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58% of the days where flows are below 120 cfs, which also equates to a long term average of37 days where the flows at the State line are supplemented by releases. More severe assumptions in the modeling reduced these benefits down to approximately 24 days of augmentation to compact flows, e D. Water Supply The construction of an off channel reservoir would be dependent on a junior water right for diverting water or the transfer of a senior right. Flows in the South Platte River significantly increase during the winter months due to return flows entering the river. There is generally no call on the river during the winter months, which would allow a junior diversion to the reservoir to operate. This would provide the prime opportunity for diverting water to the reservoir, On a number of days during the April 1- October 15 period, the historic gage flows of the South Platte River at Julesberg have exceeded 120 cfs, this would indicate that a reservoir could divert some water during this time period as well and store it for re-regulation when the flow in the river drops below 120 cfs at the state line. This would allow the reservoir to fill and refill. An analysis of the gage flows was done using thi$ concept. Several diversion capacities were assumed to optimize the effectiveness of a 2,000 acre-foot reservoir. Using the fill and refill concept, a 2,000 acre-foot reservoir would be able to re-regulate up to 2,400 acre-feet annually. The flows in the river increase above 120 cfs enough in the lat~ summer months to usually allow filling by the end of October, The primary control on filling and refilling the reservoir is the outlet capacity back to the river. Optimization modeling will be necessary at the preliminary design stage for selecting final sizes of the reservoir components, Flows in the river would also be augmented by returns flows entering the river from the Tamarack .a Project. These flows will also provide some ben~fit to the river which could be enhanced by the .. operation of this project. The exact relationship t)fhow these projects are operated is still in the formative stages. This study conclusively shows that the winter flows in this reach of the river are adequate to fill a reservoir of this size the majority of the time, Flow records for the South Platte River were analyzed for a 30 year historic study period (1966-1996). One can assume from historic records that any other diversions that occurred during the winter months would be reflected in the gage and that the recorded flows would be in priority for a junior diversion on the lower reach. of the river. The information available indicated that in all but two of the years (1977 & 1978), there would be adequate flows in the river to allow for the diversions needed to fill the reservoir. The information and assumptions that were used to estimate the quantity of water, which was available for diversion, are as follows: . Average daily stream gage records were obtained for the USGS Julesburg gage station for the period of 1966-1996, . . Criteria for when diversions would be allowed were based on the requirements of the South Platte River Compact and the Colorado prior appropriation doctrine. Table 2 entitled "Summary of Reservoir Operations - FilllRefill Scenario" is a summary of the flows estimated to be available for diversion into the reservoir site. The USGS gage records were recorded as average daily flows at the Julesburg gage, Flows in the river during the winter months were assumed to be available for storage, In addition, flows during the period April 1 - October 15 were assumed to be available for diversion if the Julesberg gage was above 120 cfs, Flows during this time period would be diverted under a refill right for the reservoir, There were no detailed e~ I . 7