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NATUREOFFLOODPROBLEl1S INTRODUCIION Flooding from large. frontal type rainstorms approa~hing from the so~thwest ~onstitutes the most serioua flood hazard in the study area. Storms of this type can be expe~ted from mid-June through December, but records ahowthatmajor flood producing rainstorlllS in southwestern Colorado most frequently occur during September and October. Flood probleIIIS 1n the study area are intensified by high velocity flow and by manmade encroachment in strearnways. A report on the flood hazard along Vallceito Creek and other streams at the northernmost end of Vallecito Reservoir waa requested by the Animas Regional Planning Commission, the Colorado Land Uae Commisaion, and the Colorado Water Conservation Board. Ita purpose is to provide a baaia for planning the best use of landa subject to inundation from 100- and 500-year flOOds, The report wss prepared by the Corps of Engineer" ~~der continuing authority provided in Section 206 of the 1960 Flood Control Act, Sa uended. The Corpa gratefully acknowledgea the proviaion of detailed topographic mapa jointly by ~ Plata County and the ColoradO Water Conservation Board; aaaistance by the Animas Regional Planning Commission and the Colorado Land Use CommissIon; and provision of flood photographs. information on past flooda, and other useful data by local residents. , , I , , I Although 40 percent of the normal seaaonal precipitation (about 40 inChes) in the watershed tributary to the study area occurs as snOll and a deep snowpack accumulates in the higher elevations, snowmelt runoff in the spring or early 6~er conatitutes a frequent but comparatively minor flood threat. Conve~tive type cloudburst storms can be expected to occur frequently during the suuuer montha. Theintenaityofcloud- burst storms is quite high and they Can produce enough precipitation to result in sign1ficant runoff. l:Iowever,duetotheirslllallarealextent, short duration of runoff, channel storage effecta. and other factors, flooding from cloudburst storma 1s not considered to be a significant hazard in the study area. the atudy area for this report includes the flood plains along the lover 3.0 miles of Vallecito Creek and the lover reaches of Grimes and Re~orda indicate that five ..ajar flood events have occurred on the >I S.",Ju..nRl"..r",\J. "aria..." of itstributarie,.- since the t\lrnof the century. However, little definitive information on flooding in localized areas such as Vallecito Creek is available due to spsrse popul..t1on sod remotenes" from urban centers. A flood that o~curred on Vellecito Creek in September 1970 was the largest and most damaging kno",,\ siuce stre.... g..glng waa started (1963). It resulted from extre..ely heavy rainfall that totaled more than ~ inches during a 6-12 Earri Creeks, Middle Creak, a ~i8trihutary of Valletitc Creek, and along other minor atreams that influence, or are influenced by flooding on the brger Streams under study. 'Ihe setting of the "tudy area is mountainous with typiCAl glaciated peaks rising to the west, north, and east. Elevations in the region range from about 7700-g000 feet in the atudy area to around 14,000 feet in the high headwater areas of Vallecito Creek. Stream gradients of 100-300 feet per mile are cO~n. Vegetation is characterized by growths of ..lder, cottonwood, pine agd willow with ground cOVer of hardy native graasea .nd shrubs ill the valley botto~, and by dense st3nds of spr~cc, pine and ASpell helow timberline on the mountain alopes. Cr~ek total. 9~ square ~il~B. 'Ihe tributaty .re~ of V.1I~cit~ l/'Ill~ "'all~cit" Cr~..k dtain"!>e 1.. plOtt a! tho: Lu.. P1nu" Riv~r Ba;;i'l, ..-hi.;I\, in turn, is tributary to th~ San Juan River, 4 major tributary 1n the Colorado River syat~m. ,