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I II I I II I I I II I I I I I I I I I I I II 3.4 Reservoirs There are several reservoirs in Costilla County. The major reservoirs are listed in Table 6. Their locations are shown on Figure 4. None of these reservoirs were constructed to provide flood control. Some of the major reservoirs do. however. provide inadvertent flood protection because their capacity is so large relative to projected (and experienced) peak inflows. There is a stream gage on Trinchera Creek upstream of Mountain Home Reservoir. Although there is no gage downstream of Mountain Home Reservoir. between it and Smith Reservoir. it was decided that no credit would be given to Mountain Home Reservoir for peak flow reduction since it is not a flood control reservoir. Based on analyses of stream gages upstream and downstream of Smith Reservoir. it did not appear that outflows on Trinchera Creek are significantly reduced by that reservoir. Ventero Creek upstream of Sanchez Reservoir was not identified as a stream of concern for floodplain delineation. Since Sanchez Reservoir is not a flood control reservoir. any delineation downstream of the reservoir must assume no flood detention. In its 81 year history Sanchez Reservoir has never been filled. In fact. the closest it has ever come to being filled is approximately 60.000 acre-feet in 1987 out of a capacity of approximately 103.000 acre-feet. On the average about 15.000-20.000 acre-feet are stored in Sanchez Reservoir. Despite that history. the hydrologic assumption of no flood detention was observed for Ventero Creek. Downstream of its confluence with Ventero Creek. Culegra Creek's drainage area was assumed to include all of the area tributary to Sanchez Reservoir. The other reservoirs listed in Table 6 are much smaller. They have not been specifically considered to have beneficial flood routing effects. One reservoir was not listed in Table 6 because it is not in Costilla County. Costilla Reservoir is in northern New Mexico. Garcia. Colorado is about 17 miles downstream of the reservoir. just north of the state line. At that point the drainage area is about 200 square miles. Although Costilla Reservoir is large. only 25 sguare miles out of those 200 square miles are tributary to it. The decision was made to consider all 200 square miles. and to assume the reservoir has no significant effect on flows at Garcia. 3.5 lrriqation Ditches Besides the reservoirs. several irrigation ditches reduce average flows and. to some degree. peak flows along the streams from which they divert water. Perhaps the most significant of these is the Eastdale Ditch on Culebra Creek just downstream of San Luis. Around Fort Garland and Blanca there are large agricultural diversions from Trinchera Creek. There are also significant diversions from Costilla Creek. both in New Mexico and Colorado. Because not enough is known about the operation of these facilities during peak flows. and because this is an approximate study. no specific flow reductions downstream of any of the diversion structures have been included in the hydrologic analysis. -23-