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3.0 HYDROLOGIC ANALYSIS In conducting the historical review. USGS Water Resources Data Reports for Col?radO ,:,ere examined. t'rom those reports the dates that are listed in Table 3 were obtained. They are the dates when large flows were measured at stream gaqes in the county. Newspapers from the two to three week periOdS around those dates were examined. In every case evidence of heavy' snowpack and/or rain somewhere in Southern CoLorado was found. In most cases eviden~e of flooding in ncarby areas of Colorado or elsewhere in Colorado was found. Historic peak flows in Costilla County are presented in <able 3. 3.1 Flood Characteristics Flooding in Costilla County results from both heavy snowmelt runoff and summer thunderstorms. occasionally snowmelt runoff is augmented by rainstorms. In the e~stern pact of the county, whiCh is more mountainous, snowmelt is the predominant cause of flooding on the tributaries and the larger subtdbutaries. On the smaller subtdbutaries thunderstorms generally cause flooding. A review of the USGS Water Resources Data reports for Colorado showed that most peak historic flows had occurred in May, June, or early July. These reports do not, however. include data for the smaller aubtributaries. The description of their flood characteristics is based on discussions with county residents. As was mentioned earlier, snowmelt_induced flows from the mountains or flows from mountain thunderstorms are rarely siqnificant by the time the streams reach the western part of the county. In that area floods are mostly caused by localized rainstorms. I Snowmelt floods are characterized by sustained periods of high flows, large total volumes. and marked diurnal fluctuations. Rainstorm floods generally Last only a few hours with a rapid rise and a rapid fall in flows and smalL total volumes. In southwestern and south-central Colorado rainfall floods can result from general rain conditions caused by Pacific storms in the late summer or early fall. Those floods may be marked by sustained periods Of moderately high base flows with shorter periodS of very high flows when intense storms occur within the less intense general rains. ., , " , 3.2 Historical Review , " , Although there is a record of siqniflcant flows ~t several qaqes in Costilla County. historical accounts of flooding in the county are very sparse. A review of n~wspaper [rom Pueblo. Alamosa. and Costilla County reveolLed that there has been major flooding on the Rio Grande and many of its tributaries in the past. There were also accounts of major flOOding in four </djac..nt or nearby counties (Huerfallo. Conejos. ALamosa, and Rio Grande Counties). Although no detailed accounts of flOOding in Costilla County were found, there were brief accounts of highway or railroad damage in ilnd around tile county. L , I , , -17- ~ 18-