antidunes migrate upstream because the stream flow is shallow and fast in the trough and slows and deepens over the crest. as a result, the shear stress on the bed decreases from trough to crest, allowing sedimentation, and increases from crest to tough, causing erosion. the inertia of the flow moves the shear stress maximum and minimum slightly downstream of the trough and crest. this allows the bedform to amplify with time as erosion occurs in the trough and deposition occurs at the crest.