The apparent long-term persistence and short-term variability of the Siple Coast ice streams of West Antarctica are tied to regional thermal conditions and local basal lubrication. Numerical simulations indicate that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of fast-moving modern and ice-age ice streams despite their tendency to freeze to the bed, and would allow additional thinning and grounding-line retreat.