The critical fluid-shear stress for the onset of sediment transport, θc, varies with the history of applied shear. This effect has been primarily attributed to compaction; the role of shear jamming is less explored. We examine the response of a granular bed to fluid-shear stress cycles of varying magnitude and direction, and determine isotropic and anisotropic contributions. Creep and bed-load transport result in direction-dependent strain hardening for θ/θc<4. Dilation-induced weakening, and memory loss, occur for larger stresses that fluidize the bed. Our findings provide a granular explanation for the formation and breakup of hard-packed riverbed “armor.”Received 29 October 2021Accepted 23 March 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L022055Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFluid-particle interactionsJammingSediment transportFluid Dynamics