Riparian forest is a major source of particulate organic matter (POM) in rivers, but its contribution has not been quantitatively described. We sampled four size fractions of suspended and deposited organic matter along two stream sections (order: 4 and 5) in central Japan from August 2007 to January 2008. One section is located downstream of a reservoir and covered by riparian forest. The other is located downstream of forested area and flow through agricultural land having no riparian forest. In both sections the source of coarse POM (> 2 mm) drastically shifted within the 3 km section. The presence of riparian forest resulted in the increase of POM concentration larger than 63µm and the decrease for finer POM. Further, elemental and isotopic analyses indicated the POM shifts in chemical composition and in source also for fine POM (1 µm - 2 mm) within 3 km, which is an estimate of its uptake length. In conclusion, we found that the contribution of riparian forest for POM dynamics was brought a few kilometers downstream in both quantity and quality in the medium-sized streams, implying the resultant shift in microbial and benthic communities.