Underwater sand waves growing by interaction between a movable sand bed and a fluid flow were simulated in a laboratory flume. Each sand wave advances in step with a flow system involving boundary-layer separation at the wave crest and an induced countercurrent to the wave lee. Four sediment-structure elements result from this flow system, moving simultaneously downstream in the following order: (1) sand-wave foresets; (2) small-scale asymmetrical ripples formed beneath the countercurrent, pointing toward the sand wave, and overriden by the sand-wave foresets; (3) an erosion hollow where the countercurrent first impinges on the flume bed; and (4) small-scale asymmetrical ripples pointing away from the sand wave and eroded by the downstream advance of the erosion hollow. Foresets, the most important of these four elements, accumulate from a combination of grain deposition from suspension creating lee-face instability and avalanching tending to create a stable face. Avalanching may be intermittent when periods of grain-stacking separate episodes of sliding; or it may be continuous when grains are in sliding motion at all times over all parts of the lee face, depending on the flow speed over the sand wave and the rate of settling on the lee face in relation to the speed of avalanche descent. Each type of avalanching gives the sand wave a distinctive internal sediment structure and vertical grading. This could prove a useful key to regime variability in cross-stratification studies.