Size matters. History too. The rest is detail

It is long established that grain size - both absolute and relative - strongly influences sediment transport. For mixtures of sizes, the history of flow and sediment supply determine the size of grains available on the bed surface for continued transport. Despite advances in understanding the interaction between the flow, bed, and transport, we have little ability to predict sediment transport rates. God is evidently in the detail. This detail can be external (detail that must be specified) or internal (detail that might be predictable). Although the elementary mechanics of grain entrainment, motion, and distrainment can be solved, our inability to specify the full system of flow and grains inevitably leads to assumptions and averaging that obscure detail, hide nonlinear interactions, and lead to predictive error. Here, we use inverse application of a surface-based transport model (flow and bed grain size predicted as a function of the rate and grain size of sediment supply) to explore the variability in transport conditions as a function of uncertainty in sediment supply. This helps define the effect of unexplained variation in external detail, providing a framework for investigating the effect of internal details related to sorting, grain structures, and biogeochemical processes.