Differential microplastic storage in main channel and backwater sediments of the Kaskaskia River (Illinois, United States)

Rivers are major pathways for microplastic transport, yet controls on microplastic storage across different riverine environments remain poorly constrained, particularly in managed systems. Here, we quantify microplastic abundance and characteristics in water and sediments from the Kaskaskia River (Illinois), a regulated tributary of the Mississippi River, with a specific focus on differences between main channel and backwater environments. Water and sediment samples were collected from three locations downstream of Carlyle Lake and analyzed for microplastic concentration, morphology, size, and color. Microplastic (MP) concentrations averaged 2.9 MP/L in the water column and 34.4 MP/kg in sediments, with clear fibers dominating both matrices. Contrary to expectations that low‐energy backwaters would act as preferential sinks, sediment microplastic concentrations were significantly ( p < 0.05) higher in main channel environments in comparison to adjacent backwaters. In contrast, microplastic concentrations in water did not vary by site, riverine environment, or depth, and were not correlated with total suspended solids ( p > 0.05), indicating a decoupling between microplastic and suspended sediment transport. Microplastics in backwater sediments were larger than those in the main channel ( p < 0.05), suggesting enhanced fragmentation in high‐energy main channel environments. These results demonstrate that in regulated, low‐gradient rivers, main channels can function as microplastic storage zones, rather than solely as transport pathways, and backwaters may consistently serve as ecological refuges. Our findings emphasize the importance of cross‐sectional sampling and environment‐specific assessments to accurately characterize fluvial microplastic dynamics and inform freshwater monitoring and management strategies across all riverine environments.