Authors surveyed the sediment runoff actually occurring in the Miyakejima Island after 4 years have passed since the volcanic eruptions lastly occurred. The survey was carried out by repeating measuring the freeboards of sedimen control dams newly installed in the rivers. Topographic surveys of cross sections of the rivers were also carried out it the two rivers. As a result, it was confirmed the tendency that sediment runoff still persists in the rivers where the mon volcanic ash deposited at the time of the last eruptions. Observing the quality of the runoff sediment, it can be classifrec into the two types, that is, scoria and fine sediment. The result of the topographic surveys in the two rivers, one is the scoria-runoff-river and the other is the fine-sediment-runoff river, showed that the source of the scoria was the sedimen in the riverbed, while that of the fine sediment was the sediment consisting of the river banks and rill ditches. The scoria-runoff-river has abundant erodible scoria on the riverbed still now, while the fine-sediment-runoff river has solic lava bed such that riverbed erosion will no longer occur.
