<p>When considering the environmental conservation of brackish lakes, it is essential to assess the influence of tidal inflow from the sea and the associated changes in the lake bottom sediments. In conducting such assessments, methods focusing on the characteristics of sediment grain size, which reflect their sources and sedimentary processes as employed in paleolimnology, prove effective. However, grain-size distributions in lake bottom sediments with multiple sources and processes coexisting are often complex. We investigated sediment supply and transport into Lake Hinuma, a brackish lake in central Japan, through detailed analysis of bottom-sediment grain-size distributions. Surface sediments were collected from 21 offshore sites, and three gravity cores down to 34&#xa0;cm depth were obtained. The cores were divided into 1-cm intervals for grain-size analysis. The grain-size distributions were decomposed into lognormal components using the expectation–maximisation algorithm, and additive logratio analysis was applied to the mixing proportions. The distributions were separated into five or six lognormal components with consistent characteristics. The proportion of the finest component (mean grain size: ~ 9.6&#xa0;phi) varied little among the samples. Logratio analysis using this component as a reference showed that finer components exhibited little variation, whereas coarser components varied both spatially and vertically. The spatial pattern of coarser components indicates sediment supply and transport by flood tidal currents, and their upward increase suggests enhanced transport by these currents over time. This study demonstrates that component separation combined with logratio analysis can effectively extract grain-size components with distinct sources and reveal past changes in sediment transport, which is beneficial for the environmental conservation of brackish lakes.</p>