Antidune
![Surface waves forming in phase above antidunes in a small stream. Water flow is away from the camera.](/Images/godic/202412/21/AntidunesInASmallStream0142.jpg")
![Water flowing over bedforms in sand under unidirectional flow to the right. Numbers correspond broadly to increasing flow regime, i.e., increasing water flow velocity. For dunes, the water surface is low over the dune and high over the interdune. For antidunes, flow depth is roughly the same everywhere, which means the water surface is high over the antidune and low over the interdune.](/Images/godic/202412/21/page1-220px-Bedforms_under_various_flow_regimes.pdf0142.jpg")
An antidune is a bedform found in fluvial and other channeled environments. Antidunes occur in supercritical flow, meaning that the Froude number is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flow regime. In antidunes, sediment is deposited on the upstream (stoss) side and eroded from the downstream (lee) side, opposite lower flow regime bedforms. As a result, antidunes migrate in an upstream direction, counter to the current flow. Antidunes are called in-phase bedforms, meaning that the water surface elevation mimics the bed elevation; this is due to the supercritical flow regime. Antidune bedforms evolve rapidly, growing in amplitude as they migrate upstream. The resultant wave at the water's surface also increases in amplitude. When that wave becomes unstable, breaks and washes downstream, much of the antidune bedform may be destroyed.