Morphologic changes in the Paranل River channel (Argentina) in the light of the climate variability during the 20th century

Recent studies regarding the climate variability in South America during the 20th century, revealed the existence of climate cycles that influenced the hydrologic conditions in the Paranل River basin, one of the largest in the continent. How that variability affected the channel morphology of this river in its middle reach is quantitatively analyzed in this paper. The link between climate, hydrology and channel morphology is obtained through the computation of effective discharge. This discharge implicitly synthesizes the point hydrologic and bed sediment transport changes in an alluvial stream during a relatively long period. The results were obtained studying, with increasing detail, two channel reaches 373 km and 25 km long, respectively. The analysis involved the processing of more than 180 bathymetric charts, satellite images and hydraulic and sedimentologic data recorded in the Paranل River since the very beginning of the 20th century. A rather detailed description of the treatment made with this information is given in the paper. It is shown that three periods of different effective discharges fairly well correlated with reported climatic fluctuations occurred during the last 100 years, i.e. two periods of high discharges (at the century beginning and from 1970 till present) and another of relatively low discharges between 1930 and 1970. Morphologic parameters of the main channel, such as mean width, thalweg sinuosity, braided index and aspect ratio, increased or decreased in correspondence with those variations. In transitional channels (between meandering and braided) like the Paranل River, careful study of the thalweg behavior is a key issue, if a proper approach to the dynamic of morphologic processes operating on the whole channel is intended. Finally, on the basis of theoretical (extreme hypotheses approach) and empirical results, it is suggested that the Paranل River main channel would not be adjusted to the present high values of effective discharge. Thus, larger erosion of banks (channel widening) and increases in the other cited morphologic characteristics will occur if those values persist.