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<br />002336 <br /> <br />separating various flows. Differences in stratification occur in a temporal dimension as well as <br /> <br />vertical and longitudinal. The following describes the dominant seasonal inflows and circulation <br /> <br />cells of Lake Powell (bottom panel, Fig I). Timing of these events can typically vary a month or <br /> <br />more in either direction. Isopleths for temperature, conductivity and dissolved oxygen for 1975 and <br /> <br />1976 (Figs. 2a and 2b) show a sequence of generally typical snapshots of water quality, as do the <br /> <br />isopleths of the longitudinal profiles (Figs. 4-10). <br /> <br />a. Surface Processes: <br /> <br /> <br />Spring Flood: This is the yearly dominating lake event, dictated by the magnitude and timing of <br /> <br /> <br />the spring headwater snow-melt coming down Cataract Canyon and the San Juan River and into <br /> <br /> <br />Lake Powell from May to July. These waters are typically fresh (lower conductance/ salinity) and <br /> <br />initiate the warming processes in the lake since the rivers are the first to warm. The spring freshet <br /> <br />may begin injecting at the surface, but the bulk of the flood drops 10 to 20 meters below the surface <br /> <br />of the lake as spring progresses and the lake surface warming exceeds river temperatures. Though <br /> <br />not as warm as the lake's surface, the volume of relatively warm flood water will dictate the <br /> <br />thickness of the epilimnion. <br /> <br />The fresh waters of the typical spring flood reach the darn by July as the last of the snow-melt <br /> <br />enters the reservoir. A small flood moves toward the darn more slowly than a large flood and its <br /> <br />signature may not be seen at the darn before it is dissipated by convective mixing in the fall. <br /> <br />Although riverine waters are well oxygenated, the organic and chemical nutrients within the flood <br /> <br />waters place a high oxygen demand that results in a precipitous drop in oxygen levels beginning in <br /> <br />July and continuing through fall. The trace of this dissolved oxygen sag, superimposed and just <br /> <br />below the flood plume, denotes the settling of detritus that fuels the hypoxic (or low oxygen <br /> <br />concentration) cycle. <br /> <br />06/02/98 <br /> <br />DRAFT <br /> <br />p~O'~ 1~) of 62 <br />