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<br />INTRODUCTION <br />As discussed with Mr. Don Soards on October 29, 1987, the U.S. <br />Army Corps of Engineers, Albuquerque District, is developing a <br />realtime flood warning system for Colorado Springs and Manitou <br />Springs, Colorado. Drainage basins whose streams flow through these <br />communities have been identified. A general area map is shown in <br />Figure 1. Rainfall gages have been installed in these basins that <br />relay rainfall amounts and in conjunction with a rainfall-runoff model <br />will provide warning as to expected flood magnitude and timing. Mr. <br />Soards, indicated that results from the report "Flood hydrology of <br />foothill and mountain streams in Colorado" (Jarrett, 1987), would be <br />used as an aid to design and develop the flood warning system. (This <br />report is presently in review as a U.S. Geological Survey professional <br />paper). Specifically the report demonstrates that there is an upper <br />elevation limit for significant rainfall-runoff flooding of about <br />7,500 feet in Colorado. The study also indicated that below about <br />7,500 feet in Colorado streams are subject to infrequent catastrophic <br />flash floods; whereas, above 7,500 feet snowmelt flows predominate and <br />are generally within stream banks. Many reported "floods" above 7,500 <br />feet were actually debris flows. The source of these flash floods is <br />intense cloudburst storms that have been shown to be limited to small <br />areas. Some of the streams that drain into Colorado Springs and <br />Manitou Springs originate up to elevations of 14,000 feet on Pikes <br />Peak. The Pike's Peak Massive rises abruptly from the plains and <br />flooding may differ from other mountain areas. Also, site specific <br />paleohydrologic investigations have not previously been done in theses <br />streams. Site specific investigations need to be done to determine if <br />elevation limits of significant rainfall differ from those developed <br />for the Arkansas River basin (Jarrett, 1987). These results can then <br />be used to determine the contributing drainage area in the new flood- <br />warning system. <br />Because of the importance of the flood-warning system, Robert <br />Jarrett conducted paleohydrologic investigations in the streams that <br />drain into the two communities on November 7-8, 1987 to provide site <br />specific information for input into the design of the flood-warning <br />system. Since the floOd-warning system is being developed during this <br />winter season for use next year, onsite investigations were made prior <br />to snowfall t?at would cover stream channels for the winter. The <br />purpose of these investigations were to (1) determine if the elevation <br />limit of 7,500 feet applies to these basins that drain off the eastern <br />flank of Pikes Peak and (2) roughly estimate the magnitude and <br />frequency of past flooding using paleohydrologic techniques. <br />Subsequent to the paleoflood investigations a review of flooding <br />history and review of available streamflow-gaging station records was <br />made. Two points need to be made (1) these identified paleofloods are <br />estimates at this time with an error estimate of about 50 percent and <br />the estimates of time of floods is approximate (detailed <br />paleohydrologic methods can improve the estimates of the number of <br />past floods, their magnitude, and age of occurrence) and (2) the <br />magnitude of floods varies dramatically over short distances <br /> <br />I. <br />