Lake Powell Water Quality Update - April 7, 2004
Synopsis
Warm spring weather has warmed the surface of Lake Powell to around 15°C. The underflow density current, a result of cold saline winter inflows into the reservoir during the past winter, has continued moving through the reservoir, further displacing the previous hypoxic hypolimnion upwards in the water column and resulting in a continued freshening of this bottom layer, exhibited by increased dissolved oxygen concentrations, increased salinity, and decreased temperatures. The reservoir is at its lowest level since June 1970 during its filling period.
Introduction
A monthly forebay water quality survey of Lake Powell was conducted by the Grand Canyon Monitoring and Research Center (GCMRC) on April 7, 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 2004 with unseasonably warm weather. As of April 15, 2004 the projected Apr-Jul unregulated inflow forecast to Lake Powell dropped to 4.0 MAF (50% of average), down from a previous forecast on March 1, 2004 of 6.0 MAF (76 % of average) (Table 1). For comparison, the unregulated Apr-Jul inflow to Lake Powell in 2003 was 3.9 MAF, the fourth consecutive year of below average inflow volumes. Current status of hydrologic conditions at Lake Powell may be found at http://www.usbr.gov/uc/water/crsp/crsp_cs_gcd.html.
| March 1, 2004 | April 15, 2004 | |
|---|---|---|
| Snowpack levels | 96% | 65% |
| Apr-Jul unregulated inflow forecast | 6.0 MAF (76%) | 4.0 MAF (50%) |
| Reservoir Elevation | 3586.86 ft AMSL | 3583.19 ft AMSL |
| Total storage (% capacity) | 12.4 MAF (48%) | 12.1 MAF (46%) |
| Active storage (% capacity) | 10.6 MAF (43%) | 10.2 MAF (42%) |
Table 1. Recent Lake Powell hydrologic conditions
Five years of below average inflows have reduced water storage in Lake Powell. As of April 15, 2004, Lake Powell's water surface elevation was 3583.19 feet, 117 feet from full pool (Figure 1). This is up slightly from a minimum level of 3582.69 on April 2, 2004, the lowest reservoir level seen since June 2, 1970, during Lake Powell's filling period. Current total storage is approximately 12.1 MAF (46% of total capacity). Live storage is 10.2 MAF (42% of live capacity). The water surface elevation at Lake Powell is currently projected to peak at an elevation of 3589.03 ft at the end of June 2004 and drop to an elevation of 3563.52 at the end of March 2005 under the current inflow forecast as of April 15, 2004.
Figure 1. Glen Canyon Dam releases and Lake Powell surface elevation
Current Forebay Water Quality Conditions
Warm spring conditions have warmed the surface of the reservoir to 14.7°C, up from 12.0°C on March 11, 2004. The deepest portions of the reservoir were 6.9°C, down from 7.6°C on March 11, 2004. This cooling of the hypolimnion is the result of continued movement of the cold dense winter underflows that moved through the reservoir during the past several months and brought more oxygenated water to this portion of the reservoir. The hypolimnetic water of early winter has continued to be displaced upward, with portions of it entrained in dam releases (Table 2, Figure 2). Glen Canyon Dam powerplant operations tend to withdraw water 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 9.1 to 6.4 mg/L (96% to 62% of saturation), reflecting exposure of this well-mixed stratum to ambient conditions. Hypolimnetic oxygen concentrations were approximately 6.0 mg/L (57% of saturation) and have increased substantially due to the winter underflow density current.
| Depth | Temperature | Specific Conductance | Dissolved Oxygen | |
|---|---|---|---|---|
| Epilimnion | 0-12m | 14.7-11.0°C | 844-845µS | 9.0-8.8 mg/L (101%-91%) |
| Metalimnion | 35-61m | 10.0-7.7°C | 844-1020µS | 2.1-8.6 mg/L (20%-87%) |
| Hypolimnion | 61-122m | 7.5-6.9°C | 981-960µS | 5.4-6.4 mg/L (51%-59%) |
| Penstock | 35m | 8.7°C | 956µS | 3.0 mg/L (29%) |
| GCD Release | 9.0°C | 928µS | 5.5 mg/L (52%) |
Table 2. Glen Canyon Dam forebay water quality conditions, April 7, 2004
Figure 2. Glen Canyon Dam forebay water quality profile
Glen Canyon Dam Releases
On April 7, 2004, the temperature of Glen Canyon Dam releases was 9.0°C, up slightly from a minimum 8.2°C on February 23, 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 16-18°C, by early autumn. Salinity levels, as reflected by specific conductance measurements, have increased slightly during the past 2 months with values around 928 µS (603 mg/L TDS) observed (Table 2, Figure 3).
Figure 3. Water quality below Glen Canyon Dam
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