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41448-B8
River Behavior in a Modern, Tectonically Active, Extensional Basin
The Palomas basin in the southern Rio Grande rift is a tectonically active, eastward-tilted half graben floored by the axial Rio Grande. The Holocene geologic history of a 20 km segment of the basin was determined by field mapping at the scale of 1:12,000, sedimentologic analysis of 51 cores ranging from 1.5 to 4 m deep, and 20 14C dates of charcoal, molluscan shells, and soil calcite. The floodplain is divisible into three channel belts of different ages. The oldest, topographically highest channel belt consists of sinuous channels locally covered by eolian sand. The channels were active between 10,580 and 940 years ago and generally become younger eastward, perhaps as a result of fault activity and asymmetrical tilting of the basin. The channel belt of intermediate age is inset 1-3 m against the oldest channel belt and is characterized by moderately sinuous channels. Initial incision of the intermediate channel belt took place between 940 and 500 years ago, corresponding to the time of the Medieval Warm Period. The youngest channel belt, which is represented by the modern floodplain of the Rio Grande, is inset 0.5 to 3 m against the older channel belts, with initial incision occurring approximately 60 years ago. At two locations in the study area, the floodplain narrows from ~3 to <0.5 km by the convergence of large, transverse alluvial fans from opposite sides of the basin. In the summer of 2006, large floods occurred on the converging Red Canyon and Palomas Canyon alluvial fans. The floods deposited 0.5 to 2 m of coarse gravel on the floodplain, changing the location and width of the Rio Grande and causing flooding for 1 km upstream of the constriction.
This study provides important insights into tectonic and climatic controls on river behavior and documents flood processes on active alluvial fans. The P.I and undergraduate research assistant have benefited professionally by integrating geomorphic and sedimentologic concepts and by improvement of mapping skills.
