Characteristics, distribution and geomorphic role of large woody debris in a mountain stream of the Chilean Andes

The paper presents an analysis of amounts, characteristics and morphological impact of large woody debris (LWD) in the Tres Arroyos stream, draining an old‐growth forested basin (9·1 km 2 ) of the Chilean Southern Andes. Large woody debris has been surveyed along a 1·5 km long channel section with an average slope of 0·07 and a general step–pool/cascade morphology. Specific wood storage is very high (656–710 m 3 ha −1 ), comparable to that recorded in old‐growth forested basins in the Pacific Northwest. Half of the LWD elements were located on the active floodplain, and around two‐thirds of LWD elements were found in accumulations. Different types of log jam were observed, some heavily altering channel morphology (log‐steps and valley jams), while others just line the channel edges (bankfull bench jams). Log‐steps represent approximately 22% of all steps, whereas the elevation loss due to LWD (log‐steps and valley jams) results in 27% loss of the total stream potential energy. About 1600 m 3 of sediment is stored in the main channel behind LWD structures, corresponding to approximately 150% of the annual sediment yield.