In natural rivers, flow variability and sediment heterogeneity affect how sediment grains are transported. A unique law that predicts the total amount of sediment transportable by a river for a wide range of sediment mixtures and flow conditions exists, but unclear trends remain. Two improvements to this law – a standardized onset of sediment transport and a common reference transport height across all sizes – appear to be critical to having a functional multi-grain-size total sediment load.