Glacial geomorphological mapping of Shuttle Radar Topography Mission (SRTM) data from the western Canadian Prairies demonstrates that during the last (late Wisconsinan) deglaciation of the Laurentide Ice Sheet terrestrial ice streams underwent a major reorganization of their flow configuration. This reorganization involved a 90° shift in flow direction and was accompanied by a corresponding increase in the influence of topography on streaming flow. Ice streams included both topographically confined and “pure” ice streams that flowed independent of topography. Streaming flow is recorded by suites of highly elongate (>60 km long) subglacial bedforms, bounded sharply at their lateral margins by prominent moraines. Initial streaming flow was unconfined by topography but was replaced progressively, and crosscut, by younger topographically confined flows. Flow reorganization is inferred to have been caused by temporal and spatial variations in the interaction between frozen and thawed bed conditions, with thinning and shutdown of one ice stream ultimately triggering initiation of others. This highlights the role of internal glaciodynamically driven reorganization in triggering streaming flow within large ice sheets and shows that large-scale flow reorganization can occur over the time scale of a single deglaciation in terrestrial ice streams.