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<br />en "10 <br />::::l <br />en <br />....J <br />UJ <br />o <br />en <br />UJ <br />UJ <br />a: <br />CJ <br />UJ <br />Cl <br /> <br />a <br /> <br /> <br />'" c::._ <br /> <br />c::> ~ c::: <br /> <br />oJ _'" <br /> <br />~~- <br /> <br />rvI__ <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The mild winter in north-eastern Utah resulted in slow cooling of water temperatures in the Green River. <br />Water temperatures at the USGS gage near Jensen, UT did not reach freezing until February 2. Temperatures <br />below 1.0 DC lasted for only 4 days before beginning to warm. Daily water temperatures (daily instantaneous <br />values) for the Green River near Jensen during the winter of 1993-94 are plotted in Figure 5. <br /> <br />Diel changes in main-channel water temperature during the first ice measurement trip (January 31 - February <br />4) typically ranged from -0.05 to 0.30De. Green River water temperatures in each reach were similar with a <br />range of -0.05 to 0.32De in Reach 1, -0.10 to 0.38De in Reach 2, and 0.4 to o.soe in Reach 3. No significant <br />differences water temperatures were noted between reaches. During the second winter ice measurement <br />(February 21-24) water temperatures were warmer, ranging from 0.62 to 3.66De during the trip. <br /> <br />"1::2 <br /> <br />e <br /> <br />.... <br /> <br />::2 <br /> <br />o <br /> <br />Figure 5. <br /> <br />Water temperature in the Green River, 1993-94, as recorded at the USGS stream gage near <br />Jensen, UT. Temperatures are daily instantaneous values. <br /> <br />5.2 ICE CONDmONS <br /> <br />Due to the warm winter, there was very little ice formation on the Green River between Island Park and <br />Ouray, UT. Ice formation began the morning of January 30 and continued for approximately 5 days. After <br />this short cold period, air and water temperatures steadily increased, allowing no additional ice formation <br />during the year. This period of ice formation was much shorter than observed by Valdez and Masslich (1990) <br />during the winters of 1986-87 and 1987-88. Figure 6 illustrates the beginning and duration of river ice <br />formation overlaid on river water temperatures and minimum daily air temperatures recorded near Jensen. <br />UT for winters 1986-87, 1987-88 and 1993-94. <br /> <br />5.2.1 Frazil Ice <br /> <br />Frazil ice formed on the Green River during two short periods. Frazil ice was first observed during the week <br />of January 31 through February 4. During this period, frazil ice was formed in the upper sections of the study <br />area, principally in Whirlpool and Split Mountain canyons. In the lower, slow water sections below Dinosaur <br />National Monument, the suspended frazil ice floated to tbe surface forming large mats. These mats, termed <br />'lily-pad ice', were typically 5 to 100ft in diameter and 1 to 1.5-ft thick. Lily-pad ice was thick, covering most <br />of the river surface in the Jensen area. Frazil ice production stopped as air temperatures warmed leaving <br />Reacbes 1 and 2 clear of frazil and lily-pad ice by 1400 hrs. <br /> <br />8 <br />