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<br />OD23H <br /> <br />40% of the year's discharge came from the river outlet works and left diversion tunnel between <br /> <br /> <br />February and August (Fig. 11), and for 3 months the bypass releases exceeded the penstocks. The <br /> <br /> <br />effects of this discharge can be seen in the isopleths'(Fig. 4). Much of the spring flood was <br /> <br /> <br />withdrl!Wll by the high penstock and river outlet works reIeases-drawing.hocizontally across the <br /> <br /> <br />lake, while the diversion tunnel directly released the most extreme hypolimnetic waters. By <br /> <br /> <br />October of 1965, the spring flood had been drawn downstream from the Bullfrog station, 170 <br /> <br /> <br />kilometers above the dam. It was unusual that such a large spring flood (9th highest of 33 inflows <br /> <br /> <br />to Lake Powell) was no longer present as far uplake as the inflow in fall. In this instance, the <br /> <br /> <br />operation of these outlets emulated the spillways by drawing from the fresh flood waters of the <br /> <br /> <br />epilimnion, while the hypolimnion was evacuated from the lake's bottom. Dissolved oxygen <br /> <br /> <br />appears to be elevated as a result of the high throughput and a hypolimnetic underflow. <br /> <br /> <br />2) 1980'sfloodyears <br /> <br /> <br />The 1980s included one of the most distinct flooding periods in the southwest since detailed <br /> <br /> <br />records began in the early 1920's. From 1979 to 1988, the Colorado River basin received 6 of the <br /> <br /> <br />10 greatest flood events in Lake Powell's history (Table 2). The flooding of the mid-eighties is <br /> <br /> <br />attributed to the weather phenomenon ofEl Nmo, a pattern that is currently building in the Pacific <br /> <br /> <br />Ocean and which may compete with that of 1983. The results to Lake Powell were 5 years of <br /> <br /> <br />back to back above average flooding. Unfortunately this unusual period was accompanied by <br /> <br /> <br />decreasing sampling; from 5 lake-wide trips in 1983, to 4 in 1984, 3 in 1985 and 1986,2 in 1987 <br /> <br /> <br />and 1989, and 1 each in 1988, 1990 and 1991. With less than 4 trips per year, the inferences <br /> <br /> <br />become uncertain. <br /> <br /> <br />The period preceding the flood years included a strong drought in 1977 that produced a strong <br /> <br /> <br />chemocline below the penstocks, followed by 8th and 12th ranked high floods in 1979 and 1980 <br /> <br /> <br />that significantly refreshed the hypolimnion. In 1981 a fairly strong drought introduced another <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 38 of 62 <br />