We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbræ and its proglacial ice mélange. The mélange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the mélange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the mélange weakens and the terminus retreats. The mélange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the mélange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at ∼40 m d−1), whereas during calving events it can move in excess of 50 × 103 m d−1 for more than 10 min. By influencing the timing of calving events, the mélange contributes to the glacier's several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice mélange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.