Abstract. Over the past decade satellite observations have revealed that active subglacial lake systems are widespread under the Antarctic ice sheet, including the ice streams, yet we have insufficient understanding of the lake-drainage process to incorporate it into ice sheet models. Process models for drainage of ice-dammed lakes based on conventional "R-channels" incised into the base of the ice through melting are unable to reproduce the timing and magnitude of drainage from Antarctic subglacial lakes estimated from satellite altimetry given the low hydraulic gradients along which such lakes drain. We developed a process model in which channels are mechanically eroded into deformable subglacial sediment (till) instead ("T-channel"). When applied to the known lakes of the Whillans/Mercer system, the model successfully reproduced the key characteristics of estimated lake volume changes for the period 2003–2009. If our model is realistic, it implies that most active lakes are shallow and only exist in the presence of saturated sediment, explaining why they are difficult to detect with classical radar methods. It also implies that the lake-drainage process is sensitive to the composition and strength of the underlying till, suggesting that models could be improved with a realistic treatment of sediment – interfacial water exchange.