Particulate organic matter (POM) supplied from riparian forest is used as major food and habitat for shredders and gatherers of aquatic insects in rivers. Therefore, the conservation of riparian forest may be motive for maintaining species diversity of aquatic insects. However, the quantitative relationship between POM and community structure of aquatic insects in artificially modified rivers has not been reported. We investigated water and sediment quality including POM and aquatic insects during 3 years at 6 stations within 10 km section of the Miyamori River (order: 1-2) in northern Japan. The presence of riparian forests in the upper (St.1, order: 1) and middle (St.4, order: 2) reaches apparently increased the flux of POM (size: > 1 mm) in both reaches during leaf fall season. The biomass of gatherers was recovered at St.5 (order: 2) located in 1 km downstream of St.4 while the biomass of shredders were remarkably low except for St.1. The result of multivariate analyses showed that POM (< 3.35 mm) in the sediment was one of the main positive factors for the biomass of gatherers, and high amount of POM (> 1 mm) in the sediment and low velocity caused high biomass of predators.