Grain-size variability and formation process of lowstand river sediments in the Japanese Islands: A review and outlook

The downstream fining of riverbed sediments is ubiquitous; however, this pattern is controlled by multiple factors, including abrasion, sorting, characteristics of sediment sources, water discharge, and river gradient. During the Last Glacial Maximum (LGM), river characteristics (e.g., water discharge, river gradient) differed from those of modern rivers, and the nature of downstream fining in LGM rivers is poorly understood. Due to sea-level fall prior to the LGM, incised valleys formed beneath the present-day coastal lowlands in the Japanese Islands. The bases of these valleys consist of lowstand river sediments (LRSs). Most LRSs in the Japanese Islands are composed of uniform gravel, giving rise to the hypothesis that gravel discharge increased due to periglacial processes in the upper reaches of the rivers during the LGM. In this study, we compiled data on the physical characteristics of 50 Japanese rivers with coastal lowlands at their mouths. From these data, we demonstrate that LRSs span a range of grain sizes from cobble to sand. LRS grain size and thickness decrease downstream, and their maximum observed values are positively linearly correlated with the maximum water discharge, distance from the edge of the continental shelf, and LGM river gradient. These findings suggest that the production of coarse materials was not necessarily a result of periglacial processes, which occurred in a very limited area of the Japanese Islands during the LGM.

Automated summary

This study shows that riverbed sediments in Japanese rivers undergo downstream fining, with grain size and thickness decreasing towards the river mouth. The maximum grain size and thickness are linearly correlated with maximum water discharge, distance from the edge of the continental shelf, and the river gradient during the last glacial period. These findings suggest that the production of coarse materials was not solely due to periglacial processes, but occurred in a limited area of the Japanese Islands during the last glacial period.