Natural and anthropic impacts of a basin in the Andes, highly sediment producer

When important basins have its nascent on a mountain zone, they produce strong sediment discharges, natural characteristics and actions or another works developed in this region, present important impacts of local impact on the top of the basin and global impact along the total fluvial course, reaching the exit mouth. In headwater regions of these sediment producer basins, any change of natural or anthropic, presents great possibilities to produce system remarkable unbalances, due to its critical condition of vulnerability. In the Andes region that involved the north-west of Argentina Republic and the south of Bolivia, there are specific examples of this kind of phenomenon, linked specially with the high basins of Pilcomayo and Bermejo rivers, which belong to La Plata River basin. In particular, is interesting to mark that sediments in suspension transported by the river Bermejo, represent the 90% of the total solid flow of Paraná river and resulting as a great economic interest for the need of dredging for overseas navigation. The present paper refers to a particular case, chosen specially because it is the most productive sediment basin in South America area. There, is being examine the local influence of anthropic impacts to the riverbed stability and process global characteristics of natural impact of sediment production, trying .to demonstrate the need to avoid structural solutions in the production region, hardly those solutions have a percentage relation with phenomenon magnitude and it could generate greater impacts. To avoid floods in Orán city (Salta, Argentina) founded in 1794, because of the floods of Blanco river, its first fluent has been deflected (Iruya river), working the land by hand during the summer of 1865, running water into a close basin of Pescado river. The erosive power of the river have been increased because of the great unevenness that exist in the valley of both rivers, in order to 100 m, the gross alluviums that filled the upper valley of Iruya were deposed very fast, down the river, filling the ancient riverbed of Pescado river, forming a bar with these materials in the exit mouth of the river and starting a pool there. Due to the deflection, the Iruya flow formed a new river with double size basin and a greater production of sediments, in this way, the section is in a permanent evolution. To protect the water inlet, on the right margin, is being constructed defenses, those are: a longitudinal dike and three groynes with the necessary declination and length to produce a cut on the right border and to deflect flow to the middle of the river. During march 1980, with a flood of a quarter century of recurrence, a critical erosive phenomenon took place on the left margin of Pescado river, against the position of the breakwaters, this motivated a lateral flood expansion of the river. On the other hand, as a part of the Strategic Planification of Bermejo River Basin, financed by FMAM, basic studies were carried out related to sediment generation and transport. The objectives of this project were the evaluation of the relative importance, location and characteristics of the different mechanisms of sediment yield, by means of the landscape units characterization and environmental zoning of the upstream basin of Bermejo river and to determinate the influence of sediment yield of possible alternatives of sub-basins management. On this way different action scenarios, structural and no structural ones, and to determine the influence on its will produce sediment discharge of Bermejo river, reaching the determination of the effects that will have the alterations of its sediment discharges on the sedimentation process on Paraná river, Delta and La Plata river. A general conclusion obtained of the project is that there are not management trends for the upstream Bermejo river capable to reduce significantly the general volume of sediment production of the whole basin.