Disturbance and recovery of riparian and fresh water ecosystem caused by sediment transport associated with flood events

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Based on three years observation for morphological riverbed changes and sediment transport, the factors disturbing riparian and freshwater ecosystems were analyzed. Rainfall, runoff, sediment storage volume, sediment production volume, bank erosion rate, suspended sediment volume and rock size distribution along the drainage net were measured at the Oyabu creek of a headwater of the Hitotsuse River located in the middle mountain range of Kyushu island, Japan. The following results were gained.(1) Sediment accumulated after a flood event trended to restore sediment storage volume to the minimum value (4000mィイD13ィエD1/km) with time elapse. (2) Sediment slug associated with riverbed degradations was occurred with 2.65m/day in velocity and that with riverbed aggradations was occurred with 4.6m/day in velocity respectively. (3) The volume of sediment supply from tributary channels decreased exponentially against time elapse. (4) Rock size distribution along a channel related with macroscopically the distance from sediment production source, and slope and catchment area of significant tributaries (slope<0.15, area>10ha) joined with main channel. (5) Bank erosion rate of the upper channel (8m in averaged channel width) of the Oyabu creek was 10cm/year.These results imply that the recovery of riparian and freshwater ecosystems is likely to be controlled by magnitude of riverbed aggradation, frequency of flooring as the recurrence interval and the time lapse after flood event. Furthermore rock size distribution along drainage net can influence on the relaxation time of ecological property of fresh water.