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II. STOR16 AND FLOOD CHARACTERISTICS <br />A. GENERAL CONSIDERATIONS <br />32. General. The flow of Bear Creek is extremely erratic. The <br />normal or base flow is derived largely from snow -melt in the high eleva- <br />tion headwater area; and it reaches its peak during May and June. During <br />the months of June, July, and August the portion of the basin between <br />Morrison and Evergreen is subject to cloudburst storms which result in <br />flash floods on the main stem and tributaries in this area. It is note- <br />worthy that there is no evidence of past floods on the main stem at ele <br />vations of over 8,000 feet, <br />33. The extremely rugged topography and the high intensity of rain- <br />fall, cause rapid concentration of run -off with very high peak rates of <br />flow. Intense rainfall from past storms in this basin has been limited <br />to small areas, seldom exceeding 20 or 25 square miles, and as the volume <br />of flood run -off is comparatively small, flood peaks are quickly moder- <br />ated downstream by the action of channel and overbank storage. <br />34. Storm rainfall in the basin is largely orographic in character. <br />The principal source of precipitable moisture is Tropical i/laritime air <br />masses which originate over the Gulf of Mexico and break out northward <br />over the Great Plains. When these warm, moisture -laden air masses are <br />deflected upward by the abrupt uplift of the foothills of the Front Range, <br />cooling and rapid condensation of moisture result. The degree of mois <br />ture saturation and instability of the air mass and its rate of ascent <br />or overturning determine the intensity of the rainfall. <br />35. Cloudburst storms of eastern Colorado,- Previous studies of <br />cloudbursts in Colorado show that most storms of this type have occurred <br />within 50 miles of the eastern slope of the Rocky Mountains and at eleva- <br />tions of from 6,000 to 7,000 feet. These storms are characterized by <br />intense rainfall for a short period and are usually confined to a rela- <br />- 11 - <br />