Large wood mobility in mountain rivers, chile

Abstract

Previous research on large wood (LW) entrainment and mobility has shown that in small rivers most mobile pieces are shorter and thinner than bankfull width and depth, and are deposited perpendicular or oblique to the flow as individual pieces. LW size, quantity and position were investigated over several years in channel segments of four mountain experimental catchments in southern Chile. Every wood piece found within the bankfull channel with more than 10 cm in diameter and 1 m in length was measured, its position was referenced and several of these elements were tagged. The channel segments were re-surveyed after consecutive winter seasons and every tagged wood piece that had moved downstream from its initial position was re-referenced to investigate log mobility and calculate travel distance. LW mobility (the percentage of tagged wood pieces that had moved) was higher in periods where maximum water level exceeded channel bankfull depth, although the difference was not statistically significant, but there was no difference in LW diameter, length or travel distance in periods with maximum during which water levels did or did not exceed bankfull stage. A significant (R = 0.603) linear regression relationship was found between LW mobility (in %) as dependent variable and the ratio Hmax/HBk (Hmax the maximum water level registered during each winter period and HBk the bankfull stage). The ratio between LW travel distance and LW piece diameter decreased with an increase in the ratio between LW piece length and the mean channel bankfull width, although this relationship was not statistically significant.