Traps designed to simulate the framework gravel of natural streams and installed in a coarse-grained channel allow the identification and differentiation of the processes governing the infiltration of fines into bed interstices. Vertical patterns of matrix distribution reflect the relative importance of variables acting at both the surface of the channel bed and within the bed sediments. Differences in grain size (and therefore pore size) of the framework exercise a strong controlling influence. An armour layer of coarse particles superimposed on comparatively finer sub-surface bed material—common to many coarse-grained rivers—is shown to encourage the clogging of near-surface pores with matrix fines. Below the obstruction, interstices may be protected from incursion and the gravel framework is often left open. This is the reason for patchy concentrations of matrices often reported in the stratigraphical literature. In contrast, a uniform size-grading of the bed sediment, or an upwards fining, leads to a more uniform packing of the interstices with matrix. Flood history is shown to be an important determinant of both the size-distribution and the spatial distribution of matrices, and is useful in providing a dynamic explanation of the variability of matrices observed from place-to-place in ancient deposits.