The behavior of glaciers terminating in deep water is determined by the interaction of buoyancy, iceberg production, and the sensitivity of ice flow to subtle changes in resistance at the glacier bed. Modeling efforts assume rapid evacuation of icebergs, but some glaciers terminate in closed basins where icebergs may accumulate. Multibeam bathymetric survey of a glacial lake in Iceland that received outlet glaciers during the Little Ice Age document near parallel flow of narrow, ice fingers that extended up to 1 km beyond the grounding line. These previously unseen features apparently occurred when iceberg production exceeded iceberg melt, resulting in iceberg buttressing of the ice margin. Similar stabilization of ice flow may resolve long-standing debates about past continental ice-sheet behavior, including the rapid readvance of the Laurentide Ice Sheet into Lake Superior ca. 11 ka and the genesis of glacial flutes observed in the central Arctic Ocean basin.