Bedrock gorge incision via anthropogenic meander cutoff

Bedrock river-gorge incision represents a fundamental landscape-shaping process, but dearth of observational data at >10 yr timescales impedes understanding gorge formation. I quantify 102 rates and processes using historical records, field observations, topographic image analysis human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off 1900 CE, abruptly lowering local base level by 6 m forcing narrowing steepening channel across knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, low millennial erosion provide ideal boundary conditions for this gorge-formation experiment. Initial fast propagation (23 m/yr; 1900–1903 CE) slowed (4 1903–1981 to diffusion (1981–2019 as slope decreased, yielding an ~350-m-long, 6-m-deep within pre–1900 CE channel. Today, dominates 500-m-long knickzone upstream gorge, where sediment transport likely limits ongoing adjustments anthropogenic cutoff. Results elucidate width, slope, discharge, dynamics with gradual transition from detachment- transport-limited fluvial adjustment base-level lowering.