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

The Rio Grande begins 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 Pleistocene Rio Grande had therefore a very different hydrology than the Late Pleistocene and Holocene through flowing river. However, exposures of channels of different ages contain very similar sediments and are difficult to distinguish. The purpose of the study is to characterize the differences in hydrology between 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 size distributions and associated transport characteristics. The preserved sedimentary structures provided constraints on the shear stresses during deposition. Early results have shown critical shear stress decreased as moved up through the channel beds, and sediment movement in the flume matched the sediment structures observed in outcrop. One surprising result was that laminated sand, deposited from suspension formed the middle of some channel fills. These were over laid by rippled sediments that were transported in shallow waters by slower current than underlying suspended load sediments.