Among the major ice streams that drain West Antarctica, Kamb Ice Stream (formerly called Ice Stream C) is unique in that it stagnated ∼150 yr ago, but its former fast-flow conditions are virtually unknown. Here we present surface-based radar profiles of the ice stream's undulating internal stratigraphy, which records these conditions. Our analysis of the profiles indicates that pre stagnation flow velocities, averaged over a period <740 yr, exceeded 350 m·yr−1 in the trunk of the ice stream. This velocity constraint would be lower if the ice had been thickening (higher if thinning), but suggests mass loss from the ice-stream catchment that is of sufficient magnitude to reverse the gain estimated for today's Siple Coast region. Analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present-day velocities, useful for evaluating how West Antarctic ice drainage has evolved.