A shallow sill can promote the downward transport of the upper-layer freshwater outflow in proglacial fjords. This sill-driven transport reduces fjord temperature and stratification. The sill depth, freshwater discharge, fjord temperature, and stratification are key parameters that modulate the heat supply towards glaciers. Additionally, the relative depth of the plume outflow, the fjord, and the sill can be used to characterize distinct circulation and heat transport regimes in glacial fjords.