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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. [ABSTRACT FROM AUTHOR]

Geologists frequently debate the origin of iconic river canyons, as well as the extent to which they record climatic and tectonic signals. Fluvial and hillslope processes work in concert to control canyon evolution; rivers both set the boundary conditions for adjoining hillslopes and respond to delivery of hillslope-derived sediment. But, what happens when canyon walls deliver boulders that are too large for a river to carry? River canyons commonly host large blocks of rock derived from resistant hillslope lithologies. Blocks have recently been shown to control the shapes of hillslopes and channels by inhibiting sediment transport and bedrock erosion. Here, we developed the first process-based model for canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. Our model reveals that two-way negative channel-hillslope feedbacks driven by block delivery to the river result in characteristic plan-view and cross-sectional river canyon forms. Internal negative feedbacks strongly reduce the rate at which erosional signals pass through landscapes, leading to persistent local unsteadiness even under steady tectonic and climatic forcing. Surprisingly, while the presence of blocks in the channel initially slows incision rates, the subsequent removal of blocks from the oversteepened channel substantially increases incision rates. This interplay between channel and hillslope dynamics results in highly variable long-term erosion rates. These autogenic channel-hillslope dynamics can mask external signals, such as changes in rock uplift rate, complicating the interpretation of landscape morphology and erosion histories. [ABSTRACT FROM AUTHOR]

Vegetation is traditionally regarded to reduce the erosion of channels in both fluvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, flat substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the flow, leading to flow concentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our findings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, flow, and landform have an important control on landscape evolution. [ABSTRACT FROM AUTHOR]

The activity of fallout radionuclides (7Be, 137Cs, and 210Pb) was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the Yellowstone River basin. Samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of Soda Butte Creek to Billings, Montana, a 423-km-long reach of channel. The inventory of radionuclides in soil increases with precipitation and is highest in the headwaters. The activity of radionuclides in suspended sediment decreases downstream, and more activity is observed earlier than later in the flood hydrograph. The radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion. Fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment. At sites high in the drainage, soils contribute 50% to the suspended load and this value decreases to 11%–26% downstream. Fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source. Transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream. These estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure. This study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size. [ABSTRACT FROM AUTHOR]