Lake Powell Water Quality Update - March 11-16, 2004
Synopsis
The results of the December 2003 quarterly reservoir survey showed a well-mixed and oxygenated epilimnion, or surface layer, as a result of convective winter mixing, but the hypolimnion, or bottom layer of the reservoir showed a significant reduction in dissolved oxygen concentrations, about 30% of saturation. Since that time an underflow density current, a result of cold saline winter inflows into the reservoir, has displaced the hypoxic hypolimnion upwards in the water column and resulted in a freshening of this bottom layer, exhibited by increased dissolved oxygen concentrations, increased salinity, and decreased temperatures. Inflows to the reservoir have since warmed and are no longer underflowing along the bottom of the reservoir. The reservoir is at its lowest level since June 1970 during its filling period.
Introduction
A quarterly reservoir water quality survey of Lake Powell was conducted by the Grand Canyon Monitoring and Research Center (GCMRC) from March 11, 2004 to March 16, 2004. The survey crew consisted of Bill Vernieu and Nick Voichick (GCMRC), Robert Radtke (USBR), and Jesse Granet (NPS). A total of 15 stations on the main channel of the Colorado River, 7 stations on the San Juan River Arm, 4 stations of the Escalante River Arm, and 2 tailwater stations were visited (Figure 1). Samples for major ion and nutrient chemistry were collected at 15 of these stations.
Figure 1. Lake Powell Water Quality Monitoring Stations
Current Hydrology Conditions
Now in its fifth year of severe drought, the Upper Colorado River remains dry. Snowpack levels have decreased in the upper Colorado River Basin, and continue to do so with unseasonably warm weather throughout the region. Snowpack levels dropped from 96% of average on March 1, 2004 to 65% of average on April 5, 2004. The projected Apr-Jul unregulated inflow forecast to Lake Powell dropped from a value of 6.0 MAF (76% of average) on March 1, 2004 to 4.0 MAF (50 % of average) as of April 5, 2004. 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 (Table 1). 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 2, 2004 | |
|---|---|---|
| Snowpack levels | 96% | 65% |
| Apr-Jul unregulated inflow | 6.0 MAF (76%) | 4.0 MAF (50) |
| Reservoir Elevation | 3586.86 ft AMSL | 3582.75 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 2, 2004, Lake Powell's water surface elevation was 3582.75 feet, 117 feet from full pool (Figure 2). This is 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 5, 2004.
Figure 2. Glen Canyon Dam releases and Lake Powell surface elevation
Current Reservoir Water Quality Conditions
Convective winter mixing on Lake Powell has cooled the epilimnion of the reservoir forebay and mixed it to a depth of 35 meters corresponding to the depth of the powerplant penstocks. With the onset of warmer spring conditions the surface has warmed to 12 °C, with 8.3 °C seen in the deeper portion of the epilimnion nearer the penstocks. Within the last month, cold dense winter inflows have underflowed the entire reservoir and replaced the oxygen-depleted hypolimnion that was last observed on February 5, 2004 with cooler water of slightly less salinity and higher dissolved oxygen concentrations. The previous hypolimnetic water has been displaced upward. Water that was on the bottom of the reservoir at a depth of 125 m on February 5, 2004 now resides at a depth of approximately 65 m. The water previously seen between the convectively mixed epilimnion and the oxygen-depleted hypolimnion has been evacuated from the reservoir. The withdrawal zone of the penstocks now represents the boundary between these two previously separated layers (Table 2, Figure 3).
In March 2004, surface temperatures in Lake Powell ranged from 10.6°C to 14.2 °C. Inflow temperatures ranged from 12.3°C to 14.0°C and are now overflowing receiving strata in the reservoir after previously flowing along the bottom of the reservoir bed as a cold dense advective underflow current. This is the continuation of a pattern seen in the last 5 years where the winter inflows undercut the hypolimnion and extended to the forebay. Temperature in the hypolimnion of the reservoir ranged from a minimum of 5.0 °C at Lower Good Hope Bay to 7.6 °C in the forebay (Figure 5).
Oxygen concentrations in the forebay epilimnion ranged from 9.06 to 6.43 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 | 0-35m | 12.0-8.3°C | 836-861µS | 10.6-14.2°C |
| Penstock | 35m | 8.3°C | 865µS | 6.4 mg/L |
| Metalimnion | 35-85m | 8.3-8.0°C | 880-978µS | 6.0-1.9 mg/L |
| Hypolimnion | 85-123m | 8.0-7.6°C | 964-978µS | 5.0-6.0 mg/L |
Table 2. Glen Canyon Dam forebay water quality conditions, March 11, 2004
Glen Canyon Dam Releases
On March 11, 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 4). 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. Salinity levels, as reflected by specific conductance measurements, have remained fairly stable since late spring 2003 with values between 820 µS (533 mg/L TDS) and 870 µS (565 mg/L TDS) observed (Figure 4).
Figure 4. Water quality below Glen Canyon Dam
Figure 5. Water quality of Colorado River main channel
Figure 6. Water quality of San Juan River arm
Figure 7. Water quality of Escalante River arm
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