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., OO!{57 oj physical conditions that are complex and extremely variable from year to year. The primary variables to be investigated to determine the extent of the ice jamming upstream of the Blue Mesa Reservoir were: o Blue Mesa Reservoir level, o Temperature, o. River flow amount, and o River shape and gradient. 1.3 Previous Investigations In 1979, the U.S. Bureau of Reclamation published the results of an investigative study of river icing in the re9ion upstream of Blue Mesa Reservoir, covering a six-year period. The study results were summarized in Techn ica 1 Report REC-ERC-79-4, Vii nter Ice Jams on the Gunni son River, by Phil H. Burgi, February 1979. There have been other investigations of general ice formation and icing action in rivers in other areas of the United States and Canada, but no other reports to our knowledge have been prepared on the particular icing conditions on the Gunnison River. 1.4 Historical Conditions and Icing Action The Gunnison Valley commonly experiences extremely cold temperatures during the wi nter months from mi d-November through mi d-March. The meteorol ogi ca 1 conditions in the valley are such that air movement is often minimal. This results in temperature inversions which accentuate the low temperatures accompanying the broad cold fronts that enter the region. Once a regional cold air mass has been introduced into the Gunnison Valley, it seems to persist longer than in nearby regions of western Colorado and thus presents conditions with longer and more severe cold periods. It is common in the Gunnison Valley for cold periods to be extreme and to be cold enough to cause substantial freezing of streams and rivers. The phenomenon of ice jamming and rapid upstream movement of the ice jams, however, has become much more promi nent since Bl ue Mesa Reservoi r fi rst filled in 1967. Long-time residents recall winters of much greater severity than those of -2-