GCMRC - Products - Water Quality - Lake Powell Forebay Water Quality Update

Lake Powell Forebay Water Quality Update - January 8, 2004

Synopsis

Lake Powell surface elevation is currently at 3596 feet, corresponding to 50% of its total capacity. This is the lowest surface elevation since 1973. Reservoir drawdown and convective winter mixing of the epilimnion has resulted in the mixed surface layer extending below the penstock elevation, resulting in releases from the surface layer of Lake Powell. While the temperature profile through the water column is approaching isothermal conditions, the specific conductance profile indicates stratified conditions. Hypolimnetic dissolved oxygen conditions are reduced, following a downward trend for the last 6 months.

Introduction

A monthly forebay water quality survey of Lake Powell was conducted by GCMRC on January 8, 2004. The survey crew consisted of Bill Vernieu and Nick Voichick (GCMRC) and Jesse Granet (NPS). Measurements and samples were taken for the Wahweap forebay station LPCR0024, approximately 2.4 river channel kilometers upstream from Glen Canyon Dam. Samples for major ion and nutrient chemistry were collected at 3 depths at this station, in addition to 2 tailwater locations below the dam. Samples for phytoplankton and zooplankton were collected in the upper 40 m of the reservoir, as well as at the two tailwater locations.

Current Hydrology Conditions

Several years of prolonged drought have reduced water storage in Lake Powell. On January 8, 2004, Lake Powell had a surface elevation of 3596.04 ft, over 100 ft lower than its full pool elevation of 3700 ft. This corresponds to a storage of 13.0 MAF, less than 50% of the reservoir's total capacity of 26.2 MAF. The water surface elevation at Lake Powell will likely continue to decrease to a level of 3583 ft in April 2004, after which snowmelt entering the reservoir is projected to raise the surface elevation to 3609 in July 2004. Releases from Glen Canyon Dam changed on Jan 1, 2004 and fluctuate from a low of 5000 cfs to a peak of 20000 cfs. These daily high fluctuating releases are part of the Gen Canyon Dam experimental flows designed to benefit the endangered humpback and will last through March 2004.

Powerplant Releases and Lake Powell Surface Elevation

Current Reservoir Conditions

As of January 8, 2004, water quality monitoring in the forebay of Lake Powell showed conditions representative of mid-winter convective mixing and current reservoir drawdown. The epilimnion, or surface layer of the reservoir, was well-mixed to a depth of 43 m, approximately 5 m below the depth of the penstock withdrawal structures, resulting in releases from Glen Canyon Dam directly from the epilimnion, a short-lived condition usually seen during this time of year. Epilimnetic temperatures ranged from 9.6 to 9.8 ° C; specific conductance was 820 µS (530 mg/L TDS). Dissolved oxygen concentrations ranged from 7.8 mg/L to 8.1 mg/L, 77 to 81 % of saturation.

A transition zone, or metalimnion, existed in the water column from 43 to 52 m, separating the well-mixed epilimnion from deeper waters in the reservoir.

The bottom layer, or hypolimnion, exists from 52 m to the bottom depth of 126 m and is characterized by slightly cooler temperatures, higher specific conductance, and lower dissolved oxygen levels. Hypolimnetic temperatures ranged from 8.6 to 9.2 °C; specific conductance ranged from 950 to 1050 µS (620 to 680 mg/L TDS). Dissolved oxygen ranged from 3.5 to 1.3 mg/L, 34 to 12 % of saturation.

Glen Canyon Dam Releases

Releases from Glen Canyon Dam reflected the current epilimnetic withdrawal conditions, with a temperature of 9.5 °C, specific conductance of 830 µS (540 mg/L TDS), and dissolved oxygen levels of 7.8 mg/L (78% of saturation). Temperatures have cooled considerably from a high value of 13.0°C in mid-November, reflecting the cooling epilimnion. Associated with withdrawals from the well-mixed epilimnion that began in early December 2003, are higher dissolved oxygen values. These conditions are expected to last until mid-February, at which time dam releases will be influenced primarily by the hypolimnion. By this time, winter convection will have mixed the reservoir to a maximum depth and the transition zone will move upward through the water column as the epilimnion becomes depleted and the hypolimnion increases in volume from dense winter inflows.

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