The rates and patterns of bedrock channel incision significantly influence landscape evolution and long-term interactions among climate, tectonics, and erosion. Unfortunately, only sparse field data are available to quantify the controls on river incision rates. We exploit the diversion of the upper Ukak River by an ash flow in 1912 to measure rates of incision along a newly formed bedrock channel. Minimum estimates of the rate of incision into intact rock vary from 0.01 to 0.10 m ṁ yr−1. This variation reflects differences in channel slope, channel width, lithologic facies, and intensity of jointing as well as the effects of upstream knickpoint migration. A stream-power–type incision model adequately explains the incision-rate data, provided (1) variations in channel width are prescribed on the basis of field measurements, (2) the slope exponent is significantly less than unity (n = 0.4 ± 0.2), and (3) observed downstream changes in lithologic facies and the intensity of jointing account for the apparent twofold downstream decrease in the coefficient of erosion. Despite the very rapid rate of incision, calibrated stream-power erosion coefficients for the Ukak River (K = 2.4 × 10−4 m0.2 ṁ yr−1 to 9.0 × 10−4 m0.2 ṁ yr−1) are within the range of previously published estimates. Two plausible explanations for the low values of the slope exponent n are that incision rate is limited by either (1) a combination of physical weathering and hydrodynamic joint-block extraction or (2) block fracture due to bedload impacts modulated on steeper channel segments by suspension of a significant fraction of the sediment load.