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<br />DRAFr Febmary25, 1998
<br />had a flow duration of about 6 days per year.
<br />Similarly, for the Green River, Utah gauge the pre-1963 mean annual peak discharge of 32,700
<br />ft /s and post-1963 mean peak discharge of 22,300 ft /s were equaled or exceeded an average of
<br />7 days per year, but the discharge of 32,700 ft/s now only occurs on an average of 2 days per
<br />year. The duration associated with the average annual peak flow has not decreased as a result of
<br />flow regulation, only the magnitude of peak has decreased.
<br />The duration of low flows has also changed since construction of Flaming Gorge Dam. For a
<br />given low flow discharge the duration has increased since 1963. The two exceptions are that the
<br />duration of flows less than 200 f 3/s at the Green River, Utah gauge has not changed and for flow
<br />between 200 and 600 W/s at Jensen, the duration has decreased (FLO Engineering, Inc., 1996).
<br />Peak Flows - Green River
<br />The seasonal hydrograph for the Green River is characterized by a snowmelt runoff peak with the
<br />majority of the runoff occurring between May 1 and June 30. The mean date of peak flow
<br />occurrence at Jensen for the period of record is approximately May 27.
<br />• For the entire record at the Green River, Utah gauge, the mean date of peak flow occurrence was
<br />May 30. Generally, the higher the peak discharge, the later in the season the peak flow occurs.
<br />Before 1963, the mean date of peak occurrence was June 3 and after 1963, the mean day of peak
<br />occurrence was May 26. Thus, on the average, the peak discharge is occurring about one week
<br />earlier in the season when data collected since the closure of Flaming Gorge Dam is compared to
<br />data collected prior to the closure of the dam for the Green River, Utah gauge.
<br />Frequency of a peak discharge is a function of the hydrological and physiological characteristics
<br />of a basin. The mean annual peak discharge is determined by averaging all the yearly
<br />instantaneous peak discharges for the period of record. For the period of 1947 to 1962, the mean
<br />annual peak at Jensen was 24,000 W/s. Since 1963, the mean annual peak has been 17,400 ft/s
<br />(FLO Engineering, Inc., 1997b).
<br />The highest peak on record at Jensen, Utah is 40,000 ft/s, which occurred on May 18, 1984.
<br />The same year the Green River, Utah gauge peaked at 48,300 ft/s on May 20. At Green River,
<br />Utah, the highest peak on record was 68,100 ft/s on June, 27, 1917. Peak discharge at Green
<br />River, UT exceeded 60,000 ft/s in 1897, 1909, 1917 and 1921 (Schmidt, 1994).
<br />The lowest annual peaks on record were 7,090 ft/s (April 26, 1989) at Jensen, Utah and 6,460
<br />ft/s (May 17, 1934) at Green River, Utah. Both the highest and lowest peak discharges at
<br />Jensen, Utah occurred after the construction of Flaming Gorge, whereas the highest and lowest
<br />• peak discharges at Green River, Utah occurred before most of the upstream water development.
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