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<br />0323 J3 <br /> <br />c. Flooding Cycles <br /> <br /> <br />While the salinization of the lake from a drought cycle can be manifested abruptly within the <br /> <br /> <br />hypolimnion, the freshening qualities of a flood may take several years to mix throughout the lake <br /> <br /> <br />(Fig_3a). This relateslo the density and mixing properties of the SWARM flows versus the__ _ _ <br /> <br /> <br />FRESCO flows. One sinks easily to the hypolimnion and mixes with previous cells and evades <br /> <br /> <br />the penstocks, the other tends to slide over the first and is evacuated at the penstocks. When a <br /> <br /> <br />very fresh flood follows a drought, the steeper salinity gradient poses a greater challenge to the <br /> <br />mixing and sinking forces. <br />The flood years of the 1980's were a dramatic example of the wettest series of years since <br /> <br />1921. The vast volume of fresh, oxygenated water, combined with the operation of the spillways <br /> <br /> <br />and river outlet works brought the lake to as complete a mixing as it has approached in its history. <br /> <br /> <br />It was with one of the worst cases of bad timing that the Bureau of Reclamation decided to scale <br /> <br /> <br />back sampling of Lake Powell at this time. This was a great loss from the perspective of <br />understanding limnologica1 processes. <br /> <br />B. Glen Canyon Dam Releases <br /> <br /> <br />Since the filling of Glen Canyon Dam, water quality conditions in the Glen Canyon Dam <br /> <br /> <br />tailwater have changed substantially. Before Glen Canyon Dam the river was characterized by <br /> <br /> <br />wide fluctuations in discharge, sediment load, temperature and salinity content. Due to the <br /> <br /> <br />presence of the dam and the location of the penstock structures for powerplant releases there has <br /> <br /> <br />been a marked reduction in the variance of all these parameters. Discharge is now a function of <br /> <br /> <br />predetermined annual release volumes, monthly scheduled releases, and daily fluctuations in <br /> <br /> <br />response to power demands and operational constraints (Fig 11). Temperatures have been greatly <br /> <br /> <br />reduced and fluctuations now range between 7 and 12 oC, reduced from a pre-dam range of 0 to <br /> <br /> <br />nearly 30 OC (Fig. 12). Fluctuations in salinity have been similarly reduced (Fig. 13). <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 27 of 62 <br />