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<br />". <br /> <br />n01463 <br /> <br />In situations when air temperatures are sustained over a period of two or <br />three days at below OOF, and where a river reach is quite turbulent, the <br />system becomes a massive ice-making machine. <br /> <br />In addition to the temperature, wind, and degree of turbulence in the <br />flowing river, there is another major factor that controls the amount of <br />frazilice produced; that is the amount of ice cover in place at the time. <br />If there has been previous cold weather where surface freezing from the <br />s ides has formed a part ia 1 ice cover over the river, then the surface area <br />of river exposed to the air is reduced. The resulting partial insulation <br />will reduce the heat transfer from the water and, even with extremely cold <br />temperatures, can reduce the amount of frazil ice produced. If the ice <br />cover completely extends over the river surface, very little, if any, frazil <br />ice will be produced under the cover. <br /> <br />Early in the winter season, before any major ice covering has formed, a <br />condition of subzero temperatures with a fully exposed open river surface <br />will therefore result in maximized production of frazil. <br /> <br />2.4 Ice Jams <br />Ice jams occur on rivers in cold regions by a variety of actions. In some <br />cases, the jamming is more prevalent during the early stages of the spring <br />thaw. The late-season jamming occurs as a result of ice blocks and ice <br />sheets from river cover breaking up and floating downstream. <br /> <br />Another form of ice jam is of the type observed in recent years on the <br />Gunnison River. Where a section of the river or a reservoir has formed a <br />complete ice cover and where there is an open water section located <br />upstream, then there is the potential for frazil ice to accumulate against <br />this face and cause ice jamming. In this situation, when the temperatures <br />get low enough and the turbulent conditions are right, the head of the ice <br />jam will accumulate the frazil ice, continue to freeze the compacted frazil, <br />and the head of the jam will move progressively upstream. <br /> <br />-8- <br />