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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]

The active Pacific margin of the Haida Gwaii and southeast Alaska has been subject to vigorous storm activity, dramatic sea-level change, and active tectonism since glacial times. Glaciation was minimal along the western shelf margin, except for large ice streams that formed glacial valleys to the shelf break between the major islands of southeast Alaska and Haida Gwaii. Upon deglaciation, sediment discharge was extensive, but it terminated quickly due to rapid glacial retreat and sea-level lowering with the development of a glacio-isostatic forebulge, coupled with eustatic lowering. Glacial sedimentation offshore ended soon after 15.0 ka. The shelf became emergent, with sea level lowering by, and possibly greater than, 175 m. The rapid transgression that followed began sometime before 12.7 ka off Haida Gwaii and 12.0 ka off southeast Alaska, and with the extreme wave-dominated environment, the unconsolidated sediment that was left on the shelf was effectively removed. Temperate carbonate sands make up the few sediment deposits presently found on the shelf. The Queen Charlotte fault, which lies just below the shelf break for most of its length, was extensively gullied during this short period of significant sediment discharge, when sediment was transported though the glacial valleys and across the narrow shelf through fluvial and submarine channels and was deposited offshore as sea level dropped. The Queen Charlotte fault became the western terminus of the glacio-isostatic forebulge, with the fault acting as a hinged flap taking up the uplift and collapse along the fault of 70+ m. This may have resulted in the development of the distinctive fault valley that presently acts as a very linear channel pathway for sediment throughout the fault system. [ABSTRACT FROM AUTHOR]

The Jurassic-Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic- Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic-Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ~380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America. [ABSTRACT FROM AUTHOR]

Erosion related to glacial activity produces enormous amounts of sediment. However, sediment mobilization in glacial systems is extremely complex. Sediment is derived from headwalls, slopes along the margins of glaciers, and basal erosion; however, the rates and relative contributions of each are unknown. To test and quantify conceptual models for sediment generation and transport in a simple valley glacier system, we collected samples for 10Be analysis from the Kahiltna Glacier, which flows off Denali, the tallest mountain in North America. We collected angular quartz clasts on bedrock ledges from a high mountainside above the equilibrium line altitude (ELA), amalgamated clast samples from medial moraines, and sand samples from the river below the glacier. We also collected sand from nine other rivers along the south flank of the Alaska Range. In the upper catchment of the Kahiltna drainage system, toppling, rockfall, and slab collapse are significant erosional processes. Erosion rates of hundreds of millimeters per thousand years were calculated from 10Be concentrations. The 10Be concentrations in amalgamated samples from medial moraines showed concentrations much lower than those measured from the high mountainside, a result of the incorporation of thick, and effectively unexposed, blocks into the moraine, as well as the incorporation of material from lower-elevation nearby slopes above the moraines. The 10Be sediment samples from downstream of the Kahiltna Glacier terminus showed decreasing concentrations with increasing distance from the moraine, indicating the incorporation of material that was less exposed to cosmic rays, most likely from the glacier base as well as from slopes downstream of the glacier. Taken together, 10Be concentrations in various samples from the Kahiltna drainage system indicated erosion rates of hundreds of millimeters per thousand years, which is typical of tectonically active terrains. We also measured 10Be concentrations from river sediment samples collected from across the south flank of the Alaska Range. Calculation of basinwide weighted erosion rates that incorporated hypsometric curves produced unrealistically high erosion rates, which indicates that the major source of sediment was not exposed to cosmic rays and was primarily derived from the base of glaciers. Moreover, the apparently high erosion rates suggest that parts of each drainage system are not in erosional steady state with respect to cosmogenic isotope accumulation. [ABSTRACT FROM AUTHOR]

A new quantitative coastal land gained-and-lost method uses image analysis of topographic maps and Landsat thematic mapper short-wave infrared data to document accelerated coastal land loss and thermokarst lake expansion and drainage. The data span 1955-2005 along the Beaufort Sea coast north of Teshekpuk Lake in the National Petroleum Reserve in Alaska. Some areas have undergone as much as 0.9 km of coastal erosion in the past 50 yr. Land loss attributed to coastal erosion more than doubled, from 0.48 km2 yr-1 during 1955–1985 to 1.08 km2 yr-1 during 1985–2005. Coastal erosion has breached thermokarst lakes, causing initial draining of the lakes followed by marine flooding. Although inland thermokarst lakes show some uniform expansion, lakes breached by coastal erosion display lake expansion several orders of magnitude greater than inland lakes. [ABSTRACT FROM AUTHOR]