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LITERATURE REVIEW OF WINTER INVESTIGATIONS Ice Processes Ice processes in rivers and streams have been documented in numerous studies. These describe and characterize various physical aspects of ice on bridges and other manmade structures, but few report biological responses (Shumway and Springer 1992). Studies descnbing effects of ice formation and processes on fish and their environment deal mainly with game species, primarily anadromous and freshwater fishes found in more northern latitudes than the Colorado River. Understanding the effect of ice on habitat, movement, survival, feeding, and growth of these northern species could provide insight into expected responses by fishes in more southern, high elevation desert rivers, such as the Green River and Yampa River. Ice Formation Commonly recognized forms of ice in medium-sized rivers are surface ice, &azil ice, anchor ice, and snow or slush ice. Each is distinguished by characteristic form and origin, and it is common to find all four forms present at the same time. Many environmental factors affect formation and occurrence of these ice forms, and involve complex processes that are strongly interrelated and dependent on each other (Shumway and Springer 1992). Ice begins to form when water is cooled to 0°C. Water in motion may not freeze at 0°C, but continues to lose heat to the atmosphere, and becomes supercooled before crystallizing. Supercooled river water is usually not more than a few hundredths of a degree below 0°C, although air temperature necessary for supercooling must usually be below -10°C, depending on water purity (Ashton 1979). Ice crystals first form near the surface, where cold air enhances supercooling. Natural river turbulence may blend ice crystals into the water column, but when velocity is low, the crystals adhere to form a thin surface sheet of ice that may thicken to form an ice cover (Ashton 1980). At higher velocities, characteristic of most rivers, mixing supercools water throughout the water column, and drives small ice crystals deep into the flow. These small ice crystals, called frazil, are usually disc-shaped and a few millimeters in diameter, but may be irregular platlets with diameters of 10 to 15 mm. Essential conditions for frazil ice formation are an air temperature of approximately -10°C or colder, and flowing water supercooled to about -0.O1 to -0.1°C (Ashton 1983). With low flow, frazil ice forms a thin surface sheet that evolves into a surface cover. Water velocity greater than about 0.6 m/sec prevents immediate formation of surface ice, and frazil ice becomes mixed in the water column (Osterkamp 1978, Ashton 1988). 32