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This graph of annual temperature patterns demonstrates 4 eras of temperature measurements below Glen Canyon Dam. as well as the current warming trend in dam releases over post-dam cooling trends. In orange is data collected at Lees Ferry from 1949 to 1963, showing normal riverine annual warming and cooling patterns reflecting seasonal patterns; peaking in the summer in the mid to upper twenties Celsius (70's to 80's °F) and reaching winter minimas below 5°C (30's °F). The green points, from 1963 to 1973 at Lees Ferry, demonstrate the transition from riverine to reservoir thermal patterns after the dam was closed in 1963. As Powell filled, it provided a thermal buffer for the Colorado River, and the peak annual temperatures have migrated to later in the year, when fall mixing processes bring warm water from the surface to the level of the penstock releases. Dark blue points (and the thin blue line) show the trend toward minimal annual variation once the reservoir began to fluctuate at typical elevations above ~3630 feet. Peak temperatures occurring in late fall / early winter coinciding with peak convective fall mixing of the reservoir. The dense light blue points represent data collected just below Glen Canyon Dam since 1988. The red data indicates the temperature of releases in 2003. Here we can see how dropping reservoir levels have resulted once again in warmer releases in the summer. This pattern has been seen only rarely since 1973. *G.C.Dam = Monitoring site immediately below Glen Canyon Dam. Lees = Lees Ferry site 15 miles downstream of Glen Canyon Dam. |
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