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<br />. <br /> <br />30th storm in Fortification Creek at the upstream city limit was determined by using the slope-conveyance <br />method (Chow, 1959; Barnes and Davidian, 1978). Numerous high-watermarks (HWMs) consisting of <br />fine woody debris and silt line are located on both banks and rated good to excellent The peak discharge <br />for the 1995 flood was estimated to be about 1,300 fl3/S (table 1), and the unit discharge was about 43 <br />fl3/s/mi2, Local residents indicated that the 1984 flood was about 1-fl deep in the right overbank (figure 13) <br />and resulted primarily from snowmelt with some rain in the lower part of the basin. The 1984 estimated <br />peak discharge is about 2,800 fl3/S (table 1). The recurrence interval was about 10 years for the 1995 <br />flood and about 100 years for the 1984 flood, <br /> <br />Rainstorm of September 9-10,1915 at Columbine <br /> <br />In northwestern Colorado above 7,500 fl, the largest known convective rainfall amount, which was <br />reported as a total of 2.57 in. on September 9 and 10, 1915 (27 hours) at Columbine (Bureau of <br />Reclamation, unpublished data). Columbine is located at an elevation of 8,680 ft on the west flank of <br />Hahns Peak (elevation of 10,839 ft) in the upper Elk River Basin and about 27 mi. north of Steamboat <br />Springs. A rainfall-bucket survey was conducted for the 1915 storm that indicated the storm covered less <br />than 10 mi2 (Bureau of Reclamation, unpublished data). Lay, Colorado, with 0-42 in. of rainfall was the <br />only other station that had more than a trace of rainfall on September 9th and 101h in northwestern <br />Colorado. <br /> <br />. <br /> <br />Extensive paleoflOod investigations were conducted in all small tributaries in the vicinity of Columbine <br />and tributaries of Hahns Peak, which include the headwaters of the Uttle Snake River (table 1). <br />Substantial gold mining occurred on Hahns Peak beginning in the 1860s. Numerous roads and mine <br />tailings cover the mountain. If 1 to 2 in. of rain in a few hours occurred on the steep, mined slopes, <br />extensive mobilization of sediments such as occurred for a 2 in. rainstorm in about 1 hr on Virginia Canyon, <br />which is a 1 mi2 mined basin at Idaho Springs in 1991 (figure 14), which had similar mining activities. At the <br />Elk River at Clark streamflow gage, which is located about 1 0 mi downstream from Columbine, there was <br />no apparent rainfall runoff on September 9 or 1 0Ih. None of the tributaries in the Columbine area (table 1) <br />have paleoflood evidence of sediment transportl (figure 15). Runoff did not exceed bankfull stages from <br />this rainstorm. The lack of other rainfall in northwestern Colorado and evidence of flooding brings into <br />question the validity of the September 1915 rainfall amount at Columbine. <br /> <br />Rainstorm of June 14, 1921 in Steamboat Springs <br /> <br />. <br /> <br />On June 14, 1921,2.57 in. was recorded in Steamboat Springs (U,S, Weather Bureau, 1921) and is <br />the largest daily rainfall for lhis site. The moisture was associated with the extreme flooding in Pueblo <br />(Penrose storm) in eastem Colorado (Follansbee and Jones, 1922). This rainstorm occurred following a <br />week of near-record temperatures in the upper 70s to upper-80s in Steamboat Springs, which produced <br />large snowmelt runoff. The flood of record of 6,820 fla/s occurred on June 14, 1921 at the Yampa River at <br />Steamboat Springs gage (09239500; period of record is 1904-present), The occurrence of this rainstorm <br /> <br />24 <br />