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00ll5G 13 Inflows to the Yampa River, aside from those calculated for the major tributaries, were generated through simple mass balance equations. This process involved subtracting the flows at an upper gage from the flow at a lower gage and then subtracting any additional calculated tributary inflow. This produced an implied gain or loss and reflects a "depleted" gain. The upper most inflow of the model represents the natural inflow to Stagecoach Reservoir. This inflow was calculated using records from the Oak Creek gage. The flow record was extended to cover the entire study period through regression against the Steamboat Springs gage. Several USGS gages are located on tributaries on which no storage projects have been identified, Gage records are often not complete for these sites over the period 1930 . ] 982 and no effort was made to extend these records and model these tributaries independently. It was believed that a more accurate representation or hydrology <.:ould be made using the Yampa River mainslcm gages and instead modeling net aggregated gains, Elk River There were no sites on the Elk River identified as likely alternatives for a new storage facility. However, a complete inflow data set was generated so that existing reservoirs on the Elk River could be operated differently in the model than they have been operated historically. This task included generating the natural inflows to Steamboat Lake. Steamboat Lake is located on Willow Creek, one of the larger tributaries to the Elk RIver. There are no stream flow gages maintained on Willow Creek and inflow data gathered by reservoir operators are limited. To estimate the natural inflows to Steamboat Lake, gage records fTOm the north and main forks of the Elk River above Clark were used as a surrogate. Total drainage areas above the gages were measured and a runoll' per unit area was calculated. This runoff per unit area was then applie~ to the measured drainage arca above Steamboat Lake, In addition, the average elevations of these drainage areas were estimated. A correction was then made to the predicted runoff of the ungaged area based on differences in basin elevation from the surrogate gaged portions of the basin. This technique was validated using a gage on the Elk River located downstream of the confluence with Willow Creek. Synthesized stream flows on Willow Creek and Lester Creek were added to flows recorded at a gage above the confluence and then compared to the lower gage flow records. Several years of records were available from the Colorado Division of Parks on their operations of Steamboat Lake. While no information has been recorded on reservoir inflows, outflow data and water surface elevations were reviewed to establish an approximate annual inflow. These inferred inflows agreed, in approximate terms, with inflows predicted using the unit area method. Inflows were also generated below Steamboat Lake on the Elk River. These inflows were derived based on gaged records and represent the implied gains between gages. The period of record for gages on the Elk RIver were extended to cover the entire 1930. I 982 study period using regression analysis with the Steamboat Springs gage on the Yampa RIver as the independent variable. Elkhead Creek Streamflows on Elkhead Creek were generated to provide inflow information to three" reservoir sites including Estey Simpson Reservoir, Pilot Knob Reservoir and E1khead Lake, The Estey Simpson and . ;L~';1i