A continuous record reveals that the incidence of bedload in a coarse-grained river channel changes from flood to flood. Long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood-wave. But when flood's follow each other closely, the bed material is comparatively loose and offers less resistance to entrainment. In this case, substantial amounts of bedload are generated on the rising limb. This is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season. This produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse-grained channels. Besides this, the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds offinal motion. This adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory. Bedload is shown to be characterized by a series of pulses with a mean periodicity of 1.7 hours. In the absence of migrating bedforms, it is speculated that this well-documented pattern reflects the passage of kinematic waves of particles in a slow-moving traction carpet. The general pattern of bedload, including pulsations, is shown to occur more or less synchronously at different points across the stream channel.

Indirect bedload transport measurements have been made with the Swiss plate geophone system in five gravel‐bed mountain streams. These geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed. To calibrate the geophone system, direct bedload transport measurements were undertaken simultaneously. At the Erlenbach in Switzerland, a moving‐basket sampler was used. At the Fischbach and Ruetz streams in Austria, a Helley–Smith type bedload sampler provided the calibration measurements. A Bunte‐type bedload trap was used at the Rofenache stream in Austria. At the Nahal Eshtemoa in Israel, Reid‐type slot bedload samplers were used. To characterize the response of the geophone signal to bedload particles impacting on the plate, geophone summary values were calculated from the raw signal and stored at one second intervals. The number of impulses, i.e. the number of peaks above a pre‐defined threshold value of the geophone output signal, correlated well with field measured gravel transport loads and was found to be a robust parameter. The relations of impulses to gravel transport loads were generally near‐linear, but the steepness of the calibration relations differed from site to site. By comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments, it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient. The analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload. Copyright © 2013 John Wiley & Sons, Ltd.

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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.