Suitable Grain Size for Distinguishing Mainstem and Tributary Gravelly Deposits in Riverbeds and Terraces Using Roundness Image Analysis

Roundness of gravel particles is widely used as an indicator of sediment transport, yet its potential for discriminating provenance between mainstem and tributary rivers has not been fully tested. In this study, we applied an image‐analysis‐based roundness measurement (Rgrains) to gravel samples from mainstem and tributary deposits in the Tama and Doushi river basins in central Japan. We used both modern riverbed sediments and terrace deposits across four grain‐size fractions (2–4, 4–8, 8–16, and 16–32 mm) to examine suitable grain sizes for distinguishing mainstem and tributary deposits. The results suggest that terrace gravel clasts may be distinguished between mainstem and tributary sources by comparing their roundness distributions with those of modern riverbed sediments. Mainstem sediments consistently exhibited higher roundness than tributary sediments, whereas tributary deposits showed statistical characteristics biased toward specific roundness values, reflecting the restricted nature of their catchments and channels. The minimum grain size required for identification depended on the tributary gradient: in steep‐gradient tributaries dominated by debris flows, clear differences were observed even in fine fractions (2–4 and 4–8 mm), whereas coarser fractions (16–32 mm) were generally required for low‐gradient tributaries dominated by bedload transport. Furthermore, fluvial transport processes inferred from riverbed slopes appear to exert a stronger influence on clast roundness than lithology, which plays a relatively minor role. The roundness of terrace gravel clasts may preserve information on tributary gradients and transport modes at the time of deposition, and comparison with modern riverbed gravel clasts suggests that past tributary gradients may be reconstructed. These findings suggest that quantitative roundness analysis provides a useful approach for linking terrace gravel clasts to their provenance, offering insights into fluvial terrace development and sediment transport in mountainous rivers.