SIGNATURES OF CLIMATICALLY-DRIVEN HILLSLOPE EVOLUTION IN RIVER AND ALLUVIAL-FAN TERRACES IN THE SOUTHWESTERN U.S

River and alluvial-fan terraces may form by a variety of climatic, tectonic, and autogenic mechanisms. In order to determine the geometric and geochronologic signatures of different forcing mechanisms on Quaternary channel and alluvial-fan deposits in the Southwestern U.S., we carried out a set of numerical experiments using a fully-coupled model of a drainage basin and its adjacent alluvial fan. The model includes hillslope evolution by diffusive and mass-movement processes, and bedload sediment transport and bank retreat in channels and on the alluvial fan, including the moving boundaries between the hillslope, bedrock, and alluvial process domains. In the first experiment, the drainage basin was removed and the fan was fed with constant water and sediment discharges as a control case. In this experiment, analogous to the "Jurassic Tank" experiments of Paola et al., the fan evolved with autogenic cycles of entrenchment, progradation, and backfilling. The relief of these terraces was small, however, and reproduced a random pattern of terraces inconsistent with the nested pattern of regionally-correlative Quaternary terraces preserved in many drainages of the Southwest. In the second set of experiments, the linkage between climatically-driven hillslope evolution and the resulting geometry of channel and alluvial-fan terraces was evaluated. The effects of climatically-controlled variations on hillslope runoff and vegetation changes were simulated, with impacts on both hillslope sediment supply and drainage density. For representative water and sediment fluxes, the transient variations in drainage density were responsible for the largest fluctuations in sediment supply to the fan, and hence was the dominant control on the occurrence of cut-and-fill cycles on the fan. Alluvial fan terraces created by the model (1) correlate with humid-to-arid transitions, (2) have volumes and dips that correlate with the duration of the previous humid interval, and (3) are areally preserved with an inverse relationship to the duration of the subsequent dry interval. These observations provide a preliminary blueprint for correlating regional patterns in alluvial-fan deposits with Quaternary climatic changes.