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flOOD SEASON AND flOOD CHARACTERISTICS bridgcs orpiug culverts, thus creating a damming etfed andoverban kflow. AS floodfiow increases, masses of debris may break loose and a wall of water and debris may surge downstroam until another obstruction Is encoun- tered. In gener<ll, obstructions restrict flood/lows Md cause overbank flows, unpredIctable areas of flooding, possible damage to or destruction of bridges and culverts, and increased velocities of flow Immediately downstroam. Flooding In the study area 'WOuld result from general rain, snowmelt, and cloudbursts. General raInstorms <lnl the princIpal cause of flooding and property d~ge along the Animas River and Lightner and Junction Creeks. General ralnfloods usually occur during the perIod from July through October. This type of flood results from prolonged heavy raInfall over the tributary areas and Is characterized by high peak flOws of moderate volume and duration. Flooding Is more severe when antecodent rainfall has produced saturated ground oonditloos. Snowmelt floods would genorally occur on the Animas River and Lightner and Junction Creeks dur!ng the perIod from late May to early Jl,jly. Flood- flows nlsultlng frocn snowmelt are characterized by moderate peaks, long duration, large volume, and marked diurnal fluctuation, and may occasion- ally be augmented by rain. Snowmelt and general rain stonns would not constitute a serious flood threat along Dry Gulch Creek because Its tribU- taryarealssmall. Manmade obstructions to floodflow In the study area !nclude 25 bridges and culverts. Representative obstructions are shown In Figures 1-7. Per- tinent data on manmado obstructions to floodflow are contained In Table 6, page 36. FACTORS AFFECTING FLOODING AND ITS IMPACT ClOudburst stonms would produce the most severe flooding on Dry Gulch Creek. These hIgh-intensity raInstorms, generally lasting no more than a few hours, usually occur during the summ(lr months. Floodflows from cloud- burstsronns are characterized by high peak flow, short duration of flow, and small volume of runoff. ClOUdburst storms can cause high flOws on Lightner ~nd JunctIon Creoks; however, those flows would not be as dam- aging as thoso resulting from general rainstorms. Obstructions to Floodflows. Natur!llobstructlonstofloodflow Include trees, brush, and other vegetation growing In and along the streams In floodway aroas. During floods, brush and vegetation growing In floodways Impede flood/lows and result In back..ater conditions ,,"d Increased flood heIghts. Brush or trees washed out during floods and carried downstre"m could collect on FiguI'C 1 - Animas RiueI' at 1/. S. High..Jay 160 crossing. (RefeI' to PifjUfOe 9, pafj'" ~2, which shows the 1927 [I.ood neaI' this C1'088infj.) 9 10