Although it sounds endemic to the ocean shore, an antidune is a feature of shallow, swift-running rivers and creeks, an ephemeral ripple of sand that migrates upstream against the water’s ﬂow. First a wave appears on the surface while bottom currents dig out a tiny furrow in the streambed; then the water slows slightly as it climbs out of this dip, dropping some of the sediment it carries. Over a few hours a long, low hump begins to build on the streambed, perpendicular to the current. This pattern continues: Water climbs the tiny dune and slows, dropping sand, then rushes down the other side, eroding the downstream face. The ripple moves upstream, often quite rapidly. On the surface a wave of water continues to stand, a reﬂection of the wave of sand below. An antidune may travel several feet over the course of a day, or after a short time it may simply dissolve.