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Frazil crystals immersed in supercooled conditions and high velocity are driven deep into the water column, and attach to submerged objects, forming anchor ice on the river bottom and associated structure, e.g., rocks, vegetation, bridge foundations, intake screens (Ashton 1979). Anchor ice is capable of transporting bottom materials, such as large stones, when buoyancy of the ice mass exceeds the weight or adhesive force of the object. Wilson et al. (1987) modeled instream temperature and ice formation processes, and predicted that construction of a two dam hydroelectric project on the Susitna River in Alaska would result in warmer than natural winter releases that would alter ice processes. This alteration would delay formation of an ice cover, and relocate the upstream end of the ice front. One dam would delay formation of an ice cap by 2 to 6 weeks, two dams would delay formation by 4 to 7 weeks. One dam would relocate the ice edge 16 to 47 km further downstream than under normal conditions. They also predicted that the most significant effect of project operations would be a change in timing of seasonal warming and cooling, with river temperatures warming later in summer and cooling later in fall than normal. Water Temperatures Associated With Ice Cover and Frazil Ice Water flowing immediately beneath ice cover or within Frazil ice usually does not vary more than a few hundredths of a degree from freezing, but can vary one or more degrees as a cooling gradient in proximity to ice masses. Temperature of flowing water under ice cover is usually constant, but a temperature gradient is often present near submerged ice masses, with supercooled water often mixed with jam ice or Frazil ice (Calkins 1984). Carlson et al. (1978) found that ice cover in northern rivers prevents both evaporative heat transfer and direct cooling, greatly reducing conduction-convection heat exchange with low- temperature air. Ice cover actually represents an equilibrium condition, and any excess heat added to the river will most likely result in open stretches of ice (Carlson et aL 1978). Growth Of Ice Cover The growth of ice cover results Gom heat loss to the atmosphere, and thickening of the cover is often related to accumulated temperature degree-days of freezing (along with a correction factor for geographic setting or insulating snow layers). Ice cover typically originates along the margins of streams and rivers, where slower water velocity and shoreline cohesion are favorable for formation. Growth of the ice cover and thickness can be predicted by using an equation involving factors responsible for surface ice formation (Calkins 1979). 33