<p>Riverbed degradation and biodiversity loss downstream of dams are widely recognized; however, the extent to which tributaries with low sand availability can alter these impacts remains unclear. This study investigated longitudinal changes in environmental conditions and benthic macroinvertebrate communities in a dam-regulated river that receives inflow from a sand-poor tributary. Comprehensive surveys of water quality, plankton drift, riverbed substrate, and macroinvertebrates were conducted along the mainstem and the tributary. The results showed that immediately downstream of the dam, macroinvertebrate assemblages were dominated by lentic-associated filter feeders, such as <i>Macrostemum radiatum</i> and Spongillidae, due to increased plankton inputs and coarsened substrates. Downstream, tributary inflow partly altered these dam-associated conditions: dissolved oxygen increased, plankton densities decreased, and riverbed substrate shifted toward finer coarse material, supporting a rise in scraper taxa such as Heptageniidae. In contrast, inorganic case-bearing caddisflies did not respond, consistent with the tributary’s limited supply of coarse sand. The results indicate that even tributaries with low sand resupply may control certain dam-induced environmental changes and contribute to shifts in macroinvertebrate community structure; however, their influence remains constrained for sediment-dependent taxa. Understanding how sediment type shapes tributary effects is essential for managing ecological conditions downstream of dams.</p>