Samples for the determination of the volume-weight ratio of deposited sediment were collected from small sediment-retention reservoirs in a number of forested watersheds in Idaho during 1969 and 1970. Volume weights varied considerably among the various watersheds and within a given impoundment. The percentage of organic matter by weight in the samples provided a reliable basis for estimating sediment volume weight, better than the more conventional approach of relating volume weight to sediment particle size. The organic content of sediment can result in a substantial loss of storage volume for reservoirs on forested watersheds and has important ecological implications as well.