Within-channel levees: a new-recognized fluvial form in the floodplain of a large river (Paraná River, South America)

RAMONELL, CARLOS G.; MARCHETTI, ZULEICA Y.; PEREIRA MA. SOLEDAD

Paris

Conferencia; 8th International conference (AIG) on Geomorphology; 2013

Univ. Caen Basse-Normandie, entre varias

Bars, islands and benches are the forms classically recognized of intra-channel sedimentation in rivers. We studied intra-channel forms in the Paraná River between the junction with the Paraguay River (27º 17? S) and its mouth (34º 26? S). The floodplain in this reach extends over 32,000 km2, and is modelled by the Paraná main channel, several major branches (ca. 200 to 500 m-width) and a lot of minor channels. Shifting rates from 1 m/year to 25 m/year were recorded in the branches, and up-to 200 m/year in tracts of the main channel. Bar and island formation, and enlargements and closures of channels are typical processes in the reach. Inside of paleochannels or in channels with width-reduction tendencies, we found sets of parallel alluvial ridges like to natural levees, but lying below the bankfull levels. We propose the name of ?within-channel levees? (WCL) for them, in consideration of their morphology and sedimentology. WCL could attain several km in length, and form ridge and trough reliefs of ca. 1 m, few meters-width. They are straights or curves in planview, sometimes like to scroll bars. Silty sediments are dominant in WCL, which are the prevailing sizes of the wash load in the river. Inside the active channels the WCL are coupled to hydrophytes; rooted or floating vegetation in the secondary channels could cover significant lengths near the banks. Thus, the origin of the WCL could be related with the trap of suspended sediments by hydrophytes within the channels. The WCL are conspicuous features in the Paraná River: they were mapped in detail downstream from 32º 04? S, where these forms extend over 3,000 km2 forming more than 20 % of the fluvial geomorphic units of the area. On the other hand, the WCL are a form by means of river branches adjusts to diminished formative discharges, with siltation of fine sediments not restricted to overbank flows. The WCL recognition expands our interpretations in the analysis of the silty strata of the floodplain.