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I I I I I I I I I I I I I I I I I I I period the native streamflow is typically insufficient to meet irrigation demands and transbasin water (CBT) constitutes a significant portion of the water in the river. The storage season typically runs from November through April. Discharge from the Loveland Wastewater Treatment Plant and transbasin diversion of CBT water for storage may provide the only source of streamflow in some reaches. 4.3 AUGMENTATION OF HISTORIC STREAMFLOW RECORD FOR BIG THOMPSON RIVER The USGS streamflow gauge at the mouth of the canyon (6.0 miles east of Drake, Colorado) has been in operation since January of 1927 through the present time. This historic record is not continuous, the following gaps are presen t: 0 October 1928 - May 1929 0 1929 - 1932 (November through March values are missing) 0 October 1933 - April 1938 (Station discontinued; no records) 0 October 1950 - March 1951 0 August 1979 - April 1980 The first step in this study task involved correction of the gauge record to account for diversions through the Dille Tunnel for hydroelectric operations of the USBR Big Thompson Hydroelectric Plant at the mouth of the canyon. Big Thompson streamflow is diverted above the gauge and consequently not recorded at the gauge. Outflow from the Hydroelectric Plant is returned to the river below the gauge. These Dille Tunnel diversions must be added to the gauge reading each month to accurately represent the quantity of "native" water in the river and available to downstream users. Monthly diversion records were obtained by the City from the USBR and provided to CDM; this data was used to correct the gauge record. Average streamflows for native Big Thompson streamflow were computed from this corrected gauge record; these values are given in Table 4-1 and illustrate the seasonal changes described in Section 4.1. 4-4