Network Structure Control on Bed Material Grain Size Distribution

The size distribution of the sediment load of a river is a function of the size distribution that enters the river, and the grain-size dependent effects of particle breakdown and net differential storage during downstream transport. In tectonically active mountains, where no net storage of sediment occurs, the average particle size of the sediment load crossing any reach must record the transformation of the input size distribution by the travel-distance dependent particle breakdown. Particles enter at different points along the river network and gather downstream. Hence, the network structure imparts load and travel distribution functions and therefore may strongly control sediment load size distribution. We explore these effects by using the width function (density function of travel distance to an outlet), a contributing area function (per unit reach of a given distance from the outlet (giving input rates)), an exponential decay function for gravel size with travel distance, and various input size distributions. In the simplest case of the input of a single grain size throughout the network, the contributing area and travel distribution functions cause entering particles to have a wide range of travel lengths to a given reach, and consequently a broad distribution of river load particle size is predicted. The lower the attrition rate, the narrower this distribution and more normal the size distribution function. High attrition rates produced grain size distributions skewed to smaller sizes. Grain size distribution passing a reach of the river, in the single grain size input case, is the direct reflection of the travel distribution function. The use of input functions that may typify natural size distributions results in size distributions that are predicted to be skewed towards smaller sizes. This simple modeling suggests that the size of the catchment, the network structure, and the form of the particle breakdown function may strongly dictate the grain sizes available for transport. In turn, this will influence the evolution of channel slope and characteristic bed morphology.