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Release temperatures are surpassing the 2003 warming rates. Lake elevations continue to drop, bringing the thin but warm epilimnion (lake's surface layer of water) within reach of the penstock release structures. It is probable that temperatures will continue to climb above 2003 levels as the lake stage descends. Unusual conditions uplake in the winter of 2003-2004 created the atypical increase in release temperatures in February through May, 2004. This related to the low volume and smaller size of the reservoir, allowing a cold winter underflow to penetrate to the dam, uplifting a relatively warm hypolimnion, which was subsequently discharged through the penstocks. This uplifted layer was depleted by June, and cooler releases resumed temporarily until; 1) spring warming from the surface and 2) longitudinal warming from the spring inflow began to impinge on the dam. With the combined effects of fall convective mixing and the low reservoir elevations on Lake Powell, the warmer surface waters are converging on the penstock release structures in Glen Canyon Dam. In 2003 and 2004 we have seen, for the first time in nearly 25 years (since 1978), sustained warmer releases in the summer season that are not the result of spillway releases. Temperatures began to increase most steeply in June, when the epilimnion began to intersect the penstock withdrawal structures. Temperatures typically peak in November and December and may rise to 15 to 17°C, depending on how low the reservoir is drawn before fall cooling and mixing attenuates the warming processes. A scattering of other warm summer releases from 1977 to 1984 were rooted in various causes, described in the following pages. Temperature is not the only factor influenced by the lowering lake elevation. The epilimnion is not only characterized by warmer temperatures, but distinct salinity, chemistry, biological community, oxygenation and pH. As we draw progressively more water from the surface, the water is more dilute; it has less salts and nutrients, and more plankton and fish. For instantaneous water temperatures measured at Lees Ferry, you can check the U.S.G.S. Water Resources website: |
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