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3.0 HYDROLOGIC ANALYSIS <br />3.1 Flood Characteristics <br />Flooding in Costilla County results from both heavy <br />snowmelt runoff and summer thunderstorms. Occasionally <br />snowmelt runoff is augmented by rainstorms. In the eastern. <br />part of the county, which is more mountainous, snowmelt is the <br />predominant cause of flooding on the tributaries and the larger <br />subtributaries. On the smaller subtributaries thunderstorms <br />generally cause flooding. A review of the USGS Water Resources <br />Data reports for Colorado showed that most peak historic flows <br />had occurred in May, June, or early July. These reports ~o <br />naC, Yiowever, include data for the smaller subtributaries. The <br />description of their flood characteristics is based on <br />discussions with county residents. <br />As was mentioned earlier, snowmelt-induced flows ~rom the <br />mountains or flows from mountain thunderstorms are rarely <br />significant by the time the streams reach the western part of <br />the county. Tn that area floods are mostly caused by localized <br />rainstorms. <br />Snowmelt floods are characterized by sustained periods of <br />high flows, large total volumes, and marked diurnal <br />fluctuations. Rainstorm floods generally last only a few hours <br />with a zapid rise and a rapid fall in flows and small total <br />volumes. In southwestern and south-central Colorado rainfall <br />floods can result from general rain conditions caused by <br />Pacific storms in the late summer or early fall. Those floc~ds <br />may be marked by sustaine8 periods of moderately high base <br />flows with shorter periods of very high f~ows when intense <br />storms occur within the less intense general rains. <br />3.2 Historical Review <br />Although there is a record of significarit flows at several <br />gages in Costilla County, historical accounts of flooding in <br />the county are very sparse. A review of newspa~er from Pueblo, <br />Alamosa, and Costilla County revealed that there has been major <br />flooding on the Rio Grande and many of its tributaries in the <br />past. There were also accounts of major flooding in four <br />adjacent or nearby counties (Huerfano, Conejos, Alamosa, an~ <br />Rio Grande Counties). Althouqh no detailed accounts of <br />flooding in Costilla County were found, there were brief <br />accounts of highway or railroad damage in and around the county. <br />-17- <br />