Accurately predicting Greenland’s ice mass loss is crucial to understanding future sea level rise. Approximately 50% of the mass loss results from iceberg calving at the ice-ocean interface. Ice mélange, a jammed, buoyant granular material that extends for 10 kilometers or more in Greenland’s largest fjords, can inhibit iceberg calving and discharge by transmitting shear stresses from fjord walls to glacier termini. Direct measurements of these resistive force dynamics are not possible in the field, thus, we created a scaled-down laboratory experiment to elucidate the most salient features of ice mélange mechanics. We captured videos of the mélange surface motion and sub-surface profile during slow, quasistatic flow through a rectangular fjord, and recorded the total force on a model glacier terminus. We find that when the wall friction is low, the ice mélange remains jammed, but moves as a solid plug with little or no particle rearrangements. When the wall friction is larger than the internal friction, shear zones develop near the walls, and the buttressing force magnitude and fluctuations increase significantly. Associated discrete particle simulations illustrate the internal flow in both regimes. We also compare our experimental results to a continuum, depth-averaged model of ice mélange and find that the thickness of the mélange at the terminus provides a good indicator of the net buttressing force. However, the continuum model cannot capture the stochastic nature of the rearrangements and concomitant fluctuations in the buttressing force. These fluctuations may be important for short-time and seasonal controls on iceberg calving rates.