Large scale one-dimensional hybrid simulations have been performed of a quasi-parallel (&THgr;Bn=20¿) high Mach number collisionless shock. It is found that backstreaming reflected ions, i.e., upstream ions with velocities exceeding the shock ram velocity, originate from the outer part (v>1.7vth) of the velocity space of the incident distribution. The backstreaming ions produce very low-frequency magnetosonic waves which propagate upstream with about 1.3VA (Alv¿n speed). As the wave crests convect toward the shock, they steepen up and the shock reforms itself. During shock reformation a large part of the incident ions are reflected. This, in turn, slows the incident ions down. The slowed down incident particle distribution and the reflected particle distribution merge and constitute the new thermalized downstream distribution. In the interval of a relatively stationary shock low-frequency whistler waves stand at the shock front. During these time intervals the whistler waves are probably responsible for dissipation by nonadiabatic compression of the incident ions. The whistler waves are destroyed by the incoming large amplitude wave crest and reemerge at the new shock front. The reappearance seems to be due to the nonlinear steepening of the incoming wave crest at the upstream side. ¿ American Geophysical Union 1990