The Gulf Stream is a major conduit of warm surface water from the tropics to the subpolar North Atlantic. Here we observe and simulate a submesoscale (<20km) mechanism by which the Gulf Stream exchanges water with subpolar water to the north. Along isopycnals, the front has a sharp compensated temperature-salinity contrast, with distinct mixed water between the two water masses 2 and 4km wide. This mixed water does not increase downstream despite substantial energy available for mixing. A series of streamers detrain this water at the crest of meanders. Subpolar water replaces the mixed water and resharpens the front. The water mass exchange accounts for a northward flux of salt of 0.5-2.5 psum(2)s(-1), (large-scale diffusivity O (100m(2)s(-1))). This is similar to bulk-scale flux estimates of 1.2psum(2)s(-1) and supplies fresher water to the Gulf Stream required for the production of 18 degrees subtropical mode water.