Breakup transforms an ice-covered river into an open river. Two ideal forms of breakup bracket the types of breakup that commonly occur. At one extreme is thermal breakup. During an ideal thermal breakup, the river ice cover deteriorates and melts in place, with no increase in flow and little or no ice movement. At the other extreme is the more complex and less understood mechanical breakup, which is the focus of this presentation. The main driver of mechanical breakup is the flow discharge hydrograph. The increase in flow induces stresses in the cover, and the stresses in turn cause cracks and the ultimate fragmentation of the ice cover into pieces that are carried by the channel flow. Ice jams take place at locations where the ice fragments stop; severe and sudden hydraulic transients can result when these ice jams form or when they release. This presentation will focus on mechanical breakup and the historical evolution of our understanding of this topic. The presentation will include discussions of ice cover formation and the typical resulting ice structure, wave-ice interaction, the physics of the cracking, and the current status of our understanding of breakup.