Cohesive sediments (<63 μm), key vectors for pollutants, undergo flocculation within the flow field. Given the complex transport of flocs, sediment transport models often disregard flocculation and the ingress of flocs in riverine substrates. The objective of this study was to evaluate effective particle size distributions (EPSDs) of suspended and ingressed particles in a gravel-bed river in southern Alberta, Canada. Four sites along a 10 km river reach were assessed. EPSDs were measured with a LISST 200x for suspended (in-situ) and ingressed particles (collected using permeable-walled retrievable sediment traps deployed in triplicates through 7 deployment cycles). Analysis of dispersed and effective sizes showed that suspended and ingressed particles were predominantly transported in flocculated form, and modalities between EPSDs suggest that particles are from the same source material. Our results showed that ingressed particles became significantly (p<0.001) coarser during lower flow (LQ) (Q~3 m3/s) compared with higher flow (HQ) conditions (Q~7.5 m3/s), likely due to the reduced turbulence of the interstitial load favoring flocculation. Conversely, the particle size distribution of suspended macroflocs (130–500 μm) did not differ between LQ and HQ. Under both flow conditions, comparisons between suspended and ingressed particles indicate that ingress is size-selective and/or differentiated through interstitial flocculation. This study advances the understanding of flocculation mechanisms and their effects on ingress under turbulent flows. These types of data are vital for enhancing existing models to simulate the transport and fate of cohesive sediment in gravel-bed rivers draining mountainous, forested headwater regions that are crucial for water supply.