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2016 Nature Article on Moses Coulee Erosion by Isaac Larsen And

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2016 Nature Article on Moses Coulee Erosion by Isaac Larsen And LETTER doi:10.1038/nature19817 Progressive incision of the Channeled Scablands by outburst floods Isaac J. Larsen1,2 & Michael P. Lamb2 The surfaces of Earth and Mars contain large bedrock canyons that flood depths to the brim-full markers12,14,15. However, the actual flow were carved by catastrophic outburst floods1,2. Reconstructing depth is unknown10 because canyon floors must have progressively the magnitude of these canyon-forming floods is essential for lowered as a result of bedrock incision, and high- water markers may understanding the ways in which floods modify planetary surfaces1,2, have been active only early in canyon formation; therefore, the dis- the hydrology of early Mars3 and abrupt changes in climate4. Flood charges predicted from ‘brim-full’ models are reported as maximum discharges are often estimated by assuming that the floods filled estimates11,13. the canyons to their brims with water; however, an alternative In the case of other bedrock canyons that formed over millions of hypothesis is that canyon morphology adjusts during incision such years, such as the Grand Canyon, USA, water flows that were responsi- that bed shear stresses exceed the threshold for erosion by a small ble for carving the canyon had depths that were always a small fraction amount5. Here we show that accounting for erosion thresholds of the canyon relief. Recent mechanistic studies5,9,16–20 and theory for during canyon incision results in near-constant discharges that are bedrock erosion by plucking of blocks from the bed21 or toppling at five- to ten-fold smaller than full-to-the-brim estimates for Moses waterfalls22,23 indicate that, where rock is fractured, bedrock channels Coulee, a canyon in the Channeled Scablands, which was carved will incise via the entrainment of bedrock blocks from the bed when during the Pleistocene by the catastrophic Missoula floods in fluid stresses exceed the threshold for entrainment. This ‘threshold eastern Washington, USA. The predicted discharges are consistent shear stress’ model implies that flows in outburst-flood-carved with flow-depth indicators from gravel bars within the canyon. canyons will drop below brim-full as canyons deepen. Resolving In contrast, under the assumption that floods filled canyons to whether canyons carved by outburst floods should be interpreted their brims, a large and monotonic increase in flood discharge is as channels filled to the brim or as valleys with flow only near their predicted as the canyon was progressively incised, which is at odds bottoms is key for reconstructing palaeo-flood discharge. However, with the discharges expected for floods originating from glacial lake evaluating these two ‘end-member’ models is challenging; both require outbursts. These findings suggest that flood-carved landscapes in removal of rock, but information on how canyons evolve during fractured rock might evolve to a threshold state for bedrock erosion, formation is lacking. thus implying much lower flood discharges than previously thought. We conduct a quantitative evaluation of the end-member brim-full Field investigation of the Channeled Scablands, which contains vast and threshold shear stress models by numerically simulating floods in tracts of scoured bedrock, giant gravel bars and deep canyons, led to Moses Coulee, a 70-km-long canyon in the Channeled Scablands (Fig. 1) the eventual recognition that short-lived, catastrophic floods, rather with evidence of flooding 175 m above the canyon floor and a history than uniformitarian processes, were the dominant agents of canyon of at least four late-Pleistocene-epoch outburst floods24. We selected formation in the region1,6. Outburst floods caused by the sudden Moses Coulee for the study because, unlike other canyons in the drainage of lakes due to the failure of glacial or other natural dams Channeled Scablands, the planform geometry is relatively simple and have been documented in many settings on Earth7; and enormous a set of bedrock terraces enables reconstruction of the palaeo-channel bedrock canyons, morphologically similar to those in the Channeled bed during canyon incision. Plucking is the dominant erosion mecha- Scablands, provide evidence for widespread outburst flooding on the nism in the well-jointed basalt bedrock of the Channeled Scablands6; surface of Mars2. Canyons carved into planetary surfaces are hence a however, forward-modelling the co-evolution of hydraulics and bed key record of the palaeo-hydrosphere3, and constraining the magni- topography in a megaflood channel that is eroding by plucking is cur- tude of the canyon-carving floods is required to understand whether rently intractable, owing to computational expense and the fact that megafloods—immense, short-lived, catastrophic floods—have the the physics of wall and bed erosion are not well known21. Instead, we potential to trigger climate change, by altering ocean circulation on conducted simulations for the modern Moses Coulee topography Earth4 or by episodically generating oceans on Mars8. and explored how flooding evolved in concert with canyon incision Although bedrock canyons and associated depositional landforms by routeing floods through four inferred stages of canyon formation provide direct evidence for palaeo-floods, reconstructing the hydraulics (Fig. 1c), for which we reconstructed palaeo-topography guided by of outburst floods is challenging. Outburst floods often initiate sub- bedrock terraces (see Methods). Floods were simulated in discharge glacially9 or, in the case of Mars, from subsurface aquifers 10; hence, increments of 0.1 × 106 m3 s−1 using a two-dimensional, depth- canyon geometry and the grain size of flood-transported sediment averaged, hydraulic model. The elevations of high-water marks often provide the only clues from which water depth, bed shear identified in the field were used to constrain the discharges predicted stress and discharge can be reconstructed. Outburst floods in the by the brim-full model. The discharges predicted by the threshold shear Channeled Scablands and on Mars have traditionally been assumed stress model were determined from simulations in which bed stresses to fill the canyons up to the elevation of high-water marks such as only slightly exceeded the threshold for incision via bedrock-block eroded channel margins11 along the rims of the canyons. Consequently, plucking, with lower-bound estimates based on the assumption most hydraulic modelling efforts implicitly assume that the canyons of cohesionless blocks and upper-bound estimates that include were filled with water to their brims11–14 or attempt to match modelled interlocking via the inclusion of block wall friction. 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA. 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. 00 MONTH 2016 | VOL 000 | NATURE | 1 RESEARCH LETTER 120° W 119° W 118° W a b d b Fig. 2a A′ 47° N Knickzone d c 700 600 Fig. 2b 500 Terraces 400 D Shear C stress (Pa) 300 Knickzone B Elevation (m) A 200 A 6,000 010203040506070 10 km 0 1 km Distance (km) Modern topography Reconstructed Canyon rim Figure 1 | The Moses Coulee study site. a, Moses Coulee in eastern in green. c, Long profile of Moses Coulee showing the modern bed Washington, USA (location indicated by the smaller boxed region in the topography (solid black line), mapped bedrock terraces (grey circles) and inset). Blue areas were inundated by late-Pleistocene floods and the white the bed topography reconstructed from the terrace elevations (grey lines line delineates the southern extent of the Cordilleran ice sheet. Flood and labelled A–D). The rim of the canyon is indicated by the dashed black line. ice extent are interpreted from version 3.0 of the Washington Department Elevation is with respect to sea level. d, The study reach. The white circle of Natural Resources digital 1:250,000 scale state geological map. b, Moses shows the location of the field-defined high-water mark; the blue line Coulee with modelled bed shear stresses (see colour scale in d) during a shows the high-water line interpreted from aerial imagery. The line A–A′ flood with a discharge of 0.6 × 106 m3 s−1; the model domain is outlined shows the location of the cross-section in Fig. 4. The threshold shear stress model for bedrock canyons stems from the brim-full model implies a temporal increase in discharge to theoretical and field evidence from gravel-bedded rivers that demon- maintain brim-full flow while the canyon floor erodes. However, in strates that, on average, bed shear stresses can exceed the threshold the Channeled Scablands, at least tens of floods are thought to have for sediment transport by only 20% before the channel cross-section occurred24, and all but the most recent floods probably had comparable will erode and evolve to a form that maintains shear stresses near the discharges, owing to a triggering mechanism that required filling an threshold for sediment transport25. Hence, well-jointed rock that is ice-dammed lake to a threshold level sufficient to float the dam and subject to megaflooding may behave more like a bed of sediment release an outburst24. A second test uses the depositional bars within with limited or no cohesion in which individual blocks can be easily Moses Coulee, because these require certain levels of inundation and entrained and transported, rather than as massive crystalline rock sediment transport regimes to form26, by determining which model that erodes slowly through abrasion5,22,23. Such bedrock channels predictions are most consistent with the location and morphology cannot withstand large discharges or shear stresses that greatly exceed of two key landforms: the Great Gravel Bar (Fig. 2a) and a boulder the threshold for plucking. Instead, the channels will adjust their bar on an ‘abandoned channel’, a terrace 70 m above the canyon floor morphology through bedrock erosion so that bed stresses, on average, (Fig. 2b, c).
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