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not form in an area once a solid ice cover has been established because the ice cover insulates the <br />underlying water from sub-zero air temperatures and prevents supercooling. Soon after they are <br />formed, frazil ice crystals are a few tenths of a millimeter in diameter. Typically, frazil ice <br />crystals will be approximately uniformly distributed throughout the depth of the flow, especially <br />in highly turbulent, shallow reaches. The many rapids located in the high-gradient reach through <br />Split Mountain Canyon, just upstream of the study area, are particularly favorable for Brazil ice <br />production during cold weather. Following formation, Brazil ice is transported downstream by <br />the river current and evolves in form as it is transported (for a more complete description of this <br />process, see Daly 1994). Individual frazil crystals gather into larger and larger masses of ice <br />known as flocs. In areas with higher water velocities and turbulent flows, frazil ice can remain <br />mixed throughout much of the water column. Under less turbulent conditions, frazil flocs rise to <br />the water surface. Frazil slush is the collection of frazil flocs and individual Brazil crystals on the <br />water's surface in a distinct layer. Frazil slush at the water surface has a marked tendency to <br />clump together. The initial clumps, if they remain at the surface long enough, can further clump <br />together and form pans, or small floes. These pans often grind against one another, causing them <br />to become roughly circular in shape and gain upturned edges. At this point they are known as <br />pancake ice (photographs and additional descriptions of pancake ice in rivers can be seen in <br />Ashton [1986] and Beltaos [1995]). Frazil slush and floes can accumulate along or abrade the <br />edge of border ice, which forms along the banks of the river channel. It is common to see <br />parallel lines of raised frazil slush along the inside edge of border ice, marking periods when <br />Brazil ice accumulated along the border ice. <br />In slower moving areas of the river, such as regions behind islands or in the lee of <br />sandbars, where there is very little or no mixing due to the locally reduced velocity, the surface <br />of the water can cool sufficiently for ice crystals to form directly on the water surface. Ice that <br />forms directly on the water surface in areas with little or no flow velocity is said to result from <br />static ice formation. This type of ice is also formed on lakes and ponds during periods of low <br />winds. Generally the surface flow velocity must be approximately 1 foot per second or less for <br />static ice to form. Static ice formation starts in a very thin layer of supercooled water at the <br />water surface, and is probably initiated by the introduction of seed ice crystals from the air. Ice <br />thickens as a result of continued ice formation at the ice/water interface as heat is transferred <br />from the ice/water interface through the ice and into the atmosphere. <br />The formation of a stable riverine ice cover results from the interaction between the <br />transported ice pieces and the flowing water. In this case the cover is said to form dynamically. <br />Ice covers that form dynamically progress in an upstream direction from an initiation point as ice <br />is transported to and deposited at the leading edge (upstream edge) of the ice cover by the flow <br />of the river. The actual process that occurs at the leading edge depends on the hydraulic flow <br />conditions and the form of the arriving ice. The processes at the leading edge are described in <br />general below in an order which reflects the relative flow velocity at which they occur, from the <br />lowest flow velocity to the highest. However, it is more common to refer to the non-dimensional <br />-3-