GCMRC - Products - Water Quality - Lake Powell Water Quality Update December 3-7, 2003

Lake Powell Water Quality Update December 3-7, 2003

Synopsis

The surface layer of Lake Powell has begun its convective winter mixing process, in which the surface of the reservoir increases in density due to cooling and mixes with successively deeper portions of the reservoir. This has dissipated the low dissolved oxygen levels associated with the spring runoff that were observed in September 2003. The early winter inflows to the reservoir appear to be interflowing between the upper and lower layers of the reservoir. The bottom layer of the reservoir has experienced a significant reduction in dissolved oxygen levels. Severe hypolimnetic reductions in dissolved oxygen content were observed in the mid to upper Colorado River main channel and especially in the San Juan and Escalante tributary arms. Complete anoxia was observed in the Escalante and San Juan River arms, with reducing conditions and the presence of measurable hydrogen sulfide in some areas.

Introduction

A quarterly reservoir water quality survey of Lake Powell was conducted by GCMRC from December 3, 2003 to December 7, 2003. 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, 5 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. Although snowpack levels were average in November, soil moisture was very low. With the dry soil conditions, average snowpack this winter will result in runoff levels about 75 % of average. November inflow levels were 64% of average. Unregulated inflow to Lake Powell in water year 2003 was only 53% of average, the fourth consecutive year of below average inflow volumes.

These low inflows have reduced water storage in Lake Powell. Lake Powell reached a low water surface elevation this year of 3605 feet (95 feet from full pool) on May 1, 2003 . Lake Powell reached its peak elevation on June 23, 2003 at 3616.6 feet. The current elevation of Lake Powell on December 3, 2003 was 3,600.25 feet (100 feet from the full pool elevation of 3700 feet). Current storage is approximately 13.7 million acre-feet (52 percent of capacity) (Figure 2). The water surface elevation at Lake Powell will likely continue to decrease until late April, with a projected elevation of 3586.06 under the current inflow forecast.

Figure 2. Glen Canyon Dam releases and Lake Powell surface elevation

Current Reservoir Conditions

The onset on convective winter mixing on Lake Powell has cooled the surface of the reservoir and mixed it to a depth of 32 meters. This pattern has incorporated the spring runoff volume that existed about 14 meters below the surface throughout the reservoir in September 2003 into the mixed surface layer. This runoff volume exhibited very low dissolved oxygen concentrations in September 2003 from resuspension of deltaic sediments in the inflow area of the reservoir.

In December 2003, surface temperatures in Lake Powell were 13.7°C. The temperature of Glen Canyon Dam releases was 12.6°C, reflecting the highest release temperatures for this time of year since 1973, due to reservoir drawdown. Inflow temperatures were about 3°C and the cold dense advective inflow current is currently flowing along the bottom of the reservoir bed, with portions of it interflowing below a depth of 30 meters. This follows a pattern seen in the last 5 years where the winter inflows flowed beneath the hypolimnion and extended to the forebay, displacing water that was previously on the bottom of the reservoir upwards and allowing portions of it to be incorporated in Glen Canyon Dam releases. Temperature in the forebay hypolimnion was 8.7°C (Figure 3).

Due to the convective winter mixing of the epilimnion, the hypoxic inflow plume observed at about 18m in September 2003 has been incorporated into the well-mixed surface layer. In the hypolimnion, however, dissolved oxygen values have declined substantially, from values of 2.2 mg/l to 5.7 mg/l observed in September 2003 to 1.1 mg/l to 3.4 mg/l in the lower portions of the reservoir, with completely anoxic conditions seen in the mid to upper Colorado River channel and in the Escalante and San Juan River arms (Figure 5, Figure 6, Figure 7). Some of the areas of anoxia also exhibited chemically reducing conditions with the appearance of measurable amounts of hydrogen sulfide.

Glen Canyon Dam Releases

Temperature of Glen Canyon Dam releases reached a maximum value of 13.2 °C on November 14, 2003, the highest release temperatures observed from Glen Canyon Dam since August 1972, during Lake Powell's filling stage. Above average release temperatures have been observed since May 2003 due to continued drawdown of the reservoir and the placement of warmer surface water near the penstock withdrawal zone. Release temperatures have recently begun to decrease due to the influence of the cooling epilimnion. 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 3. Glen Canyon Dam forebay water quality profile

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

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