Knodus moenkhausii is a small characid widely distributed and abundant in the Doce River basin, which experienced the largest socio-environmental disaster in Brazil. This species is also recognized for its broad dietary response to various levels of degradation, making it a potential indicator of the ecosystem's impacts resulting from the rupture of the Fundão iron mining dam in 2015. Thus, the objective of this study was to investigate the trophic ecology of K. moenkhausii in the Doce River basin by analyzing its carbon (δ ¹³C) and nitrogen (δ ¹⁵N) isotopic compositions. Samplings of K. moenkhausii individuals and their potential food resources were conducted at three sites affected by the rupture of the ore tailings dam and at three unaffected (control) sites, which were distributed across the upper, middle, and lower regions of the basin. Within each evaluated region, no differences were observed in the δ ¹³C values of K. moenkhausii between the affected and their respective control sites. The δ ¹⁵N was different between regions for the control sites, but similar between the affected sites, indicating possible homogenization of the river channel conditions due to the impact. In control sites, K. moenkhausii individuals fed on more nutritious resources, such as invertebrates, while in affected sites, they assimilated more algae and periphyton. We also confirmed the importance of the regional context when assigning control sites and verified that the δ ¹⁵N values were more effective in reflecting the degradation of the Doce River basin.