We explore a mathematical model that couples together a thermomechanically evolving subglacial channel, distributed cavity drainage, and basal sliding along a subglacial flood path fed by a jökulhlaup lake. It allows water transfer between channel and cavities and a migrating subglacial water divide or ‘seal’ to form between floods. Notably, it accounts for full coupling between the lake and subglacial drainage in terms of both discharge and pressure, unlike models that neglect the pressure coupling by imposing a known history of lake discharge at the channel inlet. This means that flood hydrographic evolution and its impact on glacier motion are consistently determined by our model. Numerical simulations for a model alpine lake yield stable limit cycles simulating repeating jökulhlaups, with the channel drawing water from the cavities at a varying rate that modulates basal sliding during each flood. A wave of fast sliding propagates down-glacier at flood initiation, followed by deceleration as the growing channel sucks water from the cavities. These behaviours cannot be correctly simulated without the full coupling. We show that the flood’s peak discharge, its initiation threshold and the magnitude of the ‘fast sliding’ wave decrease with the background water supply to the cavities.