The St. Francis River of southeastern Missouri along its first 31.5 mi has an average gradient of 9.2 ft./mi in a region of hilly terrain with maximum relief of 1000 ft. The drainage area encompasses approximately 870 mi 2 in an area of temperate climate with a mean annual rainfall of 40 in. Coarse and fine-grained Precambrian granitic rocks and isolated patches of Paleozoic sedimentary rocks each contribute unique detritus to the fluvial environment. Petrographic analysis of the coarse, medium, and fine sand size fractions of main channel and tributary samples reveal the respective roles of dilution, abrasion, and selective sorting in altering sediment composition. Dilution of the main channel from a rhyolitic source terrain in the southern portion of the study area tends to mask the effects of abrasion and selective sorting. Downstream decreases in relative percentages of monocrystalline quartz, polycrystalline quartz, and granite rock fragments, however, can be attributed, in part, to abrasion and/or selective sorting. Granite rock fragments show the least amount of physical durability, being almost totally absent from the size range examined at the southern end of the study area.