River-groundwater interactions in the Brazilian Pantanal. The case of the Cuiaba River

The Pantanal is a vast evaporation plain and sediment accumulation surface that floods annually. It is located in the Upper Paraguay River Basin, a major source offloodwaters to the Pantanal. The recent construction of a large dam in the upper reach of the Cuiaba River raises questions: What will be the dam influence on the flood area and duration? What will be the consequence for groundwater replenishment and permanence of flow in the floodplain channels during the dry period? This study of the Cuiaba River, within the Pantanal, describes water flow between the river channel and its adjacent floodplain, as well as relations between the surface water and groundwater near the river. Flooding of the plain adjacent to the Cuiaba River critically depends on the river stage and proceeds through a complex hydrographic network. No free water table was encountered; groundwater was confined below clay-silt layers. Two groundwater bodies were distinguished based on their piezometric behavior. In both cases the river stage variations appeared to control the piezometric heads and the flood was the main recharge source. The groundwater moved from the river towards the floodplain where it appeared to sustain channel flow and to maintain soil humidity in depressed areas during the dry period.