Partitioning of horizontal deformation between localized and distributed modes in regions of oblique tectonic convergence is, in many cases, hard to quantify. Here we use the geometry of river basins and numerical modeling to evaluate modes and rates of horizontal deformation associated with the Arabia-Sinai relative plate motion in Lebanon. We focus on river basins that drain Mount Lebanon to the west and are bounded by the Yammouneh fault, a segment of the Dead Sea fault system that transfers left-lateral deformation across the Lebanese restraining bend. We quantify a systematic counterclockwise rotation of these basins and evaluate drainage area disequilibrium using the χ metric. The analysis indicates a systematic spatial pattern whereby tributaries of the rotated basins appear to experience drainage area loss or gain with respect to channel length. A kinematic model reveals that since the late Miocene, 23%–31% of the relative plate motion parallel to the plate boundary has been distributed along a wide band of deformation to the west of the Yammouneh fault. Taken together with previous, shorter-term estimates, the model indicates little variation of slip rate along the Yammouneh fault since the late Miocene. Kinematic model results are compatible with late Miocene paleomagnetic rotations in western Mount Lebanon. A numerical landscape evolution experiment demonstrates the emergence of a similar pattern of drainage area disequilibrium in response to progressive distributed shear deformation of river basins with relatively minor drainage network reorganization.