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• IJRAFr February25, 1998 C. Hydrology Gauge Network and History The Green River The analysis presented in this chapter is based primarily on two Green River gauges: Green River near Jensen, which is located below the mouth of Split Mountain Canyon about 151 km downstream from Flaming Gorge Dam and at the Green River at Green River, Utah located about 467 km downstream from Flaming Gorge Dam. The period of record for the Jensen, Utah gauge is water year 1947 through the present. (Two discontinuous records are available for 1904 and 1906.) The period of record at the Green River, Utah gauge is longer, extending from water years 1895 to 1899 and 1905 to the present. A third gauge, Green River near Greendale, Utah, records the release of water from Flaming Gorge. Located just downstream of the dam, the flow records for this gauge describe the hydrology of reach 1, the Green River upstream from the Yampa River to Flaming Gorge. Tributaries of the Green River The principal tributaries to the Green River that have gauge records include: Little Snake River, Yampa River, Duchesne River, White River, Price River, and the San Rafael River (Figure 1). The period of record for these gauges is variable depending upon site and season. Water developments recorded by these gauges include storage reservoirs, trans-basin diversions, consumptive uses, and irrigation return flows. Mean Annual Discharge - Green River • The Green River generally peaks in the late spring with the lowest flows occurring between September 1 to March 1. The mean annual discharge is the arithmetic mean of all the mean daily discharges for the entire period of record. The mean annual discharge is generally equaled or exceeded on less than half the number of days in the year. The mean annual discharge for the Green River at the Jensen gauge was about 4,360 ft3/s prior to 1963 and 4,210 ft3/s after 1963. The Green River, Utah gauge record begins in 1895, but the Jensen gauge record begins in 1947. To compare equal lengths of record, a mean annual discharge of 5,800 ft3/s for the Green River, Utah gauge was computed for the period 1947 through 1962 and 5,600 ft3/s after 1963. For the same period of record, the two gauges exhibit a similar small decrease in the mean annual discharge. The mean annual discharge is 8,640 ft3/s for the available record for the period water year 1895 through 1946 for the Green River at Green River, Utah. Thus, on the average, runoff during this earliest period was about 50 percent greater than during either of the later periods. 5