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As we look at the history of Lake Powell's filling, we can see the linkage between Lake Powell's elevation and discharge temperature. When the dam was closed in 1963, discharge temperature patterns were similar to pre-dam riverine seasonal patterns and temperatures. But as the reservoir filled, temperatures were attenuated as the distance from the surface of the lake and the release structures increased. By 1973, the lake's elevation rose above 3630 feet, placing the warm surface waters out of reach of the penstock release structures. Since that time, water temperatures downstream of the dam actually peak in late fall-early winter due to surface cooling and mixing of the reservoir's epilimnion, or warm surface waters. Only on rare occasions have we seen warmer water releases since 1973 (as explained later). *Temperatures in purple were measured at Lees Ferry, 15 miles below Glen Canyon Dam, and are the only temperatures available for that time. Temperatures in red are measured immediately below Glen Canyon Dam in the tailrace pool. The 15 miles of travel time provide an opportunity for the water to equilibrate with ambient air temperatures, i.e., warming may occur in summer, cooling in winter. This may bias the data a degree or two up or down, depending on the volume of flow and ambient air temperatures (and possible surface area contact). Links: The Bureau of Reclamation provides additional information about recent and projected Lake Powell elevations and inflow and outflow from Glen Canyon Dam at these website's: http://www.usbr.gov/uc/water/crsp/glda.html Most probably water supply gives projections for expected reservoir elevations and discharge/ inflow for the next 2 years are found here: http://www.usbr.gov/uc/water/crsp/crsp_gc.txt Minimum pool for next winter is projected to be below 3590 ft. |
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