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The hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows, energetic subsurface flow, and macro‐scale form drag from immobile boulders and channel and bed forms. Heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro‐scale form drag; however, little work has explored steep river hydrodynamics in the absence of complex bed geometries. Here we present theory for the vertical structure of flow velocity in steep streams with planar, rough beds that couples surface and subsurface flow. We test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes (0.4–30%). Experimental flows have a nearly logarithmic velocity profile far above the bed; flow velocity decreases less than logarithmically toward the bed and is nonzero at the bed surface. Velocity profiles match theory …