The evolution of a duned bed from an initially flat bed was observed in a large water tunnel in which water moved over a sand bed with simple harmonic motion. Bed forms were observed to be of two types, (1) rolling‐grain and (2) vortex, which are called ripples and dunes, respectively. Ripples are the initial transient bed forms that are replaced by dunes. Ripples formed spontaneously when the water‐motion amplitude was sufficiently large. Ripples can be induced to form at lesser water‐motion amplitudes by placing an obstruction on the otherwise flat bed. The history of a duned bed was observed and is described from birth to equilibrium.

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