The different velocities of flood waves and stream flow can be a major control on the relation of stream discharge to suspended sediment concentrations. Flood waves move downstream faster than the flow velocity, thus leaving the original floodwaters and their entrained sediments lagging increasingly farther behind with increasing distance downstream. This process can be modeled by routing the changes in discharge at the flood-wave celerity, while routing the sediments at the flow velocity. Testing of this model using data previously collected for the Bighorn River, Wyoming and Montana, indicates that differences in flood-wave and flow velocity can explain a large portion of the downstream variations in the relation of discharge to concentration through time. The routing model results suggest that one explanation of seasonal and storm-period variations in at-a-station sediment rating curves may be seasonal changes in the distance to sediment sources.