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I I 2.0 STUDY AREA I I I I I I I I I I I I I I I I I 2.1 Location Costilla county is located in southern Colorado. immediately north of the New Mexico state line. It lies between the Sangre de Cristo Mountains on the east an~ the Rio Grande on the west (see Figures 1 and 2). Besides the Rio Grande, which forms the county's western boundary, there are three major drainage basins in Costilla County. They are Trinchera Creek in the northern part of the county, Culebra Creek in the central part of the county. and Costilla Creek in the southern part of the county (see Figure 3). Each of these creeks is tributary to the Rio Grande. Trinchera Creek and Culebra Creek have their headwaters in the Sangre de Cristo Mountains in Colorado, flow generally to the west, and ultimately join the Rio Grande in Colorado. Costilla Creek originates in the Sangre de Cristo Mountains of northern New Mexico and southern Colorado, flows westerly in New Mexico, turns northwesterly to flow into Colorado, turns more westerly in Costilla County. then turns southwesterly to eventually join the Rio Grande in northern New Mexico. The western part of Costilla County is very dry. From somewhere near the middle of the county downstream to the west all three of the major drainages are basically dry most of the time. Only heavy snowmelt or rainstorms wil1 generate surface flows in those portions of the drainages. In the eastern part of the county mountain snowpack and mountain rainstorms assure a more or less continuous flow of water in the major drainages and some tributaries. As one continues west. the channels are dry under most conditions due to extensive irrigation for agriculture, the sandy soils which absorb much of the runoff, the very low annual precipitation. and the lack of significant tributary flow. Figure 1 shows the location of Costilla County within Colorado. Figure 2 shows the county's location within the Rio Grande drainage basin. -4-