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<br />compiled and incorporated into the data set (eg., McCain and others, 1979; Jarrett and Costa, 1986).
<br />These streams have drainage areas that range from less than an acre to several thousand square mile
<br />basins (Colorado River at Glenwood Springs; South Platte River north of Denver), Stream gradient at
<br />these sites ranges from about 0.001 to 0.2 fIIft. Sediments on the surveyed flood bars ranged in size from
<br />sand-size particles to boulders greater than 3 feet in diameter. The results of these surveys clearly show
<br />that the maximum elevation of the top of all new flood bars generally equals the 1995 HWM elevations
<br />(figures 4a and 4b). Exceptions were some streams having gradients larger than about 0.04 fIIft (studies
<br />are in progress to define a relation) where the top of the new flood bar protruded above the HWMs
<br />(figures 4c). Thus, use of the elevation of the top of flood bars located in the Cimarron River provides a
<br />hydraulically reliable and accurate indication of the maximum depth and estimation of discharge for the
<br />paleofloods.
<br />REGIONAL HYDROLOGIC INVESTIGATIONS
<br />
<br />To complement the onsite paleoflood investigations, streamflow-gaging station data for all nearby
<br />streams that were in operation in 1952 were retrieved. Annual peak-flow data for 18 stations located
<br />within about 50 miles of Cimarron were retrieved from the U.S. Geological Survey WATSTORE data
<br />base (Lepkin and others, 1979). Methods to discem snowmelt runoff from rainfall runoff are summarized
<br />by Elliott and others (1982) and Jarrett (1987b, in press), A typical snowmelt runoff hydrograph with a
<br />diumal response to daily warming is shown in figure 5, Rainfall runoff, particularly from localized
<br />convective (thunderstorm) rainfall, has a relatively rapid rise and fall. The peak of convective rainfall-
<br />runoff events generally occurs within 1 hour (eg" figures 6,7, and 8) from the start of the rise of the
<br />hydrograph and intense rainfall. For example, the June 3rd, 1952 rainstorm began at approximately 3 pm
<br />and ended at 4:30 pm (MDT). The rise in the stage hydrograph began about 3:45 pm and peaked at
<br />about 4:30 pm (MDT). Frontal rainstorms generally have much longer responses times, depending on the
<br />intensity, duration, and spatial coverage of rainfall and a corresponding smaller net peak rainfall runoff
<br />(Jarrett, 1987b).
<br />
<br />Only one station, Leroux Creek near Cedaredge (09134500), which is located about 50 miles
<br />northwest of Cimarron, had its annual peak coincident with the June 3rd rainfall. Original gage-height
<br />charts for Leroux Creek were retrieved from Federal Archives and Record Center at the Denver Federal
<br />Center. That combined peak rain-on-snowmelt peak was 815 cubic feet per second, which occurred on
<br />June 4, 1952 (figure 6), The Leroux Creek streamflow-gaging station was located at an elevation of
<br />7,255 feet and had a drainage area of 34,5 square miles, The annual peak occurred during maximum
<br />annual snowmelt runoff. Only 0,12 inches of rainfall was measured in the US Weather Bureau gage
<br />located at Cedaredge on June 4th, 1952. Antecedent diurnal-snowmelt runoff prior to the rainstorm was
<br />about 650 cubic feet per second. The net peak rainfall runoff was about 165 cubic feet per second or
<br />about 25 percent of antecedent flow and was the largest during 1952. This small net rainfall runoff shows
<br />that little rainfall occurred in the basin. Rainfall runoff of about 125 cubic feet per second occurred on June
<br />5th and 6th. No rainfall was reported at Cedaredge on June 5th or 6th, which indicated rainfall occurred in
<br />the basin but not at the Cedaredge raingage. The maximum annual-peak flow in Leroux Creek is 1,310
<br />cubic feet per second, which occurred on May 26,1942, during the 29 years of gage operation (1937-
<br />1969).
<br />
<br />Three streamflow-gaging stations have been operated on the Cimarron River since 1904. One gage,
<br />Cimarron River below Squaw Creek near Cimarron (USGS station number 09127000), was operated for
<br />10 years from 1943 to 1952. The gage was located several hundred yards downstream from Cimarron at
<br />an elevation of 6,864 feet and the drainage area was 229 square miles. The annual peak discharge during
<br />1952 was 1,840 cubic feet per second, which occurred on June 11 (figure 5). The 1952 peak flow also
<br />was the maximum recorded at this gage, The original gage-height charts also were retrieved from the
<br />National Archives and Records Center, The gage-height records show little rainfall runoff on June 3rd
<br />(fillures 7) nor on any other day during 1952. Antecedent snowmelt runoff prior to the June 3rd, 1952
<br />rainstorm was about 700 cubic feet per second. The composite snowmelt- and rainfall-runoff on June 3rd
<br />was about no cubic feet per second, There were rating-curve shifts applied to compute discharge
<br />records for most of 1952 ranging from -0.05 to +0.05 and indicating a relatively stable rating curve, Shift
<br />corrections were not deemed necessary in this analysis, particularly as the net rainfall runoff discharge
<br />calculation would not be affected as the shifts were constant throughout the rainfall-runoff event. The net
<br />rainfall-runoff peak discharge was about 70 cubic feet per second or 9 percent of the antecedent snowmelt
<br />runoff. A USGS hydrographer, who serviced this gage, was in Cimarron on June 4 and 5,1952 and
<br />noted that the gage was operating correctly. The hydrographer made no remarks of a rainfall event on the
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