Sediment transport from continuous monitoring in a perennial Mediterranean stream

Sediment transport in the Ribera Salada stream was estimated from continuous suspended and bedload monitoring between 2005 and 2008. The Ribera Salada basin is a mountainous perennial river in the Southern Pyrenees and is representative of the extensive forestry land use in this upland region. Water and sediment fluxes have been analysed with the aim of determining the thresholds, duration, ranges and relative contributions of the two sediment transport modes (suspended and bedload), and their variability in relation to the basin's annual hydrology. The stream's hydrology in the first two sampling years was average in the context of a 10-yr series of the basin, while 2007–2008 can be classified as a wet year. The specific total sediment load during the study period amounted to 12 t km−2y−1, a low value compared with similar Mediterranean counterparts. The maximum load (31.5 t km−2y−1) was observed during the wettest year of the monitoring programme. During average years most of the total load (> 90%) was transported in suspension; consequently, suspended sediment transport is more frequent through time. In contrast, bedload occurs sporadically during floods that exceed certain hydraulic thresholds, corresponding to a flow equalled or exceeded 4% of the time, that yields an average shear stress of not, vert, similar 35 N m−2. Under such conditions bedload transport becomes relevant and, if the threshold is frequently exceeded as in relatively wet years such as 2007–2008, may constitute the majority of the total sediment load (74%). This paper provides new evidence of the distinct role of sediment transport modes in stable fluvial environments where only sporadic inputs of surplus energy (flow discharge) determine the dominant mode, magnitude and duration of their respective contribution.