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<br />usually occurred during the period from May to July at these elevations. Below <br />6,000 to 7,000 ft, streamflow records of the South Platte River and the Arkansas <br />River basins generally revealed an annual snowmelt hydrograph of shorter duration <br />that occurred earlier in the year. The amplitude of daily gage-height fluctuations <br />during snowmelt runoff was less pronounced at these lower-elevation stations than <br />at the higher-elevation stations. <br /> <br />Rainfall runoff from a convective storm or frontal system affects streamflow <br />much more quickly than does meltwater from the snowpack. Therefore, discharge <br />peaks produced by rainfall are less regular in occurrence than snowmelt-runoff <br />peaks. In the foothills region of Colorado, intense precipitation from convective <br />storms may produce flash floods with sudden increases in stage and discharge. <br />This type of runoff is identified in streamflow charts by an abrupt rise in stage. <br />Generally the streamflow peak lasts for only a few hours, and within a few days, <br />the discharge diminishes to the prestorm level, as shown in figure 6. Runoff peaks <br />of this type usually occur in late spring and summer when thunderstorm activity is <br />g rea tes t. <br /> <br />The rainfall hydrograph becomes complex when two or more precipitation events <br />occur within a short span of time, as shown in figure 6, or when intermittent rain <br />falling over several days results in a progressive increase in stage. Uncertainty <br />of the type of streamflow peak was usually resolved by consulting precipitation <br />and temperature records of nearby weather stations. This information is published <br />monthly for the State of Colorado by the U.S. National Oceanic and Atmospheric Ad- <br />ministration (1961-77). <br /> <br />Once peaks were determined for snowmelt runoff and rainfall runoff, the asso- <br />ciated discharges were computed by conventional means from the applicable rating <br />tables. When a snowmelt-runoff or rainfall-runoff peak was also determined to be <br />the annual runoff peak, data were used from published U.S. Geological Survey Water <br />Supply Papers or U.S. Geological Survey Water Resources Data for Colorado reports. <br /> <br />Unusua I S i tuat ions <br /> <br />A discretionary classification of streamflow records was required when peak <br />flows were caused by a combination of meteorologic factors. In a number of cases, <br />a rainfall-runoff peak occurred at a time when streamfiow included snowmelt run- <br />off. When the rainfall occurred early in the season during a period of large di- <br />urnal discharge, and if runoff contributed by rainfall was small relative to that <br />of the total streamflow, as shown in figure 7, gage height A, the resulting runoff <br />peak was classified a snowmelt-runoff peak. If the runoff contributed by rainfall <br />was large relative to the total streamflow, the peak was classified as a rainfall- <br />runoff peak, as shown in figure 7, gage height B, <br /> <br />In some cases, particularly at higher elevation stations, runoff from a rain- <br />fall event was a very small part of the total streamflow, but because it was the <br />only rainfall runoff recorded for the year, it was classified as the annual rain- <br />fall-runoff peak. <br /> <br />11 <br />