Pleistocene river diversions caused by large landslides in the western Pyrenees (Oria River drainage basin, N Spain)

River‐blocking landslides exert a deep impact on mountain range landscapes and the organization of catchments. A blocked river diverted to another watershed modifies both original and transferred drainage networks both up‐ and downstream. Using western Pyrenees examples, a geological and geomorphic framework with diagnostic criteria to detect river diversion by landslides is presented, including the identification of elbows of diversion, eroded divides, beheaded underfit rivers, diverted overfit rivers, reversed river segments and the landslides at fault. Some landslides caused the formation of lakes that overflowed upstream at catchment divide segments with elevations lower than those of blocking landslide tops. Unravelling the presence of fan deltas at distinct sites/elevations of palaeolake shores contributed as well to identification of river damming and later diversion episodes. Reconstruction of the sedimentary organization of river palaeovalleys and of their associated fluvial terraces and palaeoriver channels (some currently submerged by the Cantabrian Sea), along with the reconstruction of river profiles, analysis of bedrock and morphology of watershed divides, identify seven river diversions caused by landslides and 14 additional slides that variably constrained river basin dynamics in the area studied. The diverting slides have current areas between 0.06 and 12.3 km 2 (thus including giant examples), thicknesses up to 300 m and translational–rotational rupture surfaces usually with low dip angles (3.5–12.3°). A combination of relative dating methods and published absolute ages suggests that diversion events occurred during the Quaternary. This study shows that river diversion by landslides can be significant in mountainous areas of moderate relief.