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Lake Powell Water Quality Update - May 3, 2004

Synopsis

Warm spring weather continues to warm the surface of Lake Powell to around 16°C. The underflow density current, a result of cold saline winter inflows into the reservoir during the past winter, has essentially stopped is 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 a small peak from early snowmelt runoff, the reservoir is at an elevation of 3853 ft, slightly above its minimum level of April 2, 2004, the 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 May 3, 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 May 5, 2004 the projected Apr-Jul unregulated inflow forecast to Lake Powell dropped down to 3.8 MAF (48% 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.

Table 1. Recent Lake Powell hydrologic conditions

Five years of below average inflows have reduced water storage in Lake Powell. As of May 3, 2004, Lake Powell's water surface elevation was 3582.93 feet, 117 feet from full pool (Figure 1). This is up slightly, following a small peak from early snowmelt, 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 3587.94 ft at the end of June 2004 and drop to an elevation of 3559.55 at the end of March 2005 under the current inflow forecast as of May 5, 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 16°C, up from 14.7°C on April 7, 2004. Temperature in the deepest portions of the reservoir were 6.5°C, down from 6.9°C on April 7, 2004. 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). 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 9.4 to 7.8 mg/L (103% to 78% of saturation), reflecting exposure of this well-mixed stratum to ambient conditions. Minimum dissolved oxygen concentration in the metalimnion below the penstock elevation was 2.5 (24% of saturation). Hypolimnetic oxygen concentrations were approximately 6.4 mg/L (59% of saturation) and have stabilized with the cessation of the winter underflow density current.

Table 2. Glen Canyon Dam forebay water quality conditions, May 3, 2004

Figure 2. Glen Canyon Dam forebay water quality profile

Glen Canyon Dam Releases

On May 3, 2004, the temperature of Glen Canyon Dam releases was 8.8°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 between 900 µS (585 mg/L TDS) observed (Table 2, Figure 3).

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

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