Changing fluvial styles and backwater flooding along the Upper Paraguay River plains in the Brazilian Pantanal wetland

The channel-floodplain relationship is used to understand flooding dynamics in the Pantanal wetland. To understand how different fluvial styles along the Upper Paraguay River (UPR) control the hydrology of the Pantanal wetland, we used a database including 11 gauge stations from the Brazilian National Agency of Waters, Landsat series satellite data, GeoCover images circa 1990 and 2000, the SRTM 90 m digital elevation model (DEM) and 34 acoustic Doppler current profiler (ADCP) sections. We defined eight very distinct fluvial zones in the UPR within the Pantanal based on the channel-floodplain morphology, channel pattern, hydrologic function, and channel slope. Three zones are narrowing plains (Amolar, Urucum and Fecho dos Morros) constrained by geological features on the western border of the Pantanal sedimentary basin, which act as bottlenecks for flood runoff and produce backwater effects that delay flood wave transmission. In each of these narrow zones, the flood wave is delayed by flow velocity reduction, generating huge water bodies upstream that store flood water and affect the duration and amplitude of floods. The existence of three bottlenecks and the occurrence of a backwater effect constrain flood waves along the river, conditioning the hydrological regime of the entire wetland and resulting in unusual rating curves at some gauge stations along the UPR. This peculiar hydrological regime is responsible for the functioning of the wetland, including the undesirable “dequada” phenomena (natural fish mortality events). The backwater effect is enhanced by the existence of large flood basins upstream of the bottlenecks, where waters can be temporarily retained for periods of months, delaying the surface runoff and prolonging the flood season, which contributes to the maintenance of the wetland and its rich ecosystem.