8028

• UCREFRP Catalog Number: 8028
• Author: Daly, S. F., et al.
• Title: Effect Of Daily Fluctuations From Flaming Gorge Dam On Formation Of Ice Covers On The Green River -Draft.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: NO

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the daily average temperature is above 32°F, a negative number of freezing degree days is found
for that day. An index of the severity of the winter can be found by accumulating the number of
freezing degree days (abbreviated AFDD) throughout the winter. The maximum number of
freezing degree days for a number of winters are listed in Table 1 and shown in Figure 6.
The daily average air temperature at Vernal and the recorded water temperatures of the
Green River at Jensen and of the water released from Flaming Gorge Dam are shown in Figure 7
for the winters from 1989 through 1997. Also shown in these figures is the daily average
discharge of the Green River at Jensen. It can be seen that the water temperature at Jensen was
influenced both by the Flaming Gorge release water temperature and the air temperature. It is
interesting to note that the Green River water temperature at Jensen reached 32°F (0°C) for at
least a brief period during each winter and remained at 32°F (0°C) for considerable periods of
time for several winters. During the periods when the water temperature was at or near 32°F
(0°C), the daily average air temperature was continuously below 32°F (0°C), often reaching
minimums of 10°F (-12°C) or less. It is during periods of cold air temperatures and with the
water temperature entering the study reach at or near 32°F that maximum ice production in the
study reaches would be expected.
Historical Ice Observations
There are only a limited number of historical ice observations available for the Green
River. These include a summary of USGS observations made at irregular times throughout the
winter, the BIO/WEST reports of Valdez and Masslich (1989) and Valdez and Cowdell (1996),
and the field survey made during the winter of 1996-97 as part of this project. A summary of the
ice observations from each source is presented below.
USGS Observations
The USGS ice observations were made in conjunction with discharge measurements at
the Jensen gage. The observations are summarized in Figure 8 (personal communication, Bryan
Cowdell, unpublished data obtained by Dr. Jack Schmidt, Utah State University, 1992) for the
' years 1946 through 1992. Shown are the first and last dates ice was observed each year at the
measurement location. The most striking feature of these data is the apparent decline of
observed ice in the years following the completion of Flaming Gorge Dam in October 1962. The
creation of the reservoir upstream of Flaming Gorge undoubtedly altered the temperature regime
of the river between the dam and Jensen. The main result of this alteration was to increase
winter water temperatures and reduce ice production downstream of Flaming Gorge for some
' distance. Unfortunately, it is very difficult to use these data to draw any quantitative results
because the data are confounded by the irregular measurement schedule of the USGS. For
example, in 11 of the 30 years following 1962, only one flow measurement was made during the
' winter. (These years are marked with a question mark.) There is no doubt that the ice observed
reflects the USGS schedule of discharge measurement as much as, if not more than, the actual
appearance of ice in the Green River at Jensen. For example, there was no ice indicated by the
' USGS in the winters of 1986-87 (Water Year 1987) and 1987-88 (Water Year 1988) even though
the reports of Valdez and Masslich (1989), discussed below, document extensive ice in the Green
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