The hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes, lithofacies and architecture of the edifices formed. Hydraulics also determines the timing, location and volume of meltwater discharge, which may be sudden and catastrophic and via subglacial and/or supraglacial routes. Increasing our knowledge of meltwater hydraulics is therefore important for understanding, predicting and mitigating the impact of meltwater release on vulnerable human communities. New observations about eruption-related meltwater hydraulics are presented for well-exposed glaciovolcanic lava-fed deltas on James Ross Island, Antarctica, and accounts of historical eruptions are also re-examined to identify the major meltwater discharge routes. The study provides the first conceptual model for how meltwater escapes supraglacially. In the absence of a crevassed layer (which will dominate any meltwater flow), overflowing may be initiated by enhanced rates of seepage, despite the intrinsically low hydraulic conductivities of snow and firn. Once overflowing is established, the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable (fast) or stable (slower). The James Ross Island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape. Subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing. It is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava-fed deltas.