River morphology and dynamics are strongly influenced by active tectonics. We report channel dynamics for the Peikang River, which flows through the Hsuehshan Range in central Taiwan. Using a digital elevation model and field surveys, we constrain channel morphology for an ∼90 km stretch of river to calculate unit stream power and boundary shear stress along the river path. Incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces. We find a strong correlation between unit stream power/shear stress and incision rate, but only if variation in channel width is considered. A calibrated river incision rule implies river incision rates of ∼9–13.5 mm/yr upstream of the Meiyuan and Tili faults and suggests that one or both of these structures are presently active. Our results indicate that the Shuilikeng fault is also actively deforming, as incision rates increase to ∼6–10 mm/yr across it, compared to 1–4 mm/yr in adjacent reaches. Prominent narrowing across the Shuilikeng fault, and the absence of significant gradient variation indicate that channel width is a first-order morphological adjustment to differential incision. Only when the channel width-to-depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level, as is the case upstream of the Meiyuan and Tili faults.