Relations between debris flows of Quebrada del Candado and fluvial dynamics of Rio Toro (Province of Salta, Northwestern Argentina).

In the Cordillera of the Andes, the Arroyo El Candado, a tributary of Rio Toro (Province of Salta, northwestern Argentina), is a braided stream with low values of discharge, exclusively recorded during summer months, when the scarce rainfalls may occur. Nevertheless, such stream has an enormous importance for fluvial dynamics of the main river. The volume of sediments moved by means of debris flows, periodically occurring within its watershed, is undoubtedly significant, but it cannot be quantified exactly, due to the lack of specific studies. Debris flows generated within various subwatersheds of El Candado move recent sediments whose origin is due to the physical weathering affecting at present the slopes, and ancient quaternary sediments (fluvial terraces, alluvial fans) which are easily eroded and, owing to the great slope of the main stream (about 10%-12%), are carried away until the confluence with Rio Toro. The consequence is the formation of a wide alluvial fan which blocks (sometimes completely) the riverbed of Rio Toro, frequently producing the formation of a temporary dam-lake. In Rio Toro such processes occur since geological times: ancient pleistocenic lacustrine sediments are scattered on the slope along the riverbed, as well as ancient alluvial fans hanging over the present riverbed. The present sedimentation of the alluvium and the formation of the lake strongly influence the behaviour of Rio Toro, contributing to a fast aggradation of the riverbed. The floods owing to debris flows affect directly the Ruta Nacional 51, two bridges and the complementary works builded for their defense, compeling the local Authorities to support onerous running expenses (in order to prevent Rio Toro floods the road), and to keep clean the riverbed of Rio Toro, in order to prevent its aggradation. Nevertheless, owing to the location of the alluvial fan produced by the Arroyo El Candado and owing to the railway bridge builded on the same cone, it is difficult to suppose the planning of corrective works. The only solution seems to reduce gradually the sediment supply from the watershed of the Quebrada de El Candado, in order to prevent further problems to the fluvial dynamics of Rio Toro, and to prevent that the stream affects the infrastructures, requiring further maintenance interventions for the security of the road.