The channel dynamics of an ephemeral sandbed river, the Metsemotlhaba in southeast Botswana, were studied over a five year period using tagged magnetic particles of pebble and cobble size, scour chains and topographic cross-sections, with particular emphasis on the three-dimensional dispersal of gravels, patterns of scour and fill and depth of active layer. Two major flow events of equivalent magnitude occurred, moving the tagged particles, in December 1987, a mean distance of 837 m at a mean burial depth of 0ร40 m, and in March 1991, a mean distance of 263 m at a mean burial depth of 0ร39 m. The volume of mobile sediment, based on scour depth and distance of travel, was 2ร7 times greater in the December 1987 event, in which the mean scour depth was almost twice the mean burial depth of the tagged particles. The distribution of distance of movement was asymmetrical in this first flood, when the tracer started from a surface location, but was monotonic thereafter. Intervening small to medium events yielded limited tracer movement with a mean burial depth equivalent to depth of scour.
The tracer moved in the low and transitional flow regimes. Burial depth distribution followed the gamma model. Field data confirm that longitudinal transport is independent of particle size and shape, and strongly skewed with respect to distance, whilst depths of scour in excess of 1 m for high magnitude events suggest that scour values predicted from the empirical equation of Leopold et al. underestimate by an order of magnitude.