Trajectories of bed load particles within the active layer of an ephemeral stream

Nahal Yael, a small, intensely instrumented, high-relief catchment in the Southern Negev Desert, Israel, has been operating as a hydrological field laboratory since 1965. Sediment transport by desert floods along the braided channel of Nahal Yael is being investigated by two complementary long-term monitoring systems: (a) surficial tracing of painted bed load particles since 1970 (red, blue and yellow generations, 870 particles in total, with each colour representing a different injection time); and (b) three-dimensional tracing of magnetized pebbles (138 particles) monitored since 1985, which enable the definition of relations between the surficial and buried particles. Since travel distance and burial depth of tagged material are often used for approximating bed load yield, it is of interest to evaluate their variability. The results indicate that short pebble trajectories are associated with a burial episode, while long paths are associated with exposure from within the active layer. Pebbles entrained from a surficial position and reaching a surficial position at the end of the step move longer distances than pebbles which are found buried at the end of the step. Pebbles entrained from a buried position and subsequently found on the surface move the longest distance. Intermediate burial has no appreciable effect on aggregate transport distance, since the short travel distance of a burial episode is compensated by a longer trajectory from a buried position to the surface. Tracers recovered from varying depositional environments (bars, inner channels), if extrapolated over a series of flood events, document the interchangeability of material between surficial and buried locations, and help to explain the isomorphic characteristics of braided channels.