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3.4 Reservoirs <br />There are several reservoirs in Costilla County. The major <br />reservoirs are listed in Table 6. Their locations are shown on <br />Figure 4. None of these reservoirs were constructed to pruvide <br />flood control. Some of the major reservoirs do, however, <br />provide inadvertent flood protection because their capacit~ is <br />so large relative to projected (and experienced} peak inf~ows. <br />Because outflows are expected to be so small, no floodplain <br />is shown on Trinchera Creek immediately downstream of Mountain <br />Home Reservoir or on Ventero Creek immediately downstream of <br />Sanchez Reservoir. Based on analyses of stream gages upstream <br />and downstream of Smith Reservoir, it did not appear that <br />outflaws on Trinchera Creek are significantly reduced by that <br />reservoir. The other reservoirs listed in Table 6 are much <br />smaller and have not been specifically considered to have <br />beneficial flood routing effects. <br />One reservoir was not listed in Table 6 because it is not <br />in Costilla County. Costilla Reservoir is in northern New <br />Mexico. Garcia, Colorado is about 17 miles downstream of the <br />reservoir, just north of the state line. At that point tlae <br />drainage area is about 200 square miles. Although Costilla <br />Reservoir is large, only 25 square miles out of those 200 <br />square miles are tributary to it. The decision was made to <br />consider all 200 square miles, and to assume the reservoir has <br />no significant effect on flows at Garcia. <br />3.5 Irra.ctation Ditches <br />Besides the reservoirs, several irrigation ditches reduce <br />average flows and, to some degree, peak flows along the streams <br />from which they dive.rt water. Perhaps the most significant of <br />these is the Eastdale Ditch on Culebra Creek just downstreaan of <br />San Luis. Around Fort Garland and Blanca there are large <br />agr~cultural diversions from Trinchera Creek. There are also <br />significant diversions from Costilla Creek, both in 1Vew Mexi.co <br />and Colorado. Because not enough is known about the operation <br />of these facilities during peak flows, and because this is an <br />approximate study, no specific flow reductions downstream of <br />any of the diversion structures have b~en included in the <br />hydrologic analysis. <br />-23- <br />