Evaluating Spatiotemporal Distribution and Interactions of Sediment Grain Sizes With Salinity, Density, and Nutrient Profile in a Subtropical River Watershed

The southern part of Bangladesh hosts a network of numerous rivers and associated streams (locally known as khal ), originating from hilly terrain and transporting diverse sediment grains toward the Bay of Bengal. Despite their ecological and geomorphic significance, the spatiotemporal distribution of sediments and their interactions with key physicochemical parameters remain poorly understood in this subtropical setting. This study focuses on the Chowfaldandi watershed within the Matamuhuri River basin in Cox’s Bazar, Bangladesh. Surface sediment samples were collected using transect methods across three designated stations from upstream to downstream during the dry (February) and wet (July) seasons of 2022, with standard replication. Significant seasonal variations ( p < 0.05, one‐way ANOVA) were observed in several parameters, namely, very coarse sand (0.05%–3.09%), medium clay (1.55%–3.99%), fine clay (0.80%–3.40%), sediment salinity (2–24 psu), particle density (2.45–2.67 g·cm −3 ), and PO 4 –P (4.13–8.96 μg·kg −1 ). Principal component analysis revealed that sediment grain size and nutrient variables jointly contributed to 76.7% of the temporal variation in sediment characteristics. No significant spatial variation was detected in the watershed. Very fine sand and silt particles predominated seasonally, with a strong positive linkage between medium and very fine silt with NO 2 –N concentration ( p < 0.01). Similar associations were also found between medium sand and bulk density, as well as between medium and very fine clay with nutrients, particle density, and salinity ( p < 0.05). Regression analysis further confirmed the seasonal influences of grain sizes on nutrient content and density. These findings enhance the understanding of sediment grain size–physicochemical interactions in subtropical river systems and offer valuable insights for evidence‐based watershed management and coastal planning.