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I I I I I I I I I I I I I I I I I I I 2.6 Culebra Creek Basin The Culebra Creek headwaters are in the Sangre de Cristo Mountains in the middle of Costilla 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 Creek 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 dating back to the early time of Spanish settlement. cluster around the streams, sometimes in the floodplain. In and around San Luis several tributaries gradually join 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 floodplain encroachment in the county. Below San Luis, Culebra Creek flows through agricultural areas until it crosses Highway 142 at San Acacio. There are some roads, bridges, culverts, and agricultural structures in the floodplain. Although agricultural activity 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 Highway 142 crossing acts partly as a darn, 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 southern Colorado, and part are in northern New Mexico. Headwater elevations are as high as 13,676 feet. The tributaries basically flow south toward Costilla Reservoir. Below that reservoir there is one major mountain stream valley with small tributary valleys. In a big arc. the stream turns toward the northwest. It emerges from the foothills near Costilla, New Mexico and Garcia, Colorado at an elevation of about 7,750 feet. There is the same pattern of urbanization and agricultural uses. with the accompanying encroachments, as in the other major streams in the county. -13-