Long-term studies are tools used to study ecological and climatic processes in lotic ecosystems. This study analyzed the long-term hydrochemical dynamics in the Lower Paraíba sub-basin, one of the sub-basins that make up the Paraíba do Sul River basin (PSR), over 10 years (2009-2019), between a period before and after a severe drought, in order to identify its controlling factors. The association between discharge and hydrochemical parameters, estimates of annual flows and the relationship of parameters with the El Niño Southern Oscillation (ENOS) index were considered in the analysis. The results highlighted: 1) The hydrochemical parameters showed a differentiated response in relation to the discharge considering the period before and after the severe drought; 2) The flow had its efficiency reduced in drought events, impacting the speed of transport and river dilution, which led to a reduction in the load of materials transported; 3) The occurrence of ENOS phenomena showed an influence on the hydrochemical dynamics at the end of the RPS basin, through changes in the local seasonality and this in the internal processes of the river channel, intensified by human activities on the soils of the basin. The results suggest that the increased occurrence and intensity of climatic phenomena will lead to changes in hydrochemical and transport patterns along the longitudinal axis of the PSR, highlighting the need for studies to understand their impacts on the availability of water resources, especially in tropical rivers.