GULLY AND STREAMBANK EROSION AND THE EFFECTIVENESS OF CONTROL MEASURES IN A SEMI-ARID WATERSHED

Quantifying the sedimentation contributed by different erosion pathways, including rill, interrill, and gully and streambank soil losses at the watershed scale is crucial to know for adopting the best management practices to reduce soil erosion and reservoir siltation. This study had two goals: (1) To quantify streambank and gully erosion along two stream orders (1st and 2nd) or stream types (perennial and intermittent channels) of the Oltu sub-watershed within the Coruh River Basin in northeastern Turkey, and (2) To assess the effects of rehabilitation efforts on headwater ephemeral channels by using dry-stone and wire-cage sills. The 2-year measurements of gully and streambank erosion rates were carried out using the "erosion pin method". The total gully and streambank areas, the lengths of stream networks, bank soil bulk densities, and erosion rates were all measured to calculate total soil losses via the stream bank and gully erosion processes. Following the stream classification system, the total stream bank soil loss rates from the 2nd order stream (i.e., main channel), 1 S' order streams, and the gullies in the Oltu sub-watershed respectively ranged from 0 to 29, 48.3 to 286.5, and 3 to 237.8 t km(-1) y(-1). Within the channel network of the Oltu watershed, the contribution by gull erosion was the highest, at 18889 t y(-1) (73%), following by 1st order at 5995 t y(-1) (23%), while the 2nd order stream had the lowest contribution, at 867 t y(-1) (3%). Soil loss from the stream channels was also estimated for the total watershed at 0.7 t ha(-1) y(-1). These low rates of erosion from the gully and stream channels indicated that the dominant pathway of soil loss in the studied watershed is via rill and interrill erosion. However, long-term research is needed to monitor the event-based gully and streambank erosion under the wet conditions with intense rainfall to better understand their temporal effects. Finally, sill structures were found to be very effective at capturing the sediment and reducing the soil transport capacity from the upland areas and/or headwaters to downstream areas, and eventually the reservoirs.