The polygenetic floodplain of the Upper Parana River

The mega-river functions differently from ordinary rivers and its floodplain is characterized by its magnitude, morphological complexity, and evolutionary history. This study analyzes the morphological and evolutionary characteristics of the Upper Parana mega-river floodplain (Qm 13,000 m(3) s(-1)), characterizing it as polygenetic owing to the various formative processes involved, many of which are no longer active and have extended since the end of the Pleistocene. The Upper Parana floodplain is adjacent to the channel and in-channel on the large islands of its lower section. The 235 km study reach river has anabranching stretches that are separated by nodal sections. The analysis used satellite images, aerial photographs, 35 colors, and 52 absolute dates. Four types of floodplains were characterized: inherited, formed by processes and morphologies no longer active in the system; bayfill, formed by overflow deposits in flooded areas; crest-and-trough, by the annexation of lateral bars; and mixed, by the combination of the types above and association of the Parana floodplain with its tributaries. Floodplain formation occurred in three phases. 1) At the end of the Pleistocene when the river developed a braided sand-gravel pattern under a dry climate. 2) It began at the Pleistocene-Holocene limit, when climatic improvement produced hydrological changes in the river leading to the vertical and lateral excavation of the old floodplain, generating a new channel and a sandy terrace 10 m above the current river level. The formation of the current floodplain began in this phase with the abandonment of the pioneer lake-islands. 3) It started with a small semi-arid event in the mid-Holocene until the present. The current floodplain and river hydrology were defined in this phase. Tectonic interventions for forming the floodplain are also addressed in the text. Considering its varied origin, the floodplain is polygenetic, implying a complex mosaic of environments whose connectivity relationships are still unknown. We believe that the concept of polygenetic floodplains presented in this study will help the understanding of hydrological and ecological functioning of the polygenetic floodplains of tropical mega-rivers.