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<br />I <br />II <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 />II <br />I <br />I <br />I <br />I <br /> <br />HYDROLOGY: <br />Montrose, Delta and Mesa Counties, Colorado <br /> <br />find the maximum runoff caused by the rain alone. On the other <br />hand, the snowmelt base flow typical of that stream is clearly not <br />going to stop contributing to the total flow. Since the object of <br />the analysis is to find the peak flow caused by a rainstorm over the <br />particular area, arguments can be advanced for using the highest <br />rain-caused peak no matter what the base flow. <br /> <br />At the time of this study, considerable rainflood-snowmelt peak <br />separation was being done on Colorado gaging station records by the <br />Denver branch of the United States Geological Survey (U.S.G.S.). <br />In order to arrive upon at least a consistency of results, if not a <br />final resolution of the separation problem, the following guide- <br />lines used by the U.S.G.S. for their work were adopted for this <br />study. <br /> <br />1. During the season of obvious snowmelt, only a very large rise <br />in gage height above the snowmelt diurnal fluctuations attri- <br />butable to rain was called a rainflood peak. During the <br />months of May and early June, rainflood peaks are considered <br />exceptions. That is; if the major portion of flow is <br />snowmelt, the peak is called a snowmelt peak even if a small <br />rainstorm is evident. <br /> <br />2. When the snowmelt runoff appears to have leveled off, even <br />small rises in gage height attributable to rain are considered <br />rainflood peaks. <br /> <br />3. "Attributable to rain" means there was evidence in the <br />precipitation data to indicate rain had occurred, or a similar <br />pattern of gage height rise occurred on several unconnected <br />streams in the area on the same day, or a storm hydrograph <br />pattern was unmistakable. <br /> <br />Separation was performed by Meurer, Serafini and Meurer for gage <br />stations in Region I. For Region II the separation done in 1977 by <br />Camp, Dresser and McKee, Inc. of Denver, Colorado for the San <br />Miguel County Flood Insurance Study was used. Tables 2 and 3 list <br />the snowmelt and rainflood peaks used. In Region I the density of <br />gaging stations is much higher than in Region II, so a smaller area <br />was selected to keep the results more specific for the study <br />streams (see Table 1 and Figure 1 for the list and location of gage <br />stations used). The history of each gage station in the area was <br />examined. If the number of years of record was less than ten, or <br />substantial regulation of the stream occurred, the station was not <br />used. One exception is Surface Creek at and near Cedaredge. <br />Although there is evidence of considerable regulation, the lengths <br />of record are relatively long (35-40 years) and it was included, <br />since it is actually one of the study streams. In some instances, <br /> <br />-5- <br />