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<br />/ <br />. 1" <br /> <br />STATE OF COLORADO <br /> <br />tit <br /> <br />MEMORANDUM <br /> <br />. <br /> <br />COLORADO WATER CONSERVATION BOARD <br />Department of Natural Resources <br /> <br />721 Stale Centennial Building <br />1313 Shennan Street <br />Denver. Colorado 80203 <br />Phone (303) 866-3441 <br />FAX (303) 86&-4474 <br /> <br />Roy Romer <br />Governor <br /> <br />TO: <br /> <br />Ken Salazar <br />Execullve Director, DNR <br /> <br />Mark Sawyer <br /> <br />Brian HYde~~ tf <br />DATE: February 2, 1993 <br /> <br />FROM: <br /> <br />Caries c. LIIe, P.E. <br />Dlreaor. CWCS <br /> <br />SUBJECT': Distribution of Clear Creek Basin Area by Altitude <br /> <br />e <br /> <br />In an attempt to discern what independent parameters might lead to differences in <br />peak flow per square mile of drainage area in the three major branches of Clear Creek <br />above the Lawson gauge I've looked at the distribution of drainage area by elevation. I <br />thought that perhaps a subbasin with a greater 'proportion of its drainage area between <br />10,000' and 11,000' would generate more runoff per square mile than one with more of its <br />drainage area either above or below that particular elevation range. The thought is that the <br />area below 10,000' is too low to contribute to snowmelt flooding and the area above 11,000' <br />or 11,500' is too high (too cold). Note that this all presumes that the 100-year event is caused <br />by snow and not rain. If rain is the cause of floods, then my reasoning would have to be <br />modified. Even after a historical search it is hard to say what happened in June of 1965. Was <br />it just a rainfall flood, or was it a rain-on-snow flood? What was the base flow before it rained, <br />what Was the distribution of the rain by elevation, did the rain melt a lot of snow and lead to <br />a hybrid kind of flood? <br /> <br />My presumption is that somewhere above 8500' or 9000' it pecomes less likely that <br />a flood-producing rainstorm of significant geographic extent will occur. That means that <br />above a gauge like Lawson (8075' in elevation) rain can still.cause floods, but somewhere <br />between Georgetown (8500') and Silver Plume (9100') snow becomes the primary cause of <br />flooding. I am not suggesting that above a certain elevation it suddenly stops raining <br />enough to cause floods. I am suggesting that as you go higher (after some threshold <br />elevation like 8500' to 9000'), the peak rainfall you can expect decreases. Certainly such a <br />generalization should be tempered by considering the importance of local or regional <br />topographic features and their orographic effects. In any event, my point is that I was <br />looking at the possibility that a subbasin with more territory in the elevation range from <br />10,000' to 11,000' might produce more runoff than a similar subbasin with less territory in <br />that elevation range. <br /> <br />_I <br /> <br />For each table there are 5 locations on the stream system for which I calculated <br />drainage areas. At each of those 5 locations I calculated drainage areas for 5 elevation <br />