Vegetation such as willows on sandbars has been known to grow rapidly and increase the flood risks. By this reason, in recent years, administration pay a lot of money and time to prevent vegetation from growing on sandbars. The objective of this study is to elucidate the characteristics of lotic seeds distribution and deposition on bare sandbars, and also to consider the riverbed morphodynamics by laboratory experiments.

In the experiments, after sandbars were created on the flume, enough amounts of alfalfa seeds were supplied from the inlet section of the flume. As for water discharge, we tried two cases with constant and variable. The result showed that alfalfa seeds settled significantly at the front edge of sandbars while water discharge was decreasing. On the other hand, seeds did not settle at all on sandbars under the constant discharge.

After the seeds germinated and grew well on sandbars, the second experiment carried out under the same hydraulic condition as the first experiment. The result showed that bed aggradation of sandbars was caused by the grown vegetation on sandbars, particularly at the front edge of sandbars unlike the case without grown vegetation.

These results implied the following mechanisms for invasion process of vegetation in rivers. First, lotic seeds of pioneer species like willows deposit intensively at the front edge of bare sandbars and form vegetation patches there. Then, it makes easier for other non-pioneer vegetation species to invade on the sandbars because the bed aggradations are induced by the vegetation.