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South Branch #1 to Soutbeast Tributary to RltoSeco South Branch #2 to Sout~east Tributary to RltO Seco South Branch #3 to Sout~east Tributary to Rlto Seco North Branch to Southeast Tributary to Rito Seco East Branch to Southeast Tributary to Rito Seco EI Poso Creek EI Pedreqoso Creek Vallejos Creek North Vallejos Creek Ditch from Vallejos Creek to Ventcro Creek Ventero Creek San Prancisco Creek Costilla Creek Basin Costilla Creek r' , Upstream of Confluence with U SQutheast Tributary " RltO Seco Upstream of Confluence with 0,' Southeast Tributary '0 Rito Seco Upstream of Confluence with 0,' Southeast Tributary to Rito Seco Upstream of Confluence with 0,' Southeast Tributary '0 Rito Seco Upstream of Confluence with 0,' Southeast Tributary '0 Rito Seco Upstream of Confluence with , Culebra Creek Upstream of Confluence with , El Poso Creek Upstream of Confluence with , Culebra Creek Upstream of Confluence with . Vallejos Creek Entire Length , Upstream of Confluence with , Culebra Creek Upstream of Confluence with . Ventero Creek and on to Texas. r'rom the vicinity Of t:l Paso it forms the border between Texas and Mexico. The flo~ that remains after irrigation diversions and reservoir storaqe enters the Gulf of Mexico near Brownsville. Texas. The Rio Grande is large enough that it is well defined throuqhout the county. At the north end of the county. as it flows south from Alamosa County, the river is broader and flatter with a very wide floodplain. on the order of two or three miles wide. Then in a stream distance of about two or three miles the floodplain narrows substantially, to a width of about 500 feet. Further south. the valley turns into a canyon that is about 80 feet deep as the river enters New Mexico. On the Rio Grande there are a few buildings in the northern part of the county. Otherwise. to the south. very widely spaced bridges are the only obstruction. [ / I' , 2.5 Trinchera Creek Basin Trinchera Creek originates in the sangre de Cristo Mountains of northern Costilla County. Elevations are as high as 14.345 feet. It flows generally to the ~est and southwest. At its confluence with the Rio Grande the elevation is about 1.500 feet. , I In the eastern part of the Trincheta Creek basin streams are narrow and steep. Development in the flOOdplain has consisted primarily of roads and bridges. In Fort Garland and a few miles to the west several tributaries gradually join Trinchera Creek to form a single major drainage. As Trinchera Creek and its tributaries emerge from the foothills of the Sangre de Cristo Mountains onto the alluvial plain of the eastern San Luis Valley, the individual stream valleys become broader and flatter. , " , I ( Upstream of New Mexico Stateline (to New Mexico Statelinej , , I " Urbanization. mostly in and around Fort Garland. and agricultural uses have led to streets. roads. houses. commercial f~cilities, agricultural buildings and facilities. and irrigation structures in and adjacent to Trinchera Creek and its tributaries. This is the area where there is the most extensive encroachment into the floodplain of Trinchera Creek and its tributaries. I ! r I ~'inally, in the western part of the Trinchera Creek basin. tne single creek tlow through aqricultural areas or currently undeveloped areas. The main channel and side channels are poorly defined. and sometimes human activity has obliterated them. Aside from hi<Jhways. roads, bridges. dnd culverts. there are few obstructions. , , -11- -12-