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<br />Oil2236 <br /> <br />Although riverine waters are well oxygenated, the organic and chemical nutrients within the <br /> <br /> <br />flood waters place a high oxygen demand that results in a precipitous drop in oxygen levels <br /> <br /> <br />beginning in July and continuing through fall. The trace of this dissolved oxygen sag, <br /> <br /> <br />superimposed and just below the flOl;ldplume,. d~note1>the settling of detritus that fuels the <br /> <br /> <br />hypoxic (or low oxygen concentration) cycle. <br />Convectively Mixed Epilimnion: Autumn surface cooling and convective mixing begins lake- <br /> <br />wide in September and reaches it deepest extent around January. The previous spring's flood is <br /> <br /> <br />mixed with past years' floods and may entrain the top of the hypolimnion; the conductance of the <br /> <br /> <br />convectively mixed epilimnion is a reflection of these layers. Almost any size flood will <br /> <br /> <br />significantly freshen this layer, but larger floods produce greater freshening. It is likely each <br /> <br />mixing event is entraining floods or the convectively mixed epilimnion from two or more <br /> <br />previous years. <br /> <br />Not only does a large flood result in a much fresher convection cell, but, again, it also <br /> <br />produces a thicker and warmer body of water. The cooler temperatures and the mixing process <br /> <br /> <br />result in progressively elevated dissolved oxygen levels in this layer as winter progresses. This <br /> <br /> <br />results from the higher oxygen-carrying capacity of cold water as well as algal productivity that <br /> <br /> <br />thrives on the mixed and composted flood waters. As winter progresses into spring, oxygen levels <br /> <br /> <br />increase, typically exceeding super-saturation from February to May. <br /> <br /> <br />b. Bottom Processes: <br />In the winter, cold temperatures combine with advective currents from the inflows to produce <br /> <br />bottom hugging density currents. In Lake Powell these cold bottom plumes repeatedly divide into <br /> <br />three distinct masses depending on the time and meteorological conditions under which they <br /> <br /> <br />form. These bottom plumes are critical to the long term chemical and physical conditions found in <br /> <br /> <br />the hypolimnion (Edinger et at. 1984, Merritt and Johnson 1978, Johnson and Merritt 1979, Gloss <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 20 of 62 <br />