<p>Nonlinear weirs are preferred for new dam constructions or the rehabilitation of existing ones because of their space-efficient design and greater specific discharge capacity compared to linear weirs. The labyrinth weir is one of the most prominent types of nonlinear weirs. The impact of downstream channel depth on the efficiency of rectangular labyrinth weirs was examined in this study. Various flow patterns were analyzed, and discharge coefficient curves were generated. Validated 3D numerical simulations, conducted using OpenFOAM, assessed flow behavior and discharge coefficients under different downstream conditions. The results demonstrate that the downstream channel depth significantly influences the rectangular labyrinth weir performance. Deeper downstream channels increased discharge efficiency by up to 17%, particularly under high upstream head conditions. This improved performance is attributed to the increased capacity of the outlet alveoli to accommodate more flow. Conversely, shallow or absent downstream depths lead to submergence issues, reducing efficiency. An optimal downstream depth-to-weir height ratio of P_D−C/<i>P</i> = 2 was identified, beyond which no further improvements in discharge capacity were observed.</p>