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Filial Report 3-34 September 2000 canyons, the Price River joins the Green River but does little to affect water temperatures of the Green River. Downstream from its confluence with the Price River, the Green River enters a second large alluvial plain, where the city of Green River, Utah, is located. The river channel widens in this area, water velocity decreases, and water temperature increases slightly (Figure 3.14). The Green River continues southward from Green River, Utah; is joined by the San Rafael River; enters Stillwater Canyon; and then flows into the Colorado River. In this section, the increase in solar radiation is significant; day and night temperatures are higher and the river is warmer here than upstream. This increase is especially noticeable in the canyon area, where massive rock structures get warm during the day and reflect heat back to the air and water at night. This process moderates diel-temperature variation as the river meanders through the canyon. 3.5.2 Ice Conditions The formation of river ice covers reflects the meteorologic and hydrologic conditions of the region through which the river flows and the hydraulic conditions of the river channel itself. The water temperature represents the balance of heat transfer into and out of the river. Ice-cover formation is initiated when frazil ice forms in high-gradient portions of the river. Frazil ice is small crystals of ice that form when air that is colder than the freezing point of water supercools (i.e., reduces temperature to slightly below the freezing point) the surface layer of water, typically in turbulent sections (e.g., rapids) of the river. The frazil ice is transported downstream, until it reaches low-velocity areas, where it consolidates into a solid ice cover. The ice cover then builds in an upstream direction from this point as additional ice floes (floating masses of consolidated frazil ice) are transported by the current. The upstream point to which the ice cover will progress depends on the continued formation of frazil ice as a result of sub-zero air temperatures and the velocity of flows at the upstream edge of the ice cover. As the flow velocity increases, there is a greater tendency for ice floes arriving at the upstream end of the ice cover to be pushed underneath the ice cover and transported downstream. For additional information about ice formation, refer to Hayse et al. (2000). Breakup transforms an ice-covered river into an open river. Two types of breakup bracket those commonly found throughout most of North America. At one extreme is thermal meltout. During thermal meltout, the ice cover deteriorates as a result of warming and the absorption of solar radiation, and it melts in place, with no increase in flow and little or no ice movement. At the other extreme is the more complex and less understood mechanical breakup. Mechanical breakup requires no deterioration of the ice cover and results from an increase in flow entering the river. The increase in flow induces stresses in the ice cover, and these stresses, in turn, cause cracks and the ultimate fragmentation of the ice cover into pieces that are transported by the channel flow. Most river-ice breakups (including those in the Green River) actually fall somewhere between the extremes of thermal meltout and mechanical breakup, because the breakup usually occurs during warming periods, when the ice-cover strength deteriorates to some degree and the flow entering the river increases as a result of snowmelt or precipitation.