THE RESPONSE OF RIPARIAN VEGETATION TO SEDIMENT DEPOSITION IN MIDSTREAM RIVER CHANNEL

Gravelly channels are typical landscapes of the midstream of rivers providing a unique habitat for pioneering species. These species are recently being replaced by intensive vegetation colonization. A better understanding of the cause of this situation and restoration strategies are requested in many Japanese rivers. We hypothesized that the increase of vegetation cover is caused by the reduction of gravelly sediment which is trapped by dams. Aerial photos of several rivers were analyzed to identify the period required for the re-colonization of vegetation after a flood, in relation to the condition of overlying sediments. Longer periods were required for recolonization at sediment deposition sites, compared to sites without deposition or eroded sites. Deposition of cleansed sediments, which are low in moisture, nutrients and seeds content, over the original substrate, delayed the germination and the growth of vegetation. The Dynamic Riparian Vegetation Model (DRIPVEM) was developed to describe this process. Simulated results reflected observations of vegetation distribution and delay for re-colonization. Results suggest that vegetation coverage increases as the size of sediment deposition area, the magnitude and the frequency of floods decreases. We concluded that the reduction of unstable sediments in the river channel was the main cause of the recent increase of vegetation colonization in riparian areas.