<br />
<br />METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS
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<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
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<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.
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<br />SEASONAL DISTRIBUTION OF FLOODS
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<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.
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<br />PRECIPITATION
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<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
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