The prominent seismic low-velocity zone (LVZ) in the oceanic low-viscosity asthenosphere is approximately coincident with a zone of high seismic attenuation (or low seismic Q). Small grain sizes in the asthenosphere could link these seismic and rheological properties as small grain sizes reduce viscosity and also lower seismic velocity and seismic Q. Because grain-size is reduced by rock deformation, the asthenosphere’s seismic properties can place constraints on asthenospheric deformation or flow. To determine dominant flow patterns, we develop a selfconsistent analytical 1-D channel flow model that accounts for upper mantle rheology and its dependence on flow-modified grain-sizes, water content and melt fraction, both for flow driven by surface plate motions (Couette flow) and/or by horizontal pressure gradients (Poiseuille flow). From our flow models, Couette flow dominates if the upper mantle is dry, and plug flow (a Poiseuille flow for power law rheology) if it is wet. A plug flow configuration spanning the upper 670 km of the mantle best explains the low seismic Q zone in the asthenosphere, which can be attributed to significant grain-size reduction due to extensive shearing across the asthenosphere. Below the asthenosphere, high water content and minimal shear deformation promote large grain sizes and high seismic Q. We suggest that asthenospheric low-Q and LVZ can be largely explained by grain-size variations associated with plug flow in the wet upper mantle.