Update on Water Quality Conditions below Glen Canyon Dam 

March 2, 2007 

Bill Vernieu, Grand Canyon Monitoring and Research Center 

Introduction 

Water-quality monitoring of Glen Canyon Dam releases, in the tailrace below the dam, 

has demonstrated progressively lower dissolved oxygen (DO) concentrations and 

increasing specific conductance (SC) during the month of February 2007, indicating 

increasing influence of deep water in the reservoir affecting dam releases. The mean 

daily DO concentration in dam releases was approximately 8.0 mg/L at the beginning of 

the month (77% of saturation); however, the concentration reached 4.4 mg/L (42% of 

saturation) on February 28, with instantaneous measurements reaching as low as 3.7 

mg/L. It is uncertain how long this trend will continue. By comparison, mean daily 

tailrace DO concentrations reached a minimum of 3.9 mg/L on September 29, 2005. 

Monitoring Methods 

The Grand Canyon Monitoring and Research Center has YSI Model 6920 multiparameter sondes measuring continuous (20-minute intervals) water temperature, specific 

conductance, pH, dissolved oxygen, and turbidity at three locations in the Glen Canyon 

Dam tailwater: inside the draft tubes of the Glen Canyon Dam generators (CRDT), in the 

tailrace approximately 100 m downstream of Glen Canyon Dam below the river outlet 

works (CRBD), and at Lees Ferry on a mid-channel buoy slightly upstream of the Lees 

Ferry boat ramp. The tailrace location, CRBD, is connected to a data collection platform, 

accessible by telephone, which was installed in September of 2005. The CRDT and 

CRLF locations are visited monthly for servicing, calibration and data retrieval. The next 

visit will be March 5, 2007. 

Depth profiles of water-quality parameters are collected in the Glen Canyon Dam 

forebay, approximately 2.4 km upstream of Glen Canyon Dam, on a monthly basis, using 

a Hydrolab Surveyor 3/H20 multi-parameter sonde. Recent forebay profiles were 

collected on January 25, 2007 (Figure 1) and February 15, 2007 (Figure 2). The next 

forebay sampling is scheduled concurrently with a full reservoir survey during the week 

of March 11, 2007 

Lake Powell Conditions 

Figure 1 shows conditions through the water column of the reservoir forebay on January 

25, 2007. At this time, the surface of the reservoir (epilimnion) had mixed to the depth of 

the penstocks, approximately 40 m. Epilimnetic DO concentrations were approximately 

8.5 mg/L, SC was approximately 725 μS/cm. The hypolimnion, or bottom layer of the 

reservoir was fairly uniform from a depth of 62 m to the bottom. Hypolimnetic DO 

concentration was approximately 1.7 mg/L near the bottom to 2.7 mg/L at 75 m. DO at 
the upper boundary of the hypolimnion was 2.6 mg/L at 62 m. The entire water column 

was nearly isothermal with temperatures ranging between 7.9 oC to 8.5 oC; however, the 

lake was stratified due to the differences in SC. There was an intermediate zone of 

relatively well-oxygenated water residing below the penstock level and separating the 

mixed epilimnion from the hypolimnion. 

Figure 2 shows conditions in the reservoir forebay on February 15, 2007. At this time, 

the intermediate zone below the penstock depth had essentially been evacuated, and 

penstocks were withdrawing water from the thin transition zone between the epilimnion 

and hypolimnion. The upper boundary of the hypolimnion has moved upwards to where 

a DO concentration of 2.5 mg/L was observed at 51 m. This condition results in the 

reservoir being susceptible to internal oscillations from wind events, which can cause the 

surface layers of the reservoir to be moved upstream, with deeper layers being displaced 

upwards to be incorporated into dam releases. 

Dam Release Water-Quality 

Figure 3 shows instantaneous water-quality measurements in the Glen Canyon Dam 

tailwater (CRBD) since February 1, 2007. Conditions were fairly stable until 

approximately February 12, at which time a gradual decrease in dissolved oxygen 

concentrations, and a corresponding increase in SC, were observed, indicating an 

increasing influence of deeper reservoir water in dam releases. With the passage of a 

storm system on February 23, 2007, DO concentrations dropped rather dramatically by 

about 1 mg/L, with a corresponding jump in SC of about 50 μS/cm. Although this rapid 

decrease was probably the result of the February 23rd storm system, it is a part of a more 

long-term decrease in DO concentrations. Release temperature has remained constant 

throughout this period. 

Discussion 

Two processes appear to be affecting the quality of reservoir releases. One is the shortterm effect of weather patterns on the reservoir where the stratification boundary is very 

near the penstock release depth. The zone of fairly oxygenated water that existed below 

the penstock elevation in January 2007 has essentially been evacuated, bringing the 

hypoxic hypolimnion closer to the penstock release level. With the passage of storm 

systems on February 23-25, and again on February 28-March 1, strong SW winds created 

short-term oscillations (internal seiches) in the stratification patterns in Lake Powell and 

resulted in some of the hypoxic hypolimnion to be entrained in the releases. This is 

shown in the Figure 3 as decreases in DO and increases in SC during these periods. 

The other and more significant is the longer-term process evidenced by the gradual DO 

decreases and SC increases since February 12 shown in Figure 3. This is likely due to an 

underflow density current, from cold winter inflows moving along the bottom of the 

reservoir and displacing the hypolimnion in the forebay upwards. A distinct underflow 

density current near the dam has been observed each spring since 1999, with the 
exception of 2006. An underflow density current was not expected this year, as its 

absence in 2006 indicated that the reservoir had increased in density to the point the 

winter inflows were expected to flow through intermediate depths in the reservoir. 

However, the sustained period of cold weather that was observed in January 2007, could 

have increased the density of the inflows to the point where a distinct underflow was 

possible. No underflow density current was observed during the forebay survey of 

February 15, 2007; however, the trend in release water quality over the past three weeks 

indicates that an underflow may indeed be occurring this year and has not arrived at the 

forebay yet. This will be verified during next reservoir sampling, March 12-17, 2007. 

If an underflow density current is occurring this year, conditions could be very similar to 

what was observed in 2004, when there was a distinct underflow in the forebay. 

Reservoir elevations are similar, 3587 ft on March 1, 2004 vs. 3598 on March 1, 2007 

and similar stratification patterns were observed in both years. Figure 4 shows a 

sequence of forebay profiles, from at the dam, from February through April 2004, 

showing the influence of the underflow in displacing the hypoxic epilimnion upwards, 

allowing incorporation into dam releases. Figure 5 shows the longer-term patterns in 

mean daily Glen Canyon Dam release water-quality, for comparison of these two periods. 

While DO concentrations were not as low in 2004 as current conditions, the minimum 

concentration in releases was not reached until April 22, 2004, indicating that current 

release concentrations may continue to decrease. 
Figure 1. Forebay conditions, January 25, 2007 Figure 2. Forebay conditions, February 15, 
NOTE: Specific Conductance in mS * 10

T (deg C) - D.O. (mg/L)

 2 3 4 5 6 7 8 9

101112131415161718

28JAN07 04FEB07 11FEB07 18FEB07 25FEB07 

Figure 3. Glen Canyon Dam release water-quality since February 1, 2007
Figure 4. Glen Canyon Dam forebay profiles, February to April 2004
NOTE: Specific Conductance in mS * 10

T (deg C) - D.O. (mg/L)

 2 3 4 5 6 7 8 9

101112131415161718

01JAN04

01APR04

01JUL04

01OCT04

01JAN05

01APR05

01JUL05

01OCT05

01JAN06

01APR06

01JUL06

01OCT06

01JAN07

Figure 5. Mean daily Glen Canyon Dam release water quality, 2004 to 2007