Abstract
To investigate the influence of river outflow on the amount of litter washed ashore, we analyzed our monthly monitoring data in Tottori Prefecture. Due to the surface flow of river water carrying the litter from the drainage basin, Japanese litter was significantly more abundant on beaches near river mouths, whereas litter of foreign origin was less abundant on such beaches. As the amount of Japanese litter is roughly inversely proportional to the distance from the river mouth, we can estimate that nearly half of the Japanese litter originated from within the river basin. This estimate was supported by analysis of addresses printed on advertising lighters washed ashore. Similarly, based on the addresses on lighters, the Japanese litter on the beaches was found to originate from an area from Nagasaki to Tottori Prefectures. Despite the fact that almost half of the total Japanese litter originated from the local river basin, only approximately 3% of the population of this area lives in the river basin, indicating that litter from river mouths strongly tends to stagnate in local coastal areas. The most effective method for reducing litter on local beaches is therefore to reduce the amount of litter dumped in local river basins.
