Lake Powell Water Quality Update June 18-23, 2003
Synopsis
Downstream areas of the reservoir, from the Glen Canyon Dam forebay to Padre Bay, are experiencing fairly high algal productivity as evidenced by high subsurface dissolved oxygen concentrations and calcium carbonate precipitation, or whiting
. Farther upstream the eplimnion is progressively dominated by the advective inflow from this season's spring runoff. With the current low reservoir levels, the inflow has resuspended a large amount of deltaic sediment. Associated with the advective inflow is a progressively marked reduction in subsurface dissolved oxygen content.
Hypolimnetic reductions in dissolved oxygen content were seen throughout the reservoir, especially in the San Juan and Escalante tributary arms. On the Escalante Arm, complete anoxia, was observed from Clear Creek upstream to Willow Canyon, with reducing conditions and the first documented presence of hydrogen sulfide in the hypolimnion at Davis Gulch.
Introduction
A quarterly reservoir water quality survey of Lake Powell was conducted by GCMRC from June 18, 2003 to June 23, 2003. The survey crew consisted of Bill Vernieu and Nick Voichick (GCMRC), Robert Radtke (USBR), Nikki Trular and Jesse Granet (NPS), with assistance by Mark Anderson (NPS). A total of 15 stations on the main channel of the Colorado River, 5 stations on the San Juan River Arm, and 5 stations of the Escalante River Arm were visited. Samples for major ion and nutrient chemistry were collected at 13 of these stations, in addition to 2 tailwater locations below the dam.
Current Hydrology Conditions
Unseasonably warm weather resulted in substantial runoff, peaking somewhat early this year on June 5, 2003 at 53,000 cfs, the highest runoff since 1999. However, snow in the basin was about gone by mid-June, and river flows dropped off rapidly. Inflow volumes are expected to be substantially below average this year with water year 2003 being the fourth consecutive year with below average inflow to Lake Powell . The July final inflow forecast issued by the National Weather Service is calling for 4.2 million acre-feet of unregulated runoff into Lake Powell during the 2003 April through July runoff period. This equates to 53 percent of average. Total unregulated inflow for water year 2003 will likely be about 56 percent of average. Total unregulated inflow into Lake Powell in water years 2000 and 2001 was 62 and 59 percent of average, respectively, and only 25 percent of average in 2002, the lowest ever observed since the completion of Glen Canyon Dam in 1963.
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 is 3,616.3 feet (83.7 feet from full pool). Current storage is approximately 13.4 million acre-feet (55 percent of capacity). The water surface elevation at Lake Powell will likely continue to decrease for the remainder of the year. Under the current inflow forecast, Lake Powell will likely be near elevation 3606 feet on January 1, 2004.
Current Reservoir Conditions
The warm spring and early summer have warmed surface temperatures in Lake Powell during June 2003, ranging from 21°C to 25°C. Inflow temperatures were about 21°C and the temperature of Glen Canyon Dam releases was 9.5°C. This warming has resulted in a well-mixed surface layer, with a thermocline extending to about 8 meters. The advective inflow current is currently flowing just beneath this layer.
The brief but large amount of runoff coming into Lake Powell suspended a large amount of deltaic sediment in the inflow areas of the reservoir. In the Hite Marina area, this sediment was redeposited as a bar about 1 meter in depth across the channel as the channel widened just above the mouth of North Wash and below Hite Marina. The upstream portions of the reservoir are dominated by this advective inflow current, which contains a large amount of the resuspended sediment. The subsequent oxygen demand from the sediment has reduced dissolved oxygen levels in the surface layer of the reservoir, with minimum dissolved oxygen concentrations of below 2mg/l occurring from Hite Basin to the San Juan confluence.
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