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2.6 Culebra Creek Basin The Culebra Creek headwaters are in the Sangre de Crist:o Mountains in the middle of Costil~a County as one goes from north to south. The highest elevation is 14,047. The creek flows generally to the west, through San Luis and on to the Rio Grande. The elevation at the confluence with the Rio Grande is about 7,480 feet. The Culebra Creek basin is similar to the Trinchera Cr.eek basin in several respects. Narrow and steep mountain streams originate in the Sangre de Cristo Mountains to the east. Occasional roads and bridges comprise the major .floodplain development. Where these streams enter the alluvial plain near the western foothills, many small settlements, mostly datirng back to the early time of Spanish settlement, cl~ster around the streams, sometimes in the floodglain. In and around San Luis several tributaries g~adually j~in Culebra Creek. Except for a narrow constriction immediately downstream of San Luis, formed by local geologic features, this transition zone is where the valleys become broader and flatter. Being the most significant urbanized area in Costilla County, it is also where there is the most extensive floo~plain encroachment in the county. Below San Luis, Culebra Creek flows through agricultural areas until it crosses Hiqhway 142 at San Acacio. There a.re some roads, bridges, culverts, and agricultural structures :in the flaodplain. Although agricultural activit~ has partially obliterated the channel and substantially reduced flows, the floodplain is still apparent. Below San Acacio the valley is sparsely inhabited. The channel is almost completely obliterated. The Hiqhway 142 crossing acts partly as a dam, contributing to the appearance of a desert valley all the way to the Rio Grande. There are almost no obstructions after San Acacio. 2.7 Costilla Creek Basin Part of the headwaters of Costilla Creek are in souther:n Colorado, and part are in northern New M~xico. Headwater elevations are as high as 13,676 ~eet. The tributaries basically flow south toward Costilla Res~rvoir. Below that reservoir there is one major mountain stream valley with sm~ll tributary valleys. In a big arc, the stream turns toward t~e northwest. Tt emerges from the footh~lls near Costi.lla, New Mexico and Garcia, Colorado at an ~levation oL about 7,750 feet. There is the same pattern of urbanization and agricultural uses, with the accompanying encroacZiments, as in the other major streams in the county. -13-