The influences of tectonic and climatic changes on upland river systems are investigated using data from Plio-Pleistocene terraces of the Rio Cinca river in the southern Pyrenees. This river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the Ebro Basin. Detailed field mapping, combined with long-profile and palaeohydraulic reconstructions, yields a comprehensive picture of changes in palaeoriver character during the Plio-Pleistocene. As the area is over 150 km from the basin outlet in the Mediterranean Sea, changes in base level are unlikely to have influenced terrace development. Although tectonic activity has exerted a strong control on the position of the river, the main period of thrust propagation pre-dates the terraces and activity has waned from the Pliocene through to the present. It is concluded that the main control on incision in this area is climate, through its influence on sediment supply. Rivers which are starved of sediment by climate change will have the power to incise, whereas aggradational phases are linked to periods of increased sediment flux.