| Sumario: | The dynamics of the torrential upper reach of the Ijuez River, Central Spanish Pyrenees, was analyzed in relation to the spatial organization of grain size. The reach is located in the Eocene flysch, which usually acts as a high sediment production area, particularly during periods of intense human activity. The Ijuez Valley was cultivated and densely populated until the middle of the 20th century, resulting in general deforestation and the development of debris flows, shallow landslides, sheet wash erosion and deep gullies. As a consequence, the alluvial plain became characterized by the presence of large quantities of coarse sediment. The spatial organization of sediment showed a progressive downstream decline in the cobble and boulder size, especially in the channel, whereas the remainder of the alluvial plain showed large variability. No trend in the occurrence of the largest boulders was evident throughout the reach, confirming the importance of debris and hyperconcentrated flows in sediment transport. The torrential reach did not have a negative exponential longitudinal profile, in contrast to that usually observed for mountain rivers. This was attributed to the large quantity of heterometric sediment derived from the hillslopes, which resulted in no marked decline in sediment size and consequent change in the longitudinal profile. A recent trend of scouring was detected, reflecting the reduction in sediment supply following farmland abandonment and reforestation, which have reduced connectivity between the hillslopes and the channel.
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