Downstream changes in mean grain size and sorting coefficient are examined for four streams flowing in similar physiographic environments. Mean grain size decreases exponentially as expected, although there is considerable scatter about regression lines which can in part be related to tributary effects. Of greater interest is the downstream variation of bed material sorting which is described in each case by a sinusoidal function offering a higher level of explanation than does linear regression. This periodic behavior of the sorting coefficient is explained in terms of sediment inputs from tributary sources being superimposed on the over-all tendency for sorting and abrasion processes acting in the main stream to improve bed-material sorting in the direction of transport. The established sinusoidal relation between mean grain size and sorting cannot by itself explain the periodic variation in this case. An alternative descriptive function for the sorting data is discussed.