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I I I I I I I I I I I I I I I I I I I HYDROLOGY: Montrose, Delta and Mesa Counties, Colorado find the maximum runoff caused by the rain alone. On the other hand, the snowmelt base flow typical of that stream is clearly not going to stop contributing to the total flow. Since the object of the analysis is to find the peak flow caused by a rainstorm over the particular area, arguments can be advanced for using the highest rain-caused peak no matter what the base flow. At the time of this study, considerable rainflood-snowme1t peak separation was being done on Colorado gaging station records by the Denver branch of the United States Geological Survey (U.S.G.S.). In order to arrive upon at least a consistency of results, if not a final resolution of the separation problem, the following guide- lines used by the U.S.G.S. for their work were adopted for this study. 1. During the season of obvious snowmelt, only a very large rise in gage height above the snowmelt diurnal fluctuations attri- butable to rain was called a rainflood peak. During the months of May and early June, rainf100d peaks are considered exceptions. That is; if the major portion of flow is snowmelt, the peak is called a snowmelt peak even if a small rainstorm is evident. 2. When the snowmelt runoff appears to have leveled off, even small rises in gage height attributable to rain are considered rainflood peaks. 3. "Attributable to rain" means there was evidence in the precipitation data to indicate rain had occurred, or a similar pattern of gage height rise occurred on several unconnected streams in the area on the same day, or a storm hydrograph pattern was unmistakable. Separation was performed by Meurer, Serafini and Meurer for gage stations in Region I. For Region II the separation done in 1977 by Camp, Dresser and McKee, Inc. of Denver, Colorado for the San Miguel County Flood Insurance Study was used. Tables 2 and 3 list the snowmelt and rainf100d peaks used. In Region I the density of gaging stations is much higher than in Region II, so a smaller area was selected to keep the results more specific for the study streams (see Table 1 and Figure 1 for the list and location of gage stations used). The history of each gage station in the area was examined. If the number of years of record was less than ten, or substantial regulation of the stream occurred, the station was not used. One exception is Surface Creek at and near Cedaredge. Although there is evidence of considerable regulation, the lengths of record are relatively long (35-40 years) and it was included, since it is actually one of the study streams. In some instances, -5-