Center on August 1, 2005. This is the largest runoff volume entering the reservoir since 1997, before the onset of the recent drought. Reservoir surface elevation increased rapidly during the runoff period, rising 16 m (55 ft) from a low level of 1083.6 m (3555.1 ft) on April 8, 2005 to a peak of 1099.8 m (3608.4 ft) on July 13, 2005.
Introduction
A monthly forebay water quality survey of Lake Powell was conducted by the Grand Canyon Monitoring and Research Center (GCMRC) on July 12, 2005. The survey crew consisted of Bill Vernieu and Susan Hueftle (GCMRC), and Jesse Granet (NPS). A profile of water quality conditions and samples for major ion and nutrient chemistry, chlorophyll, phytoplankton and zooplankton were collected at the Wahweap forebay station, located at the mouth of Wahweap Bay, 2.4 miles upstream of Glen Canyon Dam.
Current Hydrology Conditions
After five years of sustained drought, the Upper Colorado River basin has experienced a relatively wet spring. Unregualted inflow to Lake Powell from snowmelt runoff through the months of April through July 2005 was 8.813 MAF (111% of average), as reported by the Colorado Basin River Forecast Center on August 1, 2005 (Table 1). This is the largest runoff volume entering the reservoir since 1997, before the onset of the recent drought. For comparison, the unregulated Apr-Jul inflow to Lake Powell in water year 2004 was 3.6 MAF, the fifth consecutive year of below average inflow volumes. Inflows during water years 2000, 2001, 2002, 2003, and 2004 were 62, 59, 25, 51, and 46 percent of average, respectively. Reservoir surface elevation increased rapidly during the runoff period, rising 16 m (55 ft) from a low level of 1083.6 m (3555.1 ft) on April 8, 2005 to peak at 1099.8 m (3608.4 ft) on July 13, 2005 (Figure 1).
Current total storage on July 13, 2005 was approximately 14.5 MAF (55% of total capacity). Live storage was 12.6 MAF (52% of live capacity). The water surface elevation at Lake Powell will likely decrease slowly through the year, but is projected to remain above 1097 m (3600 ft) through the end of February 2006 under the current inflow forecast. Current status of hydrologic conditions at Lake Powell may be found at http://www.usbr.gov/uc/water/crsp/studies/crsp_gc.txt.
| Apr-Jul 2005 unregulated inflow | 8.813 MAF (111% of average) |
| Projected Aug 2005 unregulated inflow | 0.625 MAF (91% of average) |
| Peak Reservoir Elevation (July 13, 2005) | 3608.41 ft |
| Total storage (% capacity) | 14.6 MAF (55%) |
| Live storage (% capacity) | 12.6 MAF (52%) |
Table 1. Recent Lake Powell hydrologic conditions as of August 1, 2005
Figure 1. Glen Canyon Dam releases and Lake Powell surface elevation
Current Forebay Water Quality Conditions
Warm spring conditions, as expected, have warmed the upper 5 m of the surface of the reservoir in the forebay to 26°C, an increase of 7°C since June 4, 2005. Temperature in the deepest portions of the reservoir was 7.1°C. The downstream movement of the cold dense underflows that moved through the reservoir last winter ceased in March as advective inflow currents began flowing through the reservoir near the surface.
Oxygen concentrations in the forebay epilimnion ranged from 7.7 to 10.0 mg/L (109% to 131% of saturation), reflecting exposure of this well-mixed stratum to ambient conditions and an increase in subsurface productivity. The forefront of the inflow plume was located at depths between 10 m to 32 m in the forebay and showed a decrease in dissolved oxygen concentrations, with a minimum value of 4.7 (56% of saturation) because of oxygen demand from suspended organic material in the inflow plume. The low-oxygen hypolimnetic water of early winter that was displaced upward by the underflow current was located approximately 20 m below the penstock withdrawal elevation and had a minimum dissolved oxygen concentration of 3.2 mg/L (35% of saturation). Hypolimnetic oxygen concentrations were ranged between 3.2 and 4.7 mg/L (35-44% of saturation) and have stabilized with the cessation of the winter underflow density current.
Water is entrained into Glen Canyon Dam powerplant releases from slightly higher in the reservoir than the actual penstock elevation. Therefore, release water quality reflects conditions observed at shallower depths in the reservoir than those measured at the penstock (Table 2, Figure 2).
| Depth m | Temperature °C | Specific Conductance µS | Dissolved Oxygen mg/1 (%saturation) | |
|---|---|---|---|---|
| Epilimnion | 0-10m | 26.2-21.6°C | 867-815µS | 7.7-10.0 mg/L (109%-131%) |
| Inflow Plume | 10-32m | 20.6-16.2°C | 659-730µS | 4.7-5.8 mg/L (56%-71%) |
| Metalimnion | 32-52m | 15.8-8.7°C | 795-909µS | 4.5-6.5 mg/L (44%-71%) |
| Hypolimnion | 52-129m | 8.5-7.1°C | 919-982µS | 3.2-4.7 mg/L (35%-44%) |
| Penstock | 42m | 12.9°C | 865µS | 6.3 mg/L (68%) |
| GCD Release | 13.5°C | 830µS | 6.5 mg/L (72%) |
Table 2. Glen Canyon Dam forebay water quality conditions, July 12, 2005
Figure 2. Glen Canyon Dam forebay water quality profile
Glen Canyon Dam Releases
The low reservoir levels observed in April 2005 combined with a substantially higher snowmelt inflow volume has resulted in an increased influence of reservoir inflow on Glen Canyon Dam releases.
On July 13, 2005, the mean daily water temperature of Glen Canyon Dam releases was 13.5°C (56°F), up substantially from a mean value for the same date (1989-2002) of approximately 9.0°C (48°F). Colorado River mean daily water temperature was 15.8°C (60°F) at the mouth of the Little Colorado River and 20.0°C (68°F) at the mouth of Diamond Creek on the same date . This is a continuation of the pattern that has been observed since 2003, when reservoir elevations dropped below 1097 m (3600 ft). A 2003 maximum mean daily temperature of 12.9°C (55°F) was observed on November 14, 2003. The maximum annual mean daily temperature for 2004 reached 15.0°C (59°F) on November 6, 2004, the highest release temperatures observed from Glen Canyon Dam since August 1972, during Lake Powell's filling period. The above-average release temperatures that were observed in 2003, 2004, and 2005 were because of continued drawdown of the reservoir and the placement of warmer surface water near the penstock withdrawal zone. As this year's inflow plume moves through the reservoir, temperatures are expected to increase beyond last year's maximum to levels of 17-18°C, by early autumn.
Salinity levels, as reflected by specific conductance measurements, increase during the spring and decrease in the summer and have generally increased during the recent drought. Recent values are dropping, reflecting the influence of the inflow plume on dam releases and are currently 865 µS (562 mg/L TDS) (Table 2, Figure 3). These values are expected to decrease over the next several months.
Figure 3. Water quality below Glen Canyon Dam, January 2002 to July 2005
For more information, contact:
William Vernieu
Grand Canyon Monitoring & Research Center
2255 N. Gemini Dr.
Flagstaff, AZ 86001-1600
Phone: 928.556.7051
Fax: 928.556.7368
bvernieu@usgs.gov