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to be affecting water velocities. Large masses of frazil ice were often transported downstream and became entrapped beneath the jam ice in an 11-mile (17.7 km) reach between Chew Bridge and Brush Creek. Large accumulations of frazil ice below Chew Bridge ponded the river at the upper edge of the ice pack, and slowed and restricted river flow. Numerous holes drilled through the ice revealed that frazil ice was jammed beneath the surface ice, forcing the river to flow through a series of channels. Jam ice was prevalent from Chew Bridge (RM 316) to about the Bonanza Bridge (RM 290), and smooth surface ice prevailed downstream. The jam ice was characterized by a highly fractured and irregular surface, and varied in thickness from 50 to 70 cm. Ice breakup in the Green River in Winter 2 began in mid-February, when increased water temperatures began to erode the surface ice. This warming melted channel and shoreline ice in Rain- bow Park, and the edge of the ice pack shifted from Chew Bridge (RM 316.3) on February 2, to Jensen Bridge (RM 301.8) on February 19, to Walker Hollow (RM 293.8) by February 27. This melting rate of about 1.5 km per day continued until the entire ice pack weakened sufficiently to become transported simultaneously downstream in a major spring breakup during early March. The chronology of ice development and subsequent frazil ice entrapment is illustrated in cross-section in Figure 9. The solid ice that developed in late December quickly became fractured and jammed because of continuous flow fluctuations. Through January and February, this ice cover entrapped frazil ice and chunks of surface ice dislodged from upstream. This entrapment extended for about 11 miles from the edge of the ice pack, or the distance traveled by frazil ice and ice chunks beneath the surface ice. Ice profiles revealed about 0.5 m of surface ice with accumulations of frazil ice ranging from 0.3 to 3 m thick. Yampa River Hydrology Average monthly Mows of the Yampa River were higher in Winter 1 than in Winter 2 (Figure 10), although volume was higher in water year 1988 (October 1, 1987 -September 30, 1988). Total volume was 814,500 acre-feet in water year 1987, and 912,300 acre-feet in water year 1988 (USGS 1987, 1988). For water year 1987 (October 1, 1986 -September 30, 1987), mean, maximum, and minimum daily discharges near IVlaybcll were 1,125; 5,970, and 124 cls (32, 169, and 3.5 ems), respectively. For water year 1988 (October 1, 1987 -September 30, 1988), mean, maximum, and minimum daily discharges were 1,257; 9,800; and 37 cfs (36, 278, and 1.0 ems), respectively. 15