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<br /> <br />METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS <br /> <br />as 3,000 feet above the valleys (fig. 3). Soils, where pre. <br />sent, are shallow, consisting of coarse material derived <br />from both alluvial processes and from slope wash or <br />colluvial processes. Soils generally grade from very <br />gravelly and stoney near the ridges to a sandy to <br />gravelly assortment near stream levels. Permeability <br />of soils is rapid, ranging from 6 to 20 inches per hour <br />with available water capacity generally less than 0.10 <br />inch per inch. Soils are excessively drained with rapid <br />runoff potential and are highly susceptible to severe <br />erosion. North-facing slopes have a much denser forest <br />cover than south-facing slopes, with Ponderosa pine <br />most abundant at lower altitudes and Douglas-fir <br />predominant near the mountain tops. Grasses and <br />shrubs fill the open spaces between trees, being more <br />abundant on south-facing slopes than on the denser <br />forested north-facing slopes. Under the trees, the <br />vegetation is sparse and much of the ground surface is <br />exposed. Abundant growths of cottonwood, willow, <br />and birch occur along the stream valleys where they <br />are highly susceptible to the erosive process of the <br />streams. <br />The Big Thompson and the Cache la Poudre Rivers <br />head near the same point on the Continental Divide at <br />an altitude of about 11,000 feet. The altitude of the <br />area from which the flood derived ranges from about <br />9,000 feet to about 7,500 feet as shown in figure 4. On <br />both the Big Thompson and the North Fork Big <br />Thompson Rivers, the western limit of flooding occur- <br />red at an altitude of about 7,500 feet just west of Estes <br />Park and Glen Haven, respectively. Downstream, the <br />altitudes along the Big Thompson River range from <br />6,140 feet at Drake to 5,300 feet at the canyon mouth <br />and 4,670 feet at the confluence with the South Platte <br />River. Tributaries in the Big Thompson River basin <br />near the storm center west of Drake range in altitude <br />from about 7,000 feet to about 9,000 feet along the <br />ridges. An area-altitude relation for the approximate <br />storm area of 53 square miles in the Big Thompson <br />River basin upstream from Drake is shown in figure 5. <br />About 64 percent, or 33.5 square miles, lies in the <br />range of 7,500-8,500 feet while the area above 8,500 <br />feet comprises only 8.5 square miles, or about 16 per- <br />cent of the total storm area in the Big Thompson River <br />basin. <br />Streambed gradients along the Big Thompson River <br />average about 107 feet per mile in the canyon reach <br />and about 10 feet per mile near the mouth at LaSalle. <br />On the North Fork Big Thompson River, the average <br />streambed gradient is 128 feet per mile in the reach <br />between Glen Haven and Drake. Most of the small <br />tributaries west of Drake are extremely steep with <br />streambed gradients as much as 700 feet per mile. <br />In the Cache la Poudre River basin, altitudes along <br /> <br />I <br /> <br />5 <br /> <br />the main stem are about 5,700 feet near the western <br />limit of flooding at Poudre Park, 5,240 feet at the <br />canyon mouth, and 4,610 feet at the mouth near <br />Greeley. The streambed gradient from Poudre Park to <br />the canyon mouth is about 46 feet per mile and about 9 <br />feet per mile near the mouth. Altitudes along the <br />North Fork Cache la Poudre River range from about <br />8,000 feet near the Wyoming border to 5,360 feet at the <br />mouth. Streambed gradients on the North Fork are <br />about 48 feet per mile in the northern part of the flood <br />area and 43 feet per mile near the mouth. In the vicini- <br />ty of Bellvue, small tributaries of the Cache la Poudre <br />River head at about 8,000 feet. These small streams <br />are fairly steep, averaging about 330 feet per mile. <br /> <br />SEASONAL DISTRIBUTION OF FLOODS <br /> <br />Three types of floods occur in the Colorado Front <br />Range: snowmelt floods, floods produced by a com- <br />bination of rain on snow, and rainfall floods. Snowmelt <br />floods predominantly occur during May and June of <br />each year and usually cause little or no damage. In <br />fact, this type of runoff is usually welcomed as it is <br />stored in off-channel reservoirs and provides a water <br />supply during the dry summer months. Occasionally, <br />low-intensity rainfall associated with frontal activity <br />occurs over large areas of the Front Range hastening <br />the snowmelt and producing severe flooding, especially <br />on large streams. The third type, into which classifica- <br />tion the July 31,1976 flood falls, is the flash flood pro- <br />duced by convective thunderstorms usually during the <br />months of June, July, and August. Rainfall associated <br />with this type of flooding is very intense and occurs in <br />short periods. Surface runoff rapidly concentrates in <br />nearby channels and flash flooding occurs in <br />downstream areas. Both overland and stream <br />velocities are swift, causing severe erosion along <br />hillsides and in streams. Property damage is usually <br />high and fatalities frequently occur. The short period <br />of time between the intense rainfall and flash flooding <br />frequently precludes advance warning to downstream <br />areas. Often associated with this type of flood is the <br />reported "virtual wall of water." In almost all aspects, <br />the flash flood is the most dangerous of the three types <br />of floods. <br /> <br />PRECIPITATION <br /> <br />A smoothed analysis of the average precipitation oc- <br />curring during July in Colorado is shown in figure 6 <br />(National Oceanic and Atmospheric Administration, <br />1973). Typically, summer precipitation in north- <br />eastern Colorado is light and comes from afternoon <br />and early evening thunderstorms that form over the <br />mountains and move eastward over the plains. While <br />