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/) .J nn~ ,e::s f; ~":"".i '=' GUNNISON RIVER ICING INVESTIGATION 1.0 INTRODUCTION AND BACKGROUND 1.1 General and Authorization During the past decade, the Gunnison River, just downstream from the City of Gunni son, has been forced from its banks by ice jams duri ng the wi nter season. In response to concerns by ranchers, homeowners, and other property owners along the river in this area, the Upper Gunnison River Water Conservancy District (UGRWCD) decided to fund and direct an investigative study of the river icing problem. The flooding in the winter of 1983-84, due to the ice j am movement, was one of the more severe in recent years. The hi gh snowmelt of the 1984 runoff season 1 ater increased the concern about potential flooding along the river between Gunnison and the Blue Mesa Reservoir. On November 14, 1984, the UGRWCD authorized Morrison-Knudsen Engineers, Inc. (MKE)* to conduct a study in accordance with a proposal submitted to the District earlier in the year. Soon thereafter, engineering staff of MKE began to observe and to document action of the icing process for the 1984-85 winter season. The movement of the ice jam in the 1984-85 season was unique in many respects, and caused some very serious fl oodi ng of developed areas. Thi s report documents the details of the 1984-85 winter icing action and presents the results of the overall evaluation of the Gunnison River icing problem. 1.2 Study Objective The objective of the study was to gain firsthand knowledge of the icing mechanics by making detailed in-the-field observations, to investigate the cause-and-effect relationships, to outline a predictive model of future icing, and to outline potential solutions for relieving the icing problem. It was recognized from the start of the study that the mechanism for the annual build-up of ice in the subject area is a result of a combination of *Morrison-Knudsen Engineers, Inc. operated under the name International Engineers, Inc. prior to January 1, 19B5. -1-