Progressive incision of the Channeled Scablands by outburst floods

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Abstract

The surfaces of Earth and Mars contain large bedrock canyons that were carved by catastrophic outburst floods1,2. Reconstructing the magnitude of these canyon-forming floods is essential for understanding the ways in which floods modify planetary surfaces1,2, the hydrology of early Mars3 and abrupt changes in climate4. Flood discharges are often estimated by assuming that the floods filled the canyons to their brims with water; however, an alternative hypothesis is that canyon morphology adjusts during incision such that bed shear stresses exceed the threshold for erosion by a small amount5. Here we show that accounting for erosion thresholds during canyon incision results in near-constant discharges that are five- to ten-fold smaller than full-to-the-brim estimates for Moses Coulee, a canyon in the Channeled Scablands, which was carved during the Pleistocene by the catastrophic Missoula floods in eastern Washington, USA. The predicted discharges are consistent with flow-depth indicators from gravel bars within the canyon. In contrast, under the assumption that floods filled canyons to their brims, a large and monotonic increase in flood discharge is predicted as the canyon was progressively incised, which is at odds with the discharges expected for floods originating from glacial lake outbursts. These findings suggest that flood-carved landscapes in fractured rock might evolve to a threshold state for bedrock erosion, thus implying much lower flood discharges than previously thought.

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Figure 1: The Moses Coulee study site.
Figure 2: Depositional landforms in Moses Coulee.
Figure 3: Simulated discharge and water depths.
Figure 4: Co-evolution of topography and flood level predicted by the brim-full and threshold shear stress models.

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Acknowledgements

This research was supported by a Caltech Texaco Prize Postdoctoral Fellowship, collaborative NSF (1529528, 1529110) funding to I.J.L. and M.P.L., and a NASA (NNX13AM83G) award to M.P.L. We thank E. Simon, M. Lapotre and S. Roberts for assistance and advice with the hydraulic modelling, and participants in the Caltech field methods course for field assistance.

Author information

I.J.L. and M.P.L. designed the study and wrote the manuscript. I.J.L. conducted the hydraulic simulations.

Correspondence to Isaac J. Larsen.

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Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks J. Venditti and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Computational mesh resolution.

a, Map of the computational domain (green outline). Black square shows the location of b. b, Map showing the different triangular mesh resolutions within the computational domain, with a maximum triangle area of 900 m2 within the white polygon in a and 5,000 m2 throughout the rest of the domain. We used a smaller triangle area in the study reach (within the white polygon) to better resolve spatial variability in shear stresses and a slightly larger triangle area elsewhere for computational efficiency.

Extended Data Figure 2 Hillshade maps of the study reach for each bed elevation.

These maps depict the reconstructed topography in Moses Coulee, which is based on the elevation of terraces and the modern channel bed upstream from the knickzone. a, b, Bed elevation D; c, d, bed elevation C; e, f, bed elevation B; g, h, bed elevation A; i, j, modern topography.

Extended Data Figure 3 Bed shear stress as a function of discharge for sites on the Moses Coulee knickzone.

Data correspond to locations shown in Extended Data Fig. 7. For nearly all simulated discharges, the modelled shear stresses on the knickzone exceed the thresholds for plucking via block sliding, indicating that the knickzone would probably have been rapidly eroding during floods or that it is made up of stronger rock.

Extended Data Figure 4 Flood depth versus time for a typical simulation.

The example shown is a flood with a discharge of 106 m3 s−1 routed through the modern topography. The grey box shows the final ten time-steps, from which the model results (stage, velocity, shear stress and so on) were extracted and used to produce time-averaged grids to reduce the influence of transient waves relative to data from a single time-step.

Extended Data Figure 5 Cumulative bed stress probability density functions.

The light grey bar shows the lower (467 Pa) and upper (751 Pa) threshold shear stress bounds for plucking of 0.5-m blocks. The dark grey bar shows the threshold stress bounds (117 Pa and 1,242 Pa) assuming a wider distribution of block sizes of 0.13–0.83 m, based on the D16 and D84 (16th and 84th percentiles) of basalt clasts at the Ephrata Fan and Drumheller Channels sites, respectively (Extended Data Fig. 6). Cumulative probability density functions are shown for the brim-full flood (discharge of 3.0 × 106 m3 s−1; black line) and the lower (0.3 × 106 m3 s−1; red line) and upper (0.6 × 106 m3 s−1; blue line) flood discharge bounds predicted by the threshold shear stress model for the modern Moses Coulee topography. Data are extracted from each grid cell within the entire length of Moses Coulee (Fig. 1b). For the brim-full flood, only 8% of the terrain of Moses Coulee has modelled bed shear stresses within the bounds for plucking 0.5-m blocks, and this increases to 33% when considering the wider range of block sizes; some areas have extremely high bed stresses of more than 10,000 Pa. In contrast, for discharges predicted by the threshold shear stress model, 25% of the terrain has shear stresses within the bounds for plucking 0.5-m blocks, and this increases to 67% when considering a wider distribution of block sizes. This high proportion of bed stresses within the plucking threshold range, relative to the brim-full flood, is consistent with the hypothesis that the channel adjusts so that a large portion of the terrain is near the threshold state during megaflood incision. Note that the study locations in Extended Data Fig. 7 were used to define the discharge bounds for the threshold shear stress model, so all of those locations have bed stresses within the plucking threshold range by definition (see Extended Data Fig. 8).

Extended Data Figure 6 Boulder size data.

Cumulative grain-size distribution for the Moses Coulee abandoned channel boulder bar (circles), Ephrata Fan basalt clasts (squares) and the Drumheller Channels boulder bar (triangles). Measurements were made of the intermediate grain diameter using a Wolman-style pebble count. The Ephrata Fan and Drumheller Channels sites are to the southwest of Moses Coulee and in a different flood pathway. The larger boulders at the Ephrata Fan and Drumheller Channels are assumed to originate as basalt columns that have been rounded by fluvial transport.

Extended Data Figure 7 Example model output showing the upper and lower shear stress thresholds for block sliding.

Round symbols depict the mean and bars depict one standard deviation (n = 33) of bed shear stress values extracted from one of the twelve 30,000-m2 polygons shown in Extended Data Fig. 7. In this example, the lower discharge bound (red symbol) for the threshold shear stress model is defined as the lowest discharge for which modelled shear stresses exceed the lower shear stress threshold for sliding. The high discharge bound (blue symbol) is defined as the lowest discharge with modelled shear stresses that exceed the upper shear stress threshold for sliding.

Extended Data Figure 8 Locations of the areas used to calculate mean bed shear stress.

The 30,000-m2 rectangles near the A–A′ cross-section were used to constrain discharges predicted by the threshold shear stress model. The larger 90,000-m2 polygons (labelled 1–6) were used to determine the shear stresses on the knickzone; the numbers correspond to the data in Extended Data Fig. 3.

Extended Data Figure 9 Schematic illustrating the method for defining the discharge range predicted by the threshold shear stress model.

For a given topographic reconstruction we first used the upper and lower bed stress thresholds for block plucking via sliding to define the range of possible discharges that are consistent with the threshold stress model at the lowest elevation site (Extended Data Fig. 7). The lowest elevation was always the canyon floor. We then followed the same procedure to determine the discharge range at sites with progressively higher elevations (for example, sites 2–5) that produced bed stress that were within the bounds for plucking at the site of interest and for all lower elevation sites. The range of discharges determined by these two criteria is shown by the green box. These two criteria are important because, for a flood discharge to be consistent with the threshold shear stress model, all sites that are inundated and eroding, from the canyon floor upward, must have bed stresses that are within the bounds for plucking. If the discharge required to pluck blocks at a higher elevation site exceed that of a lower elevation site, then the higher sites were considered to be dry and abandoned and so were not used to define the range of discharges for that stage of canyon incision.

Extended Data Figure 10 Predicted channel cross-sectional areas and widths.

a, b, Cross-sectional area (a) and channel width (b) predicted by the brim-full model and threshold shear stress model. The shading shows the range of predicted values based on upper- and lower-bound parameterizations of the critical dimensionless shear stress for bedrock incision by block sliding (see Methods). For the initial topography (bed elevation D), where basalt is primarily overlain by loess, modelled shear stresses are lower than the threshold for plucking. At bed elevation C, modelled shear stresses at brim-full discharge are within 5% of the lower threshold for plucking; hence, bed stresses are assumed to be sufficient for plucking when flow was brim-full. For all other bed topographies, brim-full discharge greatly exceeds predicted values for plucking and sediment initial motion. The letters A–D denote simulation results for bed elevations A–D; M denotes results for the modern topography.

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Larsen, I., Lamb, M. Progressive incision of the Channeled Scablands by outburst floods. Nature 538, 229–232 (2016) doi:10.1038/nature19817

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