Study region: Meridional plateau, Uruguay River Basin, Southern Brazil Study focus: This study investigates the relative impacts of rainfall and LULC changes on streamflow variations over the Meridional plateau in the region comprising the Uruguay River basin in Southern Brazil. We used MapBiomas annual LULC data set and the CAMELS-BR data set by aggregating daily observations from gauging stations (rainfall and streamflow) to annual indices from 1985 to 2018. We employed time series and trend analysis across three identified clusters with 78 gauging stations through an unsupervised machine learning algorithm. New hydrological insights for the regions: We found an extensive and statistically significant agricultural expansion for soybean croplands following a transition from other temporary crops (i.e., maize, wheat, oats, etc.), which was accompanied by statistically significant runoff coefficient decrease (i.e., 34 %), while the results for rainfall and flow were statistically insignificant. Furthermore, we observed a 27 % rise in water demand linked to the expansion of soybean croplands, a phenomenon corroborated by existing research. The growth in the number and capacity of dams further may reinforce these findings across the analyzed periods. The identified remarkable decrease in runoff coefficient may, therefore, be primarily attributed to LULC changes. Our findings might provide insights for improved water resource management for future land use planning to develop more resilient catchments against changing environmental and climatic conditions.