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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />{} n 2 7l! ti <br /> <br />and the probable maximum thunderstorm (the probable maximum precipita- <br />tion or PMP)(15). <br /> <br />Historically, ice jams, rain on snow, and local thunderstorms <br />have caused the greatest flood damage in and around Rangely. Ice jams <br />are formed by ice floes piling up at some obstacle such as a bridge, <br />a river bank, or streambed irregularity. Ice blocks the channel, <br />causing the water surface elevation to rise and water to flood over <br />the banks (15). Documented ice floes date back to 1891 with flooding <br />occurring from ice jams once every seven to ten years. <br /> <br />Floods approaching the 100-year storm occurred in March 1955 <br />and February 1962. The flood for the 100-year storm is calculated by <br />the COE to be 19,400 cfs while that for the 500-year storm is 34,000 <br />cfs. The estimate of the PMP flood is 120,000 cfs (15). <br /> <br />I. Air Qua 1 Hy <br /> <br />Present sources of air pollution in the project area are: dust <br />from gravel and dirt surfaced roads; fumes from engines of vehicles <br />using roads in and through the area; smoke and fumes from stoves, fire- <br />places, furnaces, and campfires; smoke from occasional burning of trash; <br />and emanations from sagebrush,junipers, ragweed, and other vegetation. <br />In general, the air quality is good to excellent; in winter it is ex- <br />cellent to superior. <br /> <br />The service area around Rangely has greater air pollution than <br />does the reservoir area. The sources are similar to these for the <br />project area although the dust, smoke, and fumes are more profuse <br />because of more miles of gravel and dirt surfaced roads, more vehicles, <br />and more furnaces and fires. In addition, there is less cover on <br /> <br />II-26 <br /> <br />',-.-, <br />