Flooding hydrology and peak discharge attenuation along the middle Araguaia River in central Brazil

Abstract

The Araguaia River in central Brazil is the largest river draining the Brazilian savanna. Located in a tropical wet-dry climate, the middle Araguaia River has an extensive and complex alluvial floodplain composed of a mosaic of geomorphologic units, dense alluvial forest, and many floodplain lakes. The Araguaia River is not currently dammed, and thus it provides an opportunity to analyze flooding hydrology in a relatively un-altered tropical system. Using average daily discharge measurements from 1975 to 2011, we analyze patterns of peak discharge attenuation (defined as the reduction in absolute peak discharge in m3s−1). We link peak discharge attenuation to bankfull discharge, explore whether peak discharge attenuation results in increased base level flows in the dry season following peak discharge, and use a simplified short-term water budget to determine whether peak discharge attenuation results in the loss of discharge from the channel over the flooding season (November to May). In addition, we explore other potential factors causing peak discharge attenuation, including surface water connectivity between the channel and floodplain lakes and floodplain lake area change between the dry season and the wet season. Although fluvial connectivity between the main channel and the floodplains starts before bankfull stage, we find that large peak discharge attenuation (up to 30% reduction in peak discharge) in the middle Araguaia usually occurs when the river rises above bankfull discharge. The river flow that is lost to the floodplain and floodplain lakes when peak discharge attenuation occurs usually returns to the channel by the end of the flooding season. However, the odds of increased baseflows in the dry season after flooding seasons with peak discharge attenuation are higher compared to flooding seasons without peak discharge attenuation for one of the studied reaches. Some types of floodplain lakes greatly increase in area from the dry season to the wet season, and many floodplain lakes become connected via surface water in the wet season. We have not found similar examples of peak attenuation in this type of tropical wet-dry floodplain system, indicating that the middle Araguaia River may be a unique system or that further research is needed.