Lake Powell Water Quality Update - July 1, 2004
Bill Vernieu, Grand Canyon Monitoring and Research Center
Synopsis
Warm summer weather has warmed the surface of Lake Powell to around 24°C. The underflow density current, a result of cold saline winter inflows into the reservoir during the past winter, has essentially stopped its downstream progress and advective inflows are now flowing through the reservoir near the surface. The hypoxic hypolimnion of early winter now resides just below the penstock elevation, with portions of it becoming entrained in Glen Canyon Dam releases. After losing a substantial amount of snowpack from unseasonably warm weather in March the combined inflow to Lake Powell peaked at 21,400 cfs on May 14, 2004. The reservoir rose to a maximum elevation of 3587.35 on June 14, 2004, less than 5 feet above its minimum elevation of 3582.69 ft on April 2, 2004, which was the lowest level since June 1970 during Lake Powell's filling period. Its elevation is beginning to decline with the cessation of spring runoff.
Introduction
A monthly forebay water quality survey of Lake Powell was conducted by the Grand Canyon Monitoring and Research Center (GCMRC) on July 1, 2004. The survey crew consisted of Bill Vernieu and Nick Voichick (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
Now in its fifth year of severe drought, the Upper Colorado River remains dry. Snowpack levels in the upper Colorado River Basin decreased substantially during March and April 2004 with unseasonably warm weather. As of July 6, 2004 the projected Apr-Jul unregulated inflow forecast to Lake Powell dropped to 3.5 MAF (44% of average), down from a previous forecast on March 3, 2004 of 6.5 MAF (82 % of average) (Table 1). This value was slightly higher than the June 3, 2004 forecast of 3.4 MAF (43% of average) For comparison, the unregulated Apr-Jul inflow to Lake Powell in water year 2003 was 3.9 MAF, the fourth consecutive year of below average inflow volumes. Inflows during water years 2000, 2001, 2002, and 2003 were 62, 59, 25, and 51 percent of average, respectively. Current status of hydrologic conditions at Lake Powell may be found at http://www.usbr.gov/uc/water/crsp/crsp_cs_gcd.html.
| March 3, 2004 | July 6, 2004 | |
|---|---|---|
| Apr-Jul unregulated inflow forecast | 6.5 MAF (82%) | 3.5 MAF (44%) |
| Reservoir Elevation | 3586.86 ft AMSL | 3585.24 ft AMSL |
| Total storage (% capacity) | 12.4 MAF (48%) | 12.3 MAF (47%) |
| Active storage (% capacity) | 10.6 MAF (43%) | 10.4 MAF (43%) |
Table 1. Recent Lake Powell hydrologic conditions
Five years of below average inflows have reduced water storage in Lake Powell. As of July 6, 2004, Lake Powell's water surface elevation was 3585.24 feet, 115 feet from full pool (Figure 1). A small runoff peak of 21,400 cfs that occurred on May 14, 2004 caused a small increase in Lake Powell's surface elevation. Lake Powell reached a maximum elevation of 3587.35 on June 14, 2004, less than 5 feet above its minimum elevation of 3582.69 ft on April 2, 2004. Current total storage is approximately 12.3 MAF (47% of total capacity). Live storage is 10.4 MAF (43% of live capacity). The water surface elevation at Lake Powell will likely decrease through the year and is currently projected to drop to an elevation of 3556.85 by the end of March 2005 under the current inflow forecast as of July 6, 2004.
Figure 1. Glen Canyon Dam releases and Lake Powell surface elevation
Current Forebay Water Quality Conditions
Warm spring conditions have continued to warm the surface of the reservoir to 24°C. This is the maximum value observed on June 4, 2004, but the depth of warming now extends 10 m into the water column. Temperature in the deepest portions of the reservoir were 6.5°C. The downstream movement of the cold dense winter underflows that moved through the reservoir during the past several months has stopped and advective inflow currents are now flowing near the surface of the reservoir. The low-oxygen hypolimnetic water of early winter that was displaced upward by the underflow current is now residing just below the penstock withdrawal elevation, with portions of it entrained in dam releases (Table 2, Figure 2). Water is entrained into Glen Canyon Dam powerplant releases from slightly higher in the reservoir than the actual penstock elevation, which is why release water quality shows higher values that that measured at the penstock.
Oxygen concentrations in the forebay epilimnion ranged from 9.4 to 6.0 mg/L (103% to 60% of saturation), reflecting exposure of this well-mixed stratum to ambient conditions and an increase in subsurface productivity. Minimum dissolved oxygen concentration in the metalimnion below the penstock elevation was 3.5 (34% of saturation). Hypolimnetic oxygen concentrations were approximately 6.0 mg/L (61% of saturation) and have stabilized with the cessation of the winter underflow density current.
| Depth | Temperature | Specific Conductance | Dissolved Oxygen | |
|---|---|---|---|---|
| Epilimnion | 0-30m | 24.2-10.0°C | 837-863µS | 9.4-6.0 mg/L (103%-60%) |
| Metalimnion | 30-74m | 9.9-7.1°C | 852-982µS | 3.5-6.0 mg/L (34%-60%) |
| Hypolimnion | 74-122m | 7.0-6.5°C | 957-944µS | 5.4-5.6 mg/L (50%-52%) |
| Penstock | 36m | 8.7°C | 864µS | 5.6 mg/L (54%) |
| GCD Release | 10.0°C | 875µS | 6.0 mg/L (60%) |
Table 2. Glen Canyon Dam forebay water quality conditions, July 1, 2004
Figure 2. Glen Canyon Dam forebay water quality profile
Glen Canyon Dam Releases
On July 1, 2004, the temperature of Glen Canyon Dam releases was 10.0°C, up slightly from a 9.0°C observed on June 4, 2004. A maximum temperature of 13.2°C occurred on November 14, 2003, the highest release temperature observed from Glen Canyon Dam since August 1972, during Lake Powell's filling period (Figure 3). Above-average release temperatures were observed in 2003 due to continued drawdown of the reservoir and the placement of warmer surface water near the penstock withdrawal zone. With continued drawdown of the reservoir, temperatures are expected to increase beyond last year's maximum to levels of 15-16°C, by early autumn. Salinity levels, as reflected by specific conductance measurements, increased during the spring and have since decreased to 875 µS (569 mg/L TDS) (Table 2, Figure 3). These values are expected stabilize over the next several months.
Figure 3. Water quality below Glen Canyon Dam
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