In mountain and piedmont hydro-systems, gravel-bed rivers often transport large amounts of fine (silt-sized) sediments. The dynamics of such a sediment mixture is usually described as a simple superposition of fine sediment transport in suspension over a bed-load of gravel. However, the interaction between the two classes of sediment is strong. This study is mainly focused on one of the aspects of this interaction, which is the development of a clogged layer through the infiltration of fine sediment into a coarser bed. Clogging affects the spawning process of many fluvial fish species and has different socio-economic consequences concerning to water supply and river dam management operations. Previous studies highlight the influence of flow velocity and fine sediment concentration on the total amount and the depth distribution of fine sediment within the gravel bed. Moreover, the ratio of mobile to bed sediment size has proved to be determinant concerning to the mechanism of infiltration and the formation or not of a sealing layer at the surface of the channel bed, that prevents from deeper infiltration of fine material. This study shows the results of laboratory experiments performed in a 18-meter long and 1-meter wide channel, focused on the analysis of the infiltration process of silt-sized suspended sediment within a gravel channel bed (mean diameters of 30microns and 8mm respectively), a combination to which little attention has been paid in the past. The influence of flow velocity and suspended sediment concentration on the amount of sediment infiltrated is analyzed. On the other hand, results on depth distribution of the infiltrated sediment are provided and interpreted, discussing the mechanisms of infiltration and the physical processes involved in them from a pore scale point of view. The present study casts light into geomorphological aspects of river behavior that have important environmental and socioeconomic impacts.