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Suspended sediment loads can coarsen or fine as a function of flow. The flow-load gradation relationships can also vary nonmonotonically. This complex relationship between flow and load gradation complicates sediment model boundary condition selection and sediment rating curve analysis. This study analyzed flow-gradation data from 78 gages in the continental United States, exploring trends and tendencies in the flow-gradation relationships. Results were then compared to the flow-gradation trends generated by sediment capacity equations and a meta-analysis of sediment model data. Several systems, with abundant sediment supply fined with flow. However, most gage data and calibrated model inputs coarsened with flow. When nonmonotonic models were considered, one-third to one-half of the gages fit a second-order curve, coarsening over low-to-moderate flows (up to an average annual exceedance probability of approximate to 30%) and fining over higher flows. The low-flow-coarsening trend was generally stronger than high-flow-fining trends. Many of these second-order trends demonstrated behavior more like asymptotic coarsening. They coarsened until they reached a maximum physical grain size limit and gradation fined slightly or remained constant beyond the threshold flow. The dominance of flow-coarsening suggests most of the rivers surveyed (which included a bias toward large, regulated rivers) are supply limited. Supply limited (bed regulated) rivers tend to coarsen with flow, while capacity limited, high supply systems tend to fine with flow. Sediment capacity equations computed a variety of flow-gradation trends. Using transport functions to compute load-gradation model boundary conditions will often diverge from (or even invert) observed trends, especially in supply limited systems.

Plain Language Summary As river flow increases, the sediment load (the sand and silt the water transports) increases. Scientists and engineers try to compute relationships between flow and sediment load for a wide range of economic and environmental applications, like computing how long the river will take to fill a reservoir with sediment or if the river will burry or scour a salmon habitat restoration project. But the suspended sediment load includes particles of various size (from silt finer than 0.004 mm to coarse sand larger than 1 mm). So as the load increases, the mixture of grain sizes changes. The size of the load material can also influence economic and environmental decisions.