Junction angle as a primary control on bed sediment characteristics in bedrock river confluences

River confluences are hydrogeomorphological hotspots. We investigated the distribution and characteristics of bed sediments in bedrock river confluences exhibiting distinct junction angles within the Serra Geral Group, located in the Pedras River basin, Guarapuava, south of Brazil. Understanding sediment distribution in bedrock confluences is crucial due to their distinct erosional dynamics compared to alluvial systems. This investigation involved field surveys quantifying channel morphology (width, depth, and slope) and the diameter, sphericity, and size selectivity of bed sediments upstream and downstream of the main river, and upstream in the tributary at representative fluvial confluences with acute (28°-55°-75°), orthogonal (80°-85°-92°), and obtuse (110°-115°-135°) junction angles. Our results indicate that increasing junction angle is associated with enhanced selectivity and sphericity of bed sediments. Furthermore, the junction angle influences the odds of finding larger bed sediment diameters depending on the position relative to the confluence and the junction angle class. Bed sediment diameters were generally greater in obtuse and orthogonal confluences compared to acute confluences. These findings suggest that junction geometry dictates the transition between longitudinal and transverse momentum dominance, fundamentally altering the energy distribution available for bedrock incision and sediment transport. In contrast to the larger grain sizes and greater size selectivity observed in orthogonal and obtuse junctions, acute junctions are characterized by finer, more angular, and less selective bed sediments in their fluvial beds. A statistically significant difference in median sediment diameter was found between acute and orthogonal confluences, and between acute and obtuse confluences. The fluvial junction angle determines the differentiation in the distribution, morphology, and selectivity of sediments in the beds of these bedrock fluvial confluences. • The junction angle controls sediment distribution in bedrock fluvial confluences; • Orthogonal and obtuse junctions limit sediment transport competence downstream; • Sediment diameter and sorting vary with acute, orthogonal, and obtuse junctions; • Flow turbulence patterns and scour hole location are directly determined by the fluvial junction angle; • Bedrock confluence morphology is defined by the interaction of junction angle, bankfull erosion, and tectonic control.