Located in central Brazil, the Araguaia River is the largest river flowing through the Cerrado, the Brazilian savanna. The thesis presented here assesses the hydro-geomorphology of the middle Araguaia River-floodplain system by characterizing flooding patterns and linking these patterns to the geomorphology of the floodplain. It also determines the response of floodplain lake morphometry and surface water connectivity to the annual flooding of the river, and describes how different floodplain geomorphologic units influence changes in open water areas in the floodplain from the dry season to the wet season. Peak discharges along the middle Araguaia River can be reduced downstream despite large increases in drainage area and the contribution of tributary inputs. After analyzing average daily discharge measurements from 1975 to 2007 along an upstream reach and a downstream reach in the middle Araguaia River, four main flooding types are characterized based on the magnitude of the peak discharge and the pattern of peak discharge reduction that occurs as the flood wave moves downstream. Short-term losses of channel discharge during the flooding peak and over the flooding season from November to May are estimated, with the downstream reach displaying more short-term channel loss compared to the upstream study reach. Differences in floodplain geomorphological characteristics between the two study reaches, including the proportions of distinct geomorphologic units (a lower elevation impeded floodplain, a unit dominated by paleomeanders, and a unit of accreted banks and islands), influence the patterns of peak reduction and channel loss. Short-term losses of channel discharge during flooding peaks are usually re-gained by the channel by the end of the flooding season, although in two years about 10% of the volume input into the downstream reach was lost from the channel over the flooding season. Using satellite imagery and an open water index, changes in lake area, perimeter, and surface water connectivity with the main channel between dry season and the wet season are determined for 32 floodplain lakes. The changes in lake morphometry and connectivity are linked to how fluvial processes formed the floodplain lakes. Spatial variations in the floodplain areas that became open water from the dry season to the wet season demonstrate that distinct floodplain geomorphologic units influence the extent and location of open water areas during flooding. Floodplain lakes that expand in area and in depth and are connected to the river channel via surface water likely provide storage areas for the channel losses and peak discharge reductions observed in some of the flooding types for the middle Araguaia River. Although there have been attempts to plan the placement of dams on the Araguaia River, the river is not impounded, allowing for the analysis of a river system with an unaltered flow regime. This thesis contributes to knowledge of a large and understudied tropical river in an ecologically sensitive region. / text

Durant les 20 dernières années d'anciens bras latéraux du Rhin ont été restaurés par reconnexion au cours principal. L'objectif de cette thèse est d'évaluer l'efficacité de la restauration et de vérifier si l'écosystème aquatique a retrouvé sa fonctionnalité. Les communautés végétales aquatiques sont utilisées ici comme indicateurs de ces changements. De nombreux facteurs ont été mesurés : les communautés végétales, leurs traits biologiques, les flux et stocks de diaspores, la chimie de l'eau, du sédiment et des plantes, l'hydro-géomorphologie. Cette étude a été menée dans 9 sites restaurés et 3 sites de référence. Trois communautés végétales ont été identifiées : une "eutrophe", une "mésotrophe", et une "rhéophile". La composition de ces communautés est liée aux conditions physiques et trophiques, et aux flux de diaspores. Les reconnexions ont permis aux sites restaurés de retrouver des communautés et une fonctionnalité écologique semblables à celles des sites de référence. / During the last 20 years, side channels of the Rhine have been reconnected to the main course. The objective of this thesis is to assess the efficiency of restoration and the recovery of the ecosystems functionality. The aquatic plant communities were used as indicators of changes. Many factors were monitored: plant communities, their biological characteristics, flux and bank of propagules, the chemistry of water, sediment and plants, hydro-geomorphology. This study was conducted in 9 restored sites and 3 reference sites. Three aquatic plant communities were identified: eutrophic, mesotrophic and rheophilic. The composition of these communities is first linked to physical conditions and second to trophic level, together with flux of propagules. Reconnections allowed the restored sites to exhibit communities and ecological functioning similar to reference sites, in a relatively short time.