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<br /> <br />. <br /> <br />e <br /> <br />./ <br /> <br /> <br />Mark Sawyer <br />February 2, 1993 <br /> <br />DRAINAGE AREA ABOVE 12,000' <br /> <br />With regard to territory above U,OOO' there is a clear trend. The South Oear Creek <br />subbasin has a greater proportion of its area in that higher elevation range when it is <br />compared to the other two subbasins. The West Clear Creek subbasin has a smaller <br />proportion of its area in that higher elevation range when it is compared to the other two <br />subbasins. That leaves the mainstem of Clear Creek in the middle. Even though there is <br />a difference between the three subbasins in this elevation range, it probably does not <br />significantly affect flood hydrology. That is because we assume that it is so high that it is too <br />cold for snow to melt rapidly. Being above treeline and more exposed to the wind, it may <br />be too high for snow to stay in large amounts until flood season. <br /> <br />DRAINAGE AREA BE1WEEN 10,000' AND 11,000' <br /> <br />I'll turn now to the elevation range that does appear to have an effect on snowmelt <br />hydrology, the range from 10,000' to 11,000'. There is a difference between the subbasins <br />in terms of the area of each that falls in the 10,000' - 11,000' range. South Clear Creek has. <br />the lowest percentage of area in that range (24% of the total area), and West Clear Creek <br />has the highest percentage (43% of the total area). The mainstem falls in the middle <br />(29%). By my initial reasoning and assuming all other hydrologic factors are equal, the <br />South Clear Creek subbasin should generate the least runoff per square mile and the West <br />Clear Creek subbasin should generate the most. <br /> <br />OTHER CONSIDERATIONS <br /> <br />There are obviously other factors that will affect the peak runoff from these three <br />subbasins. I know you and I looked at the isohyetals for Clear Creek (revisions to the <br />NOAA Atlas) and I know we talked about differences in the geology of the subbasins. We <br />did not talk about any possible differences in the snowfall in the three subbasins; we did not <br />really talk about any differences due to the south- north orientation of the South Clear <br />Creek valley as compared to the west-east orientation of the other two valleys. There does <br />not appear to be much difference in the shape of the three subbasins. I don't know how we <br />put all of those pieces together to arrive at a figure for the runoff from each subbasin. I <br />don't know whether the 1965 flood flow at Lawson was the result of an anomalous situation <br />or whether it was "typical" of a 100-year flood. Maybe there's a way to reconstruct what <br />happened in each subbasin that year and use that picture to guide us. <br /> <br />I don't know that this work helped to answer any of our questions. I don't even know <br />whether our questions can be answered with the current lack of data. Perhaps we can strive <br />for a better-educated guess. Give me a call after you have reviewed this. <br /> <br />4 <br />