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<br />0023J9 <br /> <br />volume of low-density water that overrides the surface of the reservoir and extends downstream, <br /> <br /> <br />reaching the vicinity of the dam later in the year. The volume and duration of the runoff appears <br /> <br /> <br />to dictate the downstream extent and the thickness of this layer of water. <br /> <br /> <br />This provides the initial conditions for convective mixing later in the winter. Convective <br /> <br /> <br />mixing involves cooling the surface of the reservoir, which increases its density. This water sinks <br /> <br />and mixes with deeper water of equal density with the assistance of wind action. The more winter <br /> <br /> <br />cooling the deeper the level of mixing. If the previous spring's runoff does not have enough <br /> <br /> <br />volume to reach the downstream portion of the reservoir, convection is mainly a result of the <br /> <br /> <br />cooling process. However, if a heavy spring runoff results in a large voluine of snow-melt water <br /> <br />near the reservoir, this thick layer is already relatively homogenous. Winter cooling can then mix <br /> <br /> <br />a large portion of the epilimnion to depths at or below the level of penstock withdrawal. This <br /> <br /> <br />results in direct withdrawal of epilimnetic water and has been seen during winter months <br /> <br /> <br />following high runoff from the previous spring. Examples of this occurred in early 1996 and <br />1997. <br /> <br /> <br />Reservoir inflows during the winter months are characterized by cold water of higher salinity <br /> <br /> <br />compared to those of other times of the year. These conditions result in inflows with the highest <br /> <br /> <br />densities of the year entering the reservoir. Therefore, this water plunges when it meets the <br /> <br /> <br />reservoir and flows along the bottom of the reservoir. Its ultimate fate depends on preexisting <br /> <br /> <br />conditions in the hypolimnion. If water in the deepest portion of the reservoir is of greater density <br /> <br /> <br />than the winter inflow, the inflow will override the hypolimnion and eventually be discharged <br /> <br /> <br />through the penstock outlet. This appears to happen after a saline hypolimnion has been <br />established, usually following years of low runoff. If, on the other hand, the inflowing water is of <br />greater density than the hypolimnion, it will displace the deeper body of water and route it <br />through the release structures. <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page33 of 62 <br />