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hour period on 5-6 September. Floodwater damaged or destroyed homes, parallel to the San Juan River curve for the Pagosa Springs station, trailers, and vehicles; severely damaged strea~ channels; and waahed out bridgeasndaectionlfofroada. vacationers caught unaware during their Unit hydrographa for Vallecito and Grimes Creeks S-graph method, ucUidllg a typieal OIountain characteri.tie of the Vallecito Creek region, were derived by the S-graph for terrain Labor Day week~d outinga were forced to evacuate h~elf, cabina, and call1Pdtea. Flood losaesexceeded $600,000, ruroREFLOODS Historically, the worst rdnflooda known in ..eatern Colorado occurred durillg September and October, Usillg deptb-area curves for the Sall Juan River Baain, a atandard project stor~1 for the Vallecito Creek Although floods that have occurred in of the s~e magnitudes and frequencies aa those the paat could recur in the future, discussion of Baain WaS developed from available precipitation records. "" future floods in this report ia limited to those designated aa 100- and SOO-year floods, A lOO-year flood is one with a peak flow magnitude that haa a 1 percent chance of being equalled or exceeded in any given depth-area curvee vere for June through October rainatorma with 48-hour duration, and were baaed on flood producing atorma that occurred in October 1911 and September 1910. The analyais produced a atorm with the following &mOUlltS of rainfall. year,anda long term frequency " average,- of occurrence of nbout once ie loo-yeara on the Similarly, a 500-year flood has a 0.2 percettt R.aittfall inch.u Location chance of being equalled or exceeded ittanygivenyear, and a frequency of occurrence of about once in SOO-yeara on the long [erm average. Aa indicated, the most aevere flood conditicns in the atudy atea would ceault from general rainator~. 10.01 7.55 9.14 Valleeito Creek below Fall Creek Vallecito Creek, lccal below Berri Creek Grimes Creek near Vallectio Guard Station A peak flow-frequency analy1lis based on recorda from the "nl!ar Bayfield"gageonVallecitoCreekandaeverallong-termatre8lllgagea in the vicinity waa ~de to determine peak flowa of the 100- and SOO-year floods. Due co the ahort p..riod of atreamflow recor~~ on Vallecito Creek,only the lower portion of 1ta frequency curve could be adequatel Y defined. Since it showed a trend slmilar to the frequency curve for the S^n Juan River atream gage at Pagoaa Springs (period of record 1935-12), and ainc.. the two drainageareaa have ai:dlar orientation, elevarion, climate, and atorm charaeteristie.., the vallecito Creek curve waa dra~ Standard pcoject flooda were alao co:oputed '0' varioua concentrarion points in the atudy area to aid in hydrograph development. Loaa ratea adopted for the baain (0.50 inchea initial and 0.20 inches constant) were eat1mated from 10sa ratea obaerved in nearby b.a.sins witb similar characteristics, The standard projsctflocd at the "near Rayfield" gage was obtained by 4pplyin!\ a concentr4tion point below Fall Creek. The wen obtained by applying a ntio of the frequency curve at the gage) to the standard drainage araa ratio at a IOO-yearHood hydrographa 100-year flow (ftom the project flood peak flow, 21 - The 100-year flood ia also known as an Intermediate Regional Flood. ]/" storm that reflecrs the 1Il0st severe cOll\bination of ..eteorologicsl conditions reaaonably ch.aracteristic of the geographlcal area in which rhe drainaKe basin is located, , 4