Bedrock gorge incision via anthropogenic meander cutoff

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Abstract

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

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Bender, A. M. (2022). Bedrock gorge incision via anthropogenic meander cutoff. Geology, 50(3), 321–325. https://doi.org/10.1130/G49479.1

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