BEYOND SEDIMENT BUDGETS: HOW ALLUVIAL ARCHITECTURE CAN INFORM MODELS FOR ROUTING SUSPENDED SEDIMENT PULSES THROUGH WATERSHEDS

For decades, geomorphologists have relied on sediment budgets to document locations of sediment storage and production across fluvial landscapes. Predicting the effects of tectonics, land use, climate change, and watershed management, however, requires routing sediments through watersheds to predict the timing of sediment delivery, which sediment budgets alone cannot do. Useful models have recently been proposed that combine event scale transport velocities, sediment budget data, and empirical “waiting time” distributions that specify timescales of sediment storage; these methods, ultimately based on reservoir theory, provide useful predictions of sediment delivery, but are only strictly valid for steady state conditions. To account for temporal changes in sediment budget components, two new hypotheses are proposed. The first hypothesis is represented by an alluvial storage accommodation function that describes a tendency of sedimentation rates to decrease with increasing sediment storage. The second hypothesis is represented by an age-exposure erosion function, which quantifies the decreasing probability of erosion of with increasing time since deposition (age). These hypotheses replace the time-invariant waiting time distribution in previous models, and when combined with appropriate initial and boundary conditions, can predict the movement and storage of suspended sediment pulses through watersheds with time-varying sediment budgets. The distributions of sediment ages and waiting times then become model predictions, rather than constant input parameters. The accommodation and age-exposure erosion functions can be determined from field observations using well-established methods of geochronology and stratigraphy.