Fluvial geomorphology and landscape evolution analyzed through the cartography of a large river. The case of the Argentinian Paraná
Résumé
The Paraná river, fifth river of the world when considering its flow and the size of its watershed is poorly known nowadays, despite the research programs on its course. That can probably be explained by the youth of its discovery and the difficulty of gathering information in a river watershed greater than 3 million kilometers square. The physical data available is often incomplete and generally covers a short period of time. It is then difficult to understand the changes in its morphology over a period of time that exceeds sixty years. We then wondered about the data which would permit to redraw the river dynamics and to show if the modification of the land cover over the past 200 years (the stigmas of the recent colonization are still visible) show in the river pattern? By centering us on the Argentinian course of the river, we collected 43 old maps which scales vary from regional (1/100,000) to continental (1/18,500,000). Through their analysis, depicting a period covering the fifteenth to the early twentieth century, it is possible to trace the causes from the changes picked out in the river bed. The methodology used is a map-to-map comparison, realized in a Geographic Information System, following the theory of river systems (Schumm, 1977). Therefore, sandbanks, islands and river planforms are considered to be markers of the river dynamics, given their sizes and the degree of revegetation (Smith, 1981, Ramonell et al., 2008). The causes are themselves related to the spatial and climatic phenomena. Early maps show a braided planform in the upper reach of the Paraná and a Paraguay channel free of deposits. The current dynamics indicate an inversion in sedimentary transportation and deposit. The deforestation mechanisms in the Misiones territory and their abandonment (Levington, 2009) could lead to a shift in the sediment load. This hypothesis will be demonstrated through analysis of the maps, its validation by texts and the results of a field study.