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The southernmost populations of Salix humboltiana are located in the Chubut River area of Argentina. However, its distribution area in the longitudinal and transverse gradient of the river system, the density of individuals, and the structure of the populations, among other aspects, are still unknown. A survey of S. humboldtiana and its hybrids with the invasive willow Salix x fragilis was carried out in the upper and middle courses of the Chubut River. For each individual greater than 5 cm DBH, location, sex, diameter, height, canopy status and hydro-geomorphological context were recorded. The DBH/height relationship, sex ratio, natural hybridization frequencies, and the relationship between DBH, height and canopy status with the different hydro-geomorphological elements and their activity (functional / afunctional) were analyzed. 41 populations with a total of 492 individuals of S. humboldtiana were recorded between Costa del Chubut and Paso de Indios. S. humboldtiana sapling recruitment was exclusively associated with functional hydrogeomorphological elements. 12 % of the populations presented a balanced sex ratio, and 2.5 % of individuals were female hybrids between S. humboldtiana and S. x fragilis. Hybridizations with S. x fragilis contribute to a process of gene dilution and degradation of S. humboldtiana populations. [ABSTRACT FROM AUTHOR]