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

Lake Powell Forebay Water Quality Update - February 5, 2004

Synopsis

Lake Powell surface elevation is currently at 3591 feet, corresponding to 49% of its total capacity. This is the lowest surface elevation since 1973. Depletion of the mixed epilimnion and the presence of an advective winter interflow has resulted in penstock releases drawing from this interflow. The temperature profile through the water column is approximately isothermal, with a slight amount of warming at depth due to higher specific conductance values in the hypolimnion. 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 February 5, 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 February 5, 2004, Lake Powell had a surface elevation of 3591.03 ft, almost 110 ft lower than its full pool elevation of 3700 ft and five feet lower than one month ago. This corresponds to a storage of 12.8 MAF, 49% 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 3603 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 February 5, 2004, water quality monitoring in the forebay of Lake Powell showed conditions representative of mid-winter convective mixing and current reservoir drawdown with the appearance of an interflow at penstock level from winter inflows. The epilimnion, or surface layer of the reservoir, was well-mixed to a depth of 35 m, approximately 2 m above the depth of the penstock withdrawal structures. Because of partial depletion of the epilimnion from the previous month's epilimnetic withdrawal and the addition of winter inflows below this point, releases are no longer from the epilimnion. Epilimnetic temperatures ranged from 8.5 to 8.7 ° C; specific conductance was 820 µS (530 mg/L TDS). Dissolved oxygen concentrations ranged from 8.2 mg/L to 8.6 mg/L, 80 to 84 % of saturation.

A transition zone, or metalimnion, existed in the water column from 35 to 56 m, separating the well-mixed epilimnion from deeper waters in the reservoir. This layer exhibited characteristics of a winter interflow, with slightly cooler temperatures, higher specific conductance, and lower dissolved oxygen than the overlying epilimnion. During the past several years, winter inflows have flowed along the bottom of the reservoir, underlying even the bottom-most layers of the water column. Because salinity levels in the hypolimnion have continued to increase as a result a the prolonged drought, the density of winter inflows is no longer sufficient to plunge below the hypolimnion, causing them to interflow through the transition zone between the hypolimnion and epilimnion.

The bottom layer, or hypolimnion, exists from 56 m to the bottom depth of 120 m and is characterized by slightly warmer temperatures, higher specific conductance, and lower dissolved oxygen levels than overlying layers. Hypolimnetic temperatures ranged from 8.6 to 8.8 °C; specific conductance ranged from 950 to 1050 µS (620 to 680 mg/L TDS). Dissolved oxygen ranged from 4.2 to 1.6 mg/L, 41 to 14 % of saturation19

Glen Canyon Dam Releases

Releases from Glen Canyon Dam reflected the influence of the advective interflow in relation to bounding strata. Temperature was 8.4 °C, about 1 degree C cooler than the previous month, specific conductance was 830 µS (540 mg/L TDS), and dissolved oxygen levels were 8.4 mg/L (85 % of saturation). Temperatures have cooled considerably from a high value of 13.0°C in mid-November, reflecting the continued cooling epilimnion and the influence of cooler winter inflows. 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 early March, at which time dam releases will be characterized primarily by hypolimnetic conditions. 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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