AbstractTwo landsliding episodes between late 1973 and early 1975 delivered about 60000 m3of sediment to six small deeply incised streams draining a 2·7 km2area. About 4700 m3of logs in the landslide debris formed major log jams in five streams, which impounded large volumes of landslide‐derived sediment. Five years after the landsliding, 42 per cent (25000 m3) of sediment was still in storage behind 35 log jams ranging from 1·4–8·2 m high. The landsliding episodes have produced multi‐stepped stream profiles, aggradation of channel reaches up to 150 m long to mean depths between 1·2 and 4·1 m, reductions in gradient, fining of bed material size, and related changes in bedforms and channel width:depth ratios that seem likely to persist for at least several decades. Sediment presently stored behind log jams is equivalent to between 50 and 220 years normal supply of sediment from hillslopes to stream channels. Long‐delayed, large magnitude impacts on higher‐order channels may occur if sudden failure of log jams is induced by a large storm at so