Riverine systems serve as a major pathway for microplastic transport, yet the continental-scale watershed attributes associated with their distribution patterns remain inadequately understood. This study investigated riverine microplastic distribution and its predictors using data from 171 watersheds across North America, incorporating hydrometeorological, pedological, and anthropogenic variables. Our analysis of 540 samples highlighted widespread microplastic pollution, with concentrations varying significantly across the continent. Microplastic concentrations in North American rivers ranged from 0 to 27.8 particles/L, which was generally lower than concentrations across global groundwater systems (0-97.0 particles/L). Microplastics were predominantly blue fibers <1.5 mm. Based on principal component analysis, we identified a soil structure gradient and a hydro-ecological gradient as key environmental templates within the watersheds that potentially influence microplastic concentrations in rivers. The strongest statistical associations were observed with saturated hydraulic conductivity (R-2 = 0.79-0.83, p <0.01, negative), silt content (R-2 = 0.47-0.60, p <0.05, positive) and runoff (R-2 = 0.41-0.49, p <0.1, negative), which were consistent and significant on the continental-scale and in humid regions. In contrast, NDVI (R-2 = 0.47, p <0.1, negative) was a more prominent predictor in arid regions. Silt content was identified as a potential key indicator influencing microplastic transport within watershed, with textural classes (USDA) high in silt associated with greater microplastic concentrations in rivers. Our findings reveal the broad patterns in microplastic pollution across North American rivers and suggest that factors such as hydrological and pedological conditions shape these patterns at continental scale. These insights provide a scientific basis for targeted monitoring and the development of effective mitigation strategies.