Abstract: Dam removal is currently considered as the best solution for improving the ecological transparency and the sedimentary transparency of rivers. Among other advantages, these operations provide an essential ecosystemic service in terms of water quality. By removing the dam, water retention located upstream is also removed, hence offering the river a current facies that enables its flow, speed, temperature and oxygenation to be improved, in particular via the recovery of the waterway's natural invert function which generates turbulent flow zones propitious to air/water exchange. This river movement recovery also improves its self-purification and, consequently, its resilience, which are factors that improve both its biological quality and its physicochemical quality. As such, in waterways where water quality was stagnant, one can hope to achieve correct conditions as described in the Water Framework Directive. A concrete example of the achievement of correct conditions by the simple additional effect of dam removal is present on the River Orne (Enfernay and Maisons Rouge dams). The spectacular improvement of millstream perpendicular to the zone where the dam was removed would appear to demonstrate the efficiency of this measure with regard to the recovery of superficial water quality. The ecosystemic service offered to the river is evaluated at €3.3M, representing an economy of around €2M compared to a theoretical water treatment solution. In waterways having benefited from intensive clean-up operations in the drainage basin, we can now consider that river delayering via the removal of a significant number of constructions as the sine qua non condition for achieving correct conditions. The quest for a maximum additional layering level of 30% enables the river to recover quality flow over at least 70% of its length can be considered as a preliminary