Understanding sedimentation at the El Molinito reservoir (NW Mexico) before and after dam construction using physical sediment analyses

Abstract

Sediments stored in reservoirs provide useful information about landscape erosion rates that are needed for understanding the geomorphologic evolution of sites and also for engineering purposes. In arid and semi-arid landscapes, the differentiation of sedimentation pulses is required since landscape erosion usually occurs in short periods and involves the mobilization of large volume of sediments. We analyzed the sediments stored at the El Molinito dam, that was built in 1991, which is located in the lower course of the Sonora river, NW Mexico. The river basin of the study area is dominated by arid to semi-arid conditions, where human activities are characterized by the extensive grazing for cattle and open-pit mining. The physical properties of sediments were analyzed using OSL, fall-out Cs-137, K-40 and magnetic susceptibility (MS) from four sediment cores. We found that the IRSL/BLSL and MS reveal how the density of grains settled at different parts of the reservoir changed prior and after the dam construction. The K-40 data exhibit a similar result as the IRSL/BLSL and MS values. The activity of Cs-137 has a similar pattern prior and after the dam construction, what indicates that this tracer is unaffected by the dam construction. We found that the Cs-137 is a reliable indicator of how rainfall relates to erosion within the river basin. We conclude that the IRSL/BLSL and MS analysis can aid the interpretation of how sediment is mobilized prior to their deposition and transport in arid to semi-arid landscapes where erosion is episodic and seasonal. Moreover, the approach used in this research can be used for engineering studies to accurately identify the depth at which the sedimentation of reservoir initiates and to estimate the different sedimentation rates. We also found that the analysis of Cs-137 in the sediments stored in the dam also yield useful information of the dominant landscape dynamics. This information is valuable using climate data since they allow the understanding of the main mechanisms related to the sediment routing from source-to-sink in dams built around the 1950s.