CHARACTERIZATION OF ANCIENT CHANNELS USING A FLUME FOR RECONSTRUCTION OF CRITICAL EROSION AND SEDIMENT TRANSPORT CHARACTERISTICS: EXAMPLE FROM THE PLEISTOCENE RIO GRANDE

The Rio Grande River flows southward from southern Colorado through New Mexico, and along the Texas-Mexico Border to the Gulf. During the early and Middle Pleistocene, the Rio Grande terminated in a series of lake basins in Southern New Mexico, West Texas and Northern Mexico. The early Rio Grande had a very different geomorphic and climatic setting than the present day river, which flows through an incised valley to the Gulf of Mexico. However, exposures of channels of different ages contain very similar sediments and are difficult to distinguish. The purpose of the study was to characterize the differences in hydrology between six channels of different ages using a unique High Shear Stress sediment erosion and transport flume. This allows determination of critical shear stresses for sediment mobility and determination of the percentage of bed load and suspended load transport under different shear stresses. Erosion rates and sediment grain size distributions were determined from discrete sediment samples collected from paleo-Rio Grande channels in the El Paso region. Profiles from the bases to the tops of the channel fills were analyzed and compared to the observed sedimentary structures. This data allows determinations of the grain sizes distributions and associated transport characteristics. The preserved sedimentary structures provided constraints on the shear stresses during deposition. Results confirm the upper portions of the channel having low critical shear stress and the sediment enter suspend load at lower shear stress, than the lower portions.