Lake Powell Water Quality Update June 4-8, 2004
Bill Vernieu, Grand Canyon Monitoring and Research Center
Synopsis
The results of the March 2004 quarterly reservoir survey showed the effects of the winter underflow density current, which flowed along the bottom on the reservoir and displaced old hyplimnetic water upwards in the water column, resulting in a freshening of the hypolimnion. Inflows to the reservoir have since warmed and are now overflowing denser strata in the reservoir. The reservoir reached a minimum elevation of 3582.69 feet on April 2, 2004. Snowmelt runoff raised the reservoir's surface to a maximum of 3587.35 on June 14, 2004. Supersaturated oxygen values from algal productivity were observed below the water surface at the Wahweap forebay station and at Willow Creek on the Escalante River arm. Complete anoxia was observed in the hyplolimnion of the Escalante River arm, with no evidence of hydrogen sulfide or reducing conditions. A metalimnetic oxygen deficit, associated with reservoir inflows and resuspended deltaic sediment, is beginning to form and will continue to develop during the coming months in the main channel of the reservoir. This year's lower reservoir inflows may reduce the amount of resuspension and may limit the extreme metalimnetic anoxia that was seen in September 2003.
Introduction
A quarterly reservoir water quality survey of Lake Powell was conducted by the Grand Canyon Monitoring and Research Center (GCMRC) from June 4, 2004 to June 8, 2004. The survey crew consisted of Bill Vernieu and Nick Voichick (GCMRC), Robert Radtke (USBR), Jesse Granet (NPS) and Kate Behn (volunteer). A total of 13 stations on the main channel of the Colorado River, 6 stations on the San Juan River Arm, 5 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 in the upper Colorado River Basin decreased drastically in March, 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. As of June 03, 2004, the projected Apr-Jul unregulated inflow forecast to Lake Powell dropped to 3.4 MAF (43% of average), down from a value of 6.5 MAF (82% of average) on March 3, 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 | June 14, 2004 | |
|---|---|---|
| Snowpack Levels | 96% | 65% |
| Apr-Jul unregulated inflow forecast | 6.0 MAF (76%) | 3.4 MAF (43%) |
| Reservoir Elevation | 3586.86 ft AMSL | 3587.35 ft AMSL |
| Total storage (% capacity) | 12.4 MAF (48%) | 12.5 MAF (48%) |
| Active storage (% capacity) | 10.6 MAF (43%) | 10.6 MAF (44%) |
Table 1. Recent Lake Powell hydrologic conditions
Five years of below average inflows have reduced water storage in Lake Powell. Lake Powell reached a seasonal minimum elevation of 3582.69 feet on April 2, 2004, 117 feet from full pool (Figure 2). This is the lowest reservoir level seen since June 2, 1970, during Lake Powell's filling period. This year's meager spring runoff increased the surface elevation of Lake Powell to 3587.35 feet on June 14, 2004, the maximum value for this year. Current total storage is approximately 12.5 MAF (48% of total capacity). Live storage is 10.6 MAF (44% of live capacity). Under the current inflow forecast, the water surface elevation at Lake Powell is projected to drop to an elevation of 3560.00 at the end of March 2005.
Figure 2. Glen Canyon Dam releases and Lake Powell surface elevation
Current Reservoir Water Quality Conditions
Warm spring conditions have warmed the surface of Lake Powell in the forebay to 24 °C, with 8.5 °C seen in the deeper portion of the reservoir near the penstocks. A dissolved oxygen maximum was observed at a depth of 15m and is associated with high algal productivity at this depth The oxygen-depleted hypolimnion that was last observed in the forebay on February 5, 2004 was replaced by a cold, dense underflow current associated with winter inflows. Its upward movement ceased once inflows began to warm and started moving through the reservoir near the surface as an overflow density current. This previous hypolimnetic water has been displaced upward and now occupies the water column just below the penstock withdrawal zone. The deepest portions of the reservoir are now occupied by the previous season's winter underflow current, which is characterized by its lower temperatures and higer dissolved oxygen concentrations (Table 2, Figure 3).
| Depth | Temperature | Specific Conductance | Dissolved Oxygen | |
|---|---|---|---|---|
| Epilimnion | 0-24m | 24.3-9.8°C | 834-863µS | 7.4-12.1 mg/L |
| Penstock | 36m | 8.5°C | 873µS | 6.2 mg/L |
| Hypolimnion | 85-124m | 6.8-6.4°C | 957-951µS | 6.2-6.4 mg/L |
| GCD Release | 9.0°C | 885µS | 6.1 mg/L |
Table 2. Glen Canyon Dam forebay water quality conditions, July 1, 2004
Figure 3. Glen Canyon Dam forebay water quality profile
Spring inflows to Lake Powell are now overflowing receiving strata in the reservoir and extend downstream to Escalante Canyon, 117 river channel kilometers upstream of Glen Canyon Dam.
In June 2004, surface temperatures in Lake Powell ranged from 21.9°C to 28.5°C, with warmer water observed in tributary inflow areas. Inflow temperatures ranged from 24.6°C to 29.8°C. Temperature in the hypolimnion of the reservoir ranged from a minimum of 6.3 °C in the forebay to 8.4 at Lower Good Hope Bay, 209 river channel kilometers upstream from Glen Canyon Dam (RKM 209).
Specific conductance values, a measure of salinity, ranged from a maximum of 990µS in the forebay metalimnion to minimums of 568µS at Scorup Canyon on the Colorado River arm, (RKM 226) and 364µS at Alcove Canyon on the San Juan River arm, 53 kilometers upstream from the San Juan Confluence. Inflow conductances were 640µS for the Colorado River, 450µS for the San Juan River, and 545µS for the Escalante River.
Dissolved oxygen values varied widely throughout the reservoir. A maximum value of 12.1mg/L (136% of saturation) was observed at 14m in the forebay and 11.1mg/L (140% of saturation) was observed at 6m at Willow Creek, 20 kilometers upstream of the confluence on the Escalante River arm. These high values represent increased algal productivity slightly below the surface in the forebay and on the Escalante River arm. A minimum dissolved oxygen value of 3.4mg/L was observed at Hite Basin, (RKM 239), associated with increased oxygen demand in the reservoir inflows from resuspended deltaic sediments. As the inflow plume moves downstream through the reservoir in upcoming months, the extent and degree of this hypoxia will increase. However, lower reservoir inflows in 2004 may result in less sediment resuspension and may limit similar conditions observed in September 2003, where complete anoxia was observed in the inflow plume at several locations. Hypolimnetic dissolved oxygen concentrations in the main Colorado River channel ranged from 6.0mg/L to 6.6 mg/L (58% to 61% of saturation) from the forebay upstream to Iceberg Canyon (RKM 140). Upstream of Iceberg Canyon to the Colorado River inflow, dissolved oxygen concentrations near the bottom of the reservoir continued to decline to a minimum value of 3.2mg/L at Scorup Canyon (RKM 226), due to settling of suspended sediments and their associated oxygen demand. Complete anoxia was observed in the hypolimnion below a depth of 30m at Davis Gulch, RKM 12 on the Escalante River arm, and extended upstream to just below Willow Creek (Escalante RKM 20). Oxidation-reduction potential measurements were positive throughout this area, indicating that, even though devoid of oxygen, these waters still exhibited oxidizing conditions, not conductive to the formation of hydrogen sulfide. However, during the coming months, this hypolimnetic anoxia is expected to further develop and reducing conditions, which indicate the formation of hydrogen sulfide, can be expected to form by the next scheduled reservoir water quality survey in September 2004 (Figure 4, Figure 5, Figure 6).
Figure 4. Water quality of Colorado River main channel
Figure 5. Water quality of San Juan River arm
Figure 6. Water quality of Escalante River arm
Glen Canyon Dam Releases
On June 4, 2004, the temperature of Glen Canyon Dam releases was 9.0°C, up slightly from a minimum 8.2 °C on February 23, 2004. 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. 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 7). Salinity levels, as reflected by specific conductance measurements, have increased during the last two months from a minimum of 815 µS on January 11, 2004 to a maximum value of 942 µS (612 mg/L TDS) on April 22, 2004, due to influence of the upwelling winter hypolimnion on the penstock withdrawal zone. A current value of 885 µS (575 mg/L TDS) was observed on June 4, 2004. (Table 2,Figure 7).
With continued drawdown of the reservoir due to below-normal inflows, Lake Powell is expected to be 10m lower than last year, bringing warm surface waters closer to the penstock withdrawal zone. Release temperatures are expected to increase until November 2004, possibly reaching levels of 15°C to 16°C.
Figure 7. 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