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Many aquatic processes (i.e., organism locomotion, macrophyte response to unsteady flow and mass/heat transfer) involve oscillatory or undulatory movements perpendicular to the direction of flow and perpendicular to the plane of oscillation (i.e., spanwise). The reduction of boundary layer (BL) thickness (δ) and increased turbulence have been identified as the mechanisms for faster heat and mass transfer on these moving surfaces. We examined the hydrodynamics of a submerged spanwise‐oscillating plate in a crossflow within turbulent open channel flow at different oscillation frequencies (f) and channel velocity (U). The BL over the plate was characterized using moving particle image velocimetry to understand the effects of f and U on boundary stress, vorticity, and transfer processes. Changes in the velocity gradient at the plate surface depend on the interactions between spanwise vorticities of opposite directions that reversed downstream along the plate. The vorticity structure reduced shear stress at the surface of the plate resulting in > 65% drag reduction near the upstream edge. Whereas δ was reduced in comparison to a stationary plate, vorticity increased the diffusive sublayer thickness (δDSL) over the oscillating plate (≲1.75 thicker than stationary), which would reduce rather than increase mass transfer based on the film model. Conversely, oscillations would enhance the mass transfer by a factor of ≤ 7 due to periodic renewal of water at the DSL. Similar arguments indicate that renewal would lead to increased convective heat transfer along the plate. Results of this work will increase our understanding of the physical and biological processes that occur in environmental boundary layers. [ABSTRACT FROM AUTHOR]