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. DRAFT 4.2.3 Ice ConcH tions . Extremely cold condi tions during most of the 1987-88 winter resulted in extensive ice formation throughout the study area. Conceptualized diagrams of ice formation are presented in Figures 3 and 4 to illustrate the types of ice observed. The following is a description of the ice conditions during the 1987-88 winter: . 1. Solid surface ice. Solid surface ice formed over a major portion of the study area during the 1987-88 winter. This ice was interspersed with jam ice below Split Mountain, from approximately RM 310 to 305 (Figure 3). Downstream of RM 305, solid ice ranging from 0.8 to 2.5 feet thick covered the entire river from late December through mid February. Small open leads of water in the ice, termed "air holes" by local residents, were present at scattered locations along the thalwag. . . Throughout the 1987-88 winter, several observations were made on the condition of the surface ice as it reached its maximum development. During mid January it was noted that the ice covering the river appeared to sag toward the middle of the channel (Figure 3). This sagging was most apparent between RM 310 and 300 near Jensen, but was noticable through most of the study area. In extreme cases, the angle of the shoreline ice was 25 to 30 degree from horizontal to a distances of 15 to 30 feet from the bank. At the point where the ice began to level out across the channel, there were often long cracks running parallel to the shoreline. Water seeping through these cracks froze atop the existing ice causing ice to thicken in these areas. It is speculated that this sagging and cracking is related to fluctuating water levels from Flaming Gorge Dam. . . Cracks in the ice were also present in the channel further from shore. Generally, orientation of these cracks was also parallel to the shoreline. Seepage of water through these midchannel cracks was not apparent until mid February when, at several locations, extensive areas of seepage created a layered effect. In these areas, the original ice layer was covered with 2 to 4 inches of free water which was then covered by a second ice layer 1 to 2 inches thick. It was again speculated that this seepage is related to flucuating water levels which increase the water pressure under the ice and force free water to the surface. . Beginning in about mid February, increased water temperatures began to erode the ice cover. This process continued for the remainder of February, eroding the ice from the upper end of the ice pack downstream at a rate of 5 to 7 miles per day. This erosion resulted in a ponding effect at the upper end of the ice pack which moved progressively downstream. . 2. Jam river ice. Jam river ice was prevalent in the 1987-88 winter, particularily between RM 316 and 307 (Figure 4). From RM 316 upstream to RM 318, large stacks of ice up to 12 feet high were piled along both shorelines, indicating that short-lived ice jams were present further upriver than actually . 18 .