Superimposed large-scale transverse bed forms are currently assigned to classes in addition to dunes. In modern rivers superimposition of the forms occurs only where the hydrological variables change rapidly with time over a large range. An analysis of the time-series of large-scale transverse forms in the Fraser River, Canada, indicates that the small forms, which here abruptly arise on the backs of large ones on the recession, are able during the flood to evolve back into the large forms. Hence superimposed large-scale transverse forms may all be classified hydrodynamically as dunes, though only the members of one order (the smallest in scale) may be expected to be active at any instant. The concept of the bed-form hierarchy remains valid but, in the context of unsteady natural flows, it should be recognized that the members of some classes of form may experience a discontinuous cyclical evolution.