Sediment trajectories through a semiarid valley

Sediment eroded from upland areas is often deposited within river valley floors. Particles follow different trajectories through valleys: some particles pass directly through the channel, while others remain in the valley for long periods in deposits such as floodplains. The trajectory of a particle can be viewed as a random process, consisting of a series of mobilization, transport, and deposition events. The probabilities of these events are determined by the rates of sediment transport and exchange in the valley floor (the sediment budget). This theory was formalized and then tested by modeling the redistribution and radioactive decay of particle-bound 137Cs in a small alluvial valley downstream of Los Alamos, New Mexico, and comparing the modeled distribution in 1997 with an independent map of 137Cs storage in that year.; The study area is a sand-dominated valley with an ephemeral channel and a narrow floodplain, of a kind often encountered in semiarid environments. Field monitoring and theoretical calculations indicate that two distinct populations of sediment follow different trajectories: a coarse fraction, which moves gradually and is frequently exchanged with the channel bed, and a fine fraction, which is supplied from outside the channel, is well-mixed in the flow, and is occasionally exchanged with the floodplain. Rates of sediment exchange with the channel and floodplain are large compared with the total sediment flux. Over several decades, events which occur on average more than once per year dominate rates of transport and exchange of both fractions. These characteristics are probably representative of many small, sand-dominated valleys in semiarid environments.; Sedimentologic evidence was used to reconstruct the release history of 137Cs, and the theory was used to simulate the movement of particles through the valley. The modeled 1997 inventory of 137Cs overpredicted the measured inventory by a factor of 3, but this discrepancy is small compared with the estimated total amount of 137Cs introduced to the system over the past 50 years. The relative spatial distribution of the contaminant matched the measured distribution. The model predicted that about 50% of the 137Cs currently in the study area will decay radioactively before leaving the valley.