<p>Since the first impoundment in 2008, there has been significant and persistent leakage on the right bank of the Renzonghai Dam in Sichuan Province, China. Despite four major repair campaigns, the issue remained unresolved through 2019. This study investigates the seepage characteristics and controlling factors by comprehensively analyzing a decade (2009–2019) of monitoring data (water levels, seepage quantities) and employing numerical simulations to evaluate the effectiveness of the dam’s impervious curtain. The analysis reveals that the primary cause is a construction defect: the lower anti-seepage curtain was not built to its design depth, which facilitates the principal leakage pathway: bypass seepage where water flows under the curtain and through the highly conductive fault and fissure network of the right abutment. The other is the internal leakage occurring through the compromised connection between the cutoff wall and the geomembrane. The strong hydraulic connection between the reservoir and the right bank is confirmed by high reduction coefficients (&gt; 0.7) and minimal head loss (as low as 9.77&#xa0;m) in downstream boreholes. Despite the significant leakage volume, a stability analysis shows that the hydraulic gradients within the dam (ranging from 0.019 to 0.119) remain below permissible limits. This indicates that, due to effective internal drainage, the immediate risk of seepage-induced failure is low. The findings provide a definitive explanation for the persistent leakage and offer a clear basis for targeted remediation.</p>