Experiments have been performed to investigate the sporadic motion of cubic and rectangular prism particles at the threshold of hydraulic conveying in a pipe inclined at various angles. The particle spends periods of time at rest at the invert of the pipe before moving variable distances downstream; the process then repeats. It is shown that successive time periods of rest auto-correlate, meaning that the particles exhibit some memory effect with regard to rest times, although there is a distinct random factor that is presumably due to turbulence. Further, it is statistically shown that the memory is real. The curve of memory factor vs. flow rate transposes up the abscissa as pipe inclination and particle density increases and exhibits a maximum in between total particle stasis and saltation. The physical basis for this memory effect is open to debate, but may be due to the fact that particles can adopt multiple orientations whilst at rest, some of which are more stable then others. The memory factor is shape dependent. (c) 2005 Elsevier B.V. All fights reserved.