Midday drift samples were collected through summer and autumn from two reaches of Jordan Creek, a second-order stream in east-central Illinois. The two-study reaches differed primarily in gradient and substrate characteristics, with silt-sand predominating in the low-gradient reach, and gravel-cobble predominating in the high-gradient reach. Aquatic insect larvae, especially dipterans, were the most abundant invertebrates in samples from the gravel-cobble reach, but were uncommon in drift from the silt-sand reach, which was composed mostly of small copepods and cladocerans. Invertebrate abundance and size declined markedly through the summer in both study reaches, then increased through autumn. Insect larvae were virtually absent from the silt-sand drift in October, but were always present in drift from the gravel-cobble reach. On the basis of invertebrate size and abundance, the gravel-cobble reach provided a more dependable food base for Jordan Creek fishes than did the silt-sand reach. Stream modifications that alter substrate composition may increase the severity of seasonal food shortages for fish, and contribute to declining diversities in stream fish assemblages of the Midwest.