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Deep canyons on Earth occur in both terrestrial and submarine environments, where they are carved by actively incising channels. Apparently similar flights of unpaired terraces, seen at the inside of bends in incised sinuous channels, are also common in both environments. Here we demonstrate a new mechanism for terrace formation that we believe is unique to settings where sediment transporting flows are only slightly denser than the ambient fluid, such as those encountered in submarine environments or on other planets and moons. Whereas it is well known that variable rates of river incision and lateral migration create bedrock terraces in river canyons, the processes responsible for their submarine counterparts are largely unexplored, limiting our ability to reconstruct canyon evolution and therefore landscape history in these environments. Tangential momentum in turbidity currents traversing canyon bends can cause currents to run far up the outer banks of bends, such that flow separates from inner banks. The result is very little current and very low velocities along the channel bottom near the inside of the bends. We present experimental results that capture terrace formation at the inner banks of bends, through sustained erosion by energetic currents outside low-velocity, flow-separation zones coupled with no erosion or weak deposition within separation zones.