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27 IIlixing of the water which eventually reaches ita grenest dens1t" at near 4 C. In the presence of decreasing ilt'r'temperature th.a surface water r~aches its freezing point of 0 C and ice is formed initialllr in' leas tUI'bulent areas, especially along the streambanks (hinge ice) ~lnd in slow and slack water area. (surface 1ce). Continued freeling often te.ults 1n . ~ildup of ice, called hinge ice that extends toward the stnaa center. As" , turbulence is decressed by the hinge ice, surface ic., foru. Thus thls less dense surface and hinge ice at'() C tllnd to fora -~,i!rtial.pr~Jul.l.,1ce cover that prevents further cooling of thE! heavier 4 C 'bOttom w.tn~"~' , The breakup of ice cover in the spring gene raIl), follows a I'ise in temperature and melting of the seasonal snowpack. A significant increase in stream discharge causes cracking of the ice from "ertical hydrostatic pressures, and the resultant blocks and plates of iCI' are carried downstream as ice floes. The movement of these floell tends to be erratic and interlllittent resulting in periodic dallll1ll1ng and flooding of low areas near the channel, extensive gouging an,d mechanical erosion of the channel banks, and sedimentation and redistribution of botto.. gravels. Underwater Ice Frazil Ice (slush). Frazil ice is one of two fc>rms of underwater ice that are closely related. Small crystals called fra.z:ll ice or slush sometimes form in flowing water streams regardless of' turbulence. It is caused by radiant loss of heat from the dark-colored streambed under water at 0 C or when snow flakes become entrained (i.e., sUlspended in water). Reports of air temperatures needed to cause frazil ice formation vary from about minus 16 to minus 23 C (Benson 1955, Needham and Jones 1959). The whole sequence may occur night after night following long periods of low precipitation, clear skies facilitating maximum radia,nt heat loss from the water and land and air temperatures below freezing (0 C). Water at 0 C, upon losing 80 calories per gram, is said 1:0 be supercooled and is initially transformed into the floating frazil or slush ice. Frazil ice may form in shallow or deep water and its presence at all levels of the water column often results in scouring of the streambed and banks. Anchor Ice. Anchor ice is bottom ice that forms from accumulation of frazil ice. It characteristically occurs on riffle and shallow w,ater areas. As frazil ice drifts along in the current and makes contact with the substrate it becomes attached. The ac(~umulation of ice particles on the bottom forms a mat that becomes more dense and more transparent. and that seelllingly defies the principle that ice is lighter than 4 C l{ater. Nevertheless, the continued buildup of anchor ice can blanket the stream bottom, impede water flow, cause flooding and significantly alter stream habitat. Anchor ice may be very destructive to aquatic life and, if frequent, lower stream productivity even though the stream may ha"e high fertility, adequate cover and water flow. Heavy mortalities have been observed in benthic communities and in fish populatIons. StreallS'subjected to frequent anchor ice formation tend to be low in productivity.