Time evolution of a single stellar stream from our simulations. The left column shows the reference run and the right column shows the merger run. Top panel : distance of the stream progenitor to the Milky Way’s centre of mass as a function of time. In the merger case, each point is colour-coded by the progenitor’s distance to the satellite, and the satellite’s own orbital radius around the Milky Way is overplotted in black. Middle panel : evolution of the density profile along the stream (zero at the progenitor; negative values denote the leading arm, positive values the trailing arm). Bottom panel : evolution of the stream asymmetry, defined as the sum of the absolute difference between the two cumulative density profiles of the leading and trailing arms (Eq. (4)).