Dynamics and management alternatives of in-channel large wood in mountain basins of the southern Andes

Abstract

Large wood has a positive effect on channel morphodynamics and freshwater biodiversity of mountain streams. However, its presence can enhance the risks associated with extreme flood events in inhabited floodplains. This study reports on wood abundance, spatial distribution, recruitment and depositional mechanisms in three third-order basins of south-eastern Andes (Tres Arroyos, Río Toro and Buena Esperanza). Major differences in large wood loads and quantity exist among the analyzed basins, due to different disturbance history and forest cover features. Marked logs in Buena Esperanza and Tres Arroyos were surveyed before and after floods. Ordinary events (recurrence interval < 2 years) moved only small and isolated logs (diameter < 0.25 m; length < 3 m) and for relatively short distances. The reported study basins are taken as paradigmatic cases to illustrate a management strategy for hazards associated with wood transport during major floods. Log removal and riparian vegetation cuts are not effective strategies because high-magnitude, infrequent events are able to recruit trees from hillslopes due to mass wasting processes. Flood-prone area should be preserved free from sensitive settlements. However, important localized infrastructures can be protected with specifically designed structural solutions. Their potential use in the context of the three analyzed Andean basins is discussed.