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3.3 Stream Gaqe Data <br />A hydrologic analysis was performed to determine 100-year <br />peak flows on streams in Costilla County. The principal data <br />sources were the USGS publication, Technical Manual No. lo <br />Manual for Estimatinq Flood Characteri.stics of Natural-Flow <br />Streams in Colorado (TM-1), published i~ 1976, and a USDA Soil <br />Conservation Service (SCS) hydrologic analysis for the Town of <br />San Luis, prepared as a part of a 1979 flood control project <br />study that was conducted by the SCS. These two sourc~s were, <br />in turn, principally based on analyses of gage records for <br />sites in Costilla County. <br />TM-1 included gage analyses for five (5) stream gages in <br />Costilla County. These gages are listed in Table 4. The SCS <br />analyzed five (5) gages also. Three of those gages had already <br />been analyzed in TM-l. The results of their analyses are <br />listed in Table 5. <br />Since it had not been analyzed for TM-1 or the SCS study, <br />one more gage was studied by the CWCB. The gage on Trinchera <br />Creek below Smith Reservoir was felt to be impor~ant becaus~ it <br />had over 50 ~ears of record and because it provided data <br />further downstream on any of the three major tributaries than <br />any other gage in the county. Even though it is located below <br />a reservoir, the gage provided helpful data. <br />The site is actually downstream of two reservoirs. <br />Mountain Home Reservoir is upstream of Smith Reservoir on the <br />mainstem of Trinchera Creek (see Figure 4). Sangre de Cristo <br />Creek does not flow through Mountain Home Reservoir, but joins <br />Trinchera Creek at Smith Reservoir, farther downstream. Sangre <br />de Cristo Creek actually contributes a much larqer drainage <br />area to the Trinchera Creek basin above Smith Reservoir than <br />does the Trinchera Creek mainstem. The large capacity of <br />Mountain Home Reservoir relative to the drainage area upstream <br />of it and its history of not being filled had to be <br />considered. The result was that the 61 square miles on the <br />Trinchera Creek mainstem that are tributary to Mountain HomP <br />Reservoir were subtracted from the total drainage area at the <br />gage below Smith Reservoir (396 square miles). The resulting <br />area (335 square miles) is still a sizable drainage area and <br />provides useful information for the discharge profile for <br />Trinchera Creek west of the foothills. <br />-20- <br />