The sediment formation mechanisms of a newly constructed reservoir in Ehime, Japan were evaluated by characterizing the soil particles (SP) and particulate phosphorus (PP) in the runoff and reservoir sediments. The SP and PP loads from the runoffs of the main river in the watershed considerably increased, when the specific discharge rates were over 300 l/s/km2 (high flow conditions). When the specific discharge rates exceeded over 300 l/s/km2, 19% of the watershed generated over 80% of the SP and PP loads. When the specific discharge rates were under 300 l/s/km2 (low flow conditions), the contributions of the previously mentioned 19% area to the SP and PP loads were smaller. Significant amounts of smectite were found in the sediments in the reservoir and in the soil samples obtained at the forest exposed area in this 19% area while it was negligible in citrus orchards and paddy fields that constituted the remaining land surfaces. The forest area exposed by recent landslides was significant for the SP and PP in the reservoir. Judging from the outcomes, land use information alone may not be sufficient to detect critical sources of SP and PP in the runoffs and reservoirs. To identify and confirm crucial areas for the SP and PP in the runoffs, the investigations should be conducted under high flow conditions and the composition of clay minerals in the sediments should be checked against the clay mineral distributions of soils in the watershed.