A network-based routing model is used to determine how spatial variations in sediment supply influence rates of downstream fining in a small watershed in central Pennsylvania. Gravel of a specified size distribution and lithology is supplied to the network at zero-order basins. Once in transport, the total sample weight and moments of the grain-size distribution are modified according to empirical functions of transport distance. Contributions from each zero-order basin are summed to determine the total population of sizes and lithologies at any location. In the study area, resistant quartz sandstone ridges supply sediment an order of magnitude larger than other lithologies. Because these ridges occur primarily in the basin headwaters, spatial variations in supply strongly influence rates of downstream fining. Model calculations indicate that spatial variations in supply alone account for over 80% of the decrease in mean grain size along the main channel. ¿ American Geophysical Union 1995