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Show water conservation. In addition, the model has been used to determine whether better long- term routing methods can be used to improve projections for salinity in the lower Colorado River. The physical limnology of Lake Powell is complex but reasonably well studied ( Johnson and Merritt 1979; Gloss et al. 1980; USDI 1983). The lake has been described as " warm monomictic" which indicates that convective mixing ( lake turnover) occurs only once a year, during the winter cool- down period. However, Johnson and Merritt ( 1979) report that advective circulation, caused by differences in water density ( which is related to changes in TDS and temperature), is significant. They further state that during a typical year the lake will receive two distinctive types of inflow: a lower- density water associated with the spring flood, and a higher- density water during the winter. The impact of the density- dependent advective circulation is important in the limnology of Lake Powell. The influx of warm, lower- density water ( spring overflow current) largely sets the thickness of the summer thermal equilibrium, especially in the upper reaches of the lake. Johnson and Merritt ( 1979) report that years with a large spring flood are associated with a deeper, more diffused metalimnion than are years with a small spring flood. As the summer progresses, the TDS content of the inflow water increases, eventually closing off the overflow current. By early winter, the water is sufficiently dense to become an underflow current ( the winter underflow current) ( Johnson and Merritt 1979). This current contains cold, saline water with a high initial dissolved oxygen concentration and flows into the deeper areas of the lake, replenishing the deep- water dissolved oxygen concentrations. This phenomenon, along with the behavior of other downlake currents, indicates that the lake still possesses much of the character of a river reach in which the water has a long residence time. Although the bottom of Lake Powell generally contains adequate dissolved oxygen for a good fishery, Johnson and Page ( 1981) describe an oxygen- depleted layer that develops in late summer below the mixed layer at about 45- 60 feet below the surface. This layer is found the full 186- mile length of the lake and is more distinct in the tributary bays. Oxygen depletion is thought to be caused by bacterial respiration and the chemical process of organic decay. These processes result in oxygen depletion below the mixing zone where the organic debris concentrates. The distribution of oxygen may have a substantial impact on the fishery of Lake Powell. In particular, the depleted oxygen layer presents a formidable barrier to the vertical migration of fish during late summer and early fall. Record high flows along the mainstem of the Colorado River in 1983 caused severe flood damage throughout parts of the Colorado River Basin ( Vandivere and Vorster 1984). Large discharges through the Glen Canyon Dam resulted in the release of large amounts of more saline water from the depths of the lake. These releases in turn reduced the TDS 14 |
