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Clogging the Mississippi? Glacial bedload from the Rockies to the Gulf. Bill Locke Department of Earth Sciences Montana State University. Conceptual “old-school” model. “Glacial terraces” What Aggradation downstream Where Entire system from ice-front to the sea When

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Clogging the Mississippi? Glacial bedload from the Rockies to the Gulf


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    1. Clogging the Mississippi? Glacial bedload from the Rockies to the Gulf Bill Locke Department of Earth Sciences Montana State University

    2. Conceptual “old-school” model • “Glacial terraces” • What • Aggradation downstream • Where • Entire systemfrom ice-frontto the sea • When • Contemporaneous with glaciation? • What really happened along the Mississippi? Chadwick et al., 1997 Locke (2005) NC-GSA

    3. “Not quite old-school” model Yuba R. • Hydraulic mining debris inthe Sierra Nevada • G. K. Gilbert (1917) • Importance of bedload • “Sediment wave” hypothesis • NOT explicit • Allan James (1989+) • Explicit attenuated wavehypothesis Sacramento R. Both from James, 1989 Locke (2005) NC-GSA

    4. Related studies Upland Major valleys Primary subaerial denudation rate • Longitudinal profile synthesis (Snow and Slingerland, 1987) • Quadratic profiles due to size-selective aggradation (Rice and Church, 2001) • Residence time of glacial sediment (Church and Ryder, 1972) • Landscape relaxation time >10 kyr (Church and Slaymaker, 1989) • Sediment slugs (Cui, Parker et al., 1997 ff) • Dispersion and translation ± particle size (Lisle et al., 2001) Locke (2005) NC-GSA

    5. “New-school” model • Finite sediment supply • Effective storage near source (attenuation) • Reworking of sediment downstream (“wave”) • Decreasing efficiency of reworking downstream (slope ) • Increasing time required (formative Q ) • Finite downstream wave propagation in infinite time! Locke (2005) NC-GSA

    6. “Sandbox” experiments “You are here” • Madison River, MT • Turner (1995) • Shoshone River, WY • Yellowstone River, MT Locke (2005) NC-GSA

    7. Madison River setting http://www.crh.noaa.gov/mbrfc • Although glacial terraces exist, examine recent terraces • Madison ‘Slide • All sediment = local • Managed by Hebgen Dam Locke (2005) NC-GSA

    8. Madison River August 19, 1959 M7.5 quake 3x107 m3 Terraces Morphology Unaffected channel Rockslide Older (“glacial”) terraces Outwash Locke (2005) NC-GSA

    9. Terraces (Turner, 1995) • Conceptual model • Sediment wave • Downstream attenuation • Unknown extent • Unknown duration Locke (2005) NC-GSA

    10. Actual Terraces • Observed to form post- 1959 • Represent proximal fill • Show progressive incision/translation • Define a single time-transgressive “terrace” Locke (2005) NC-GSA

    11. Terraces with space • Deterministic! • Represent deviation from “equilibrium” • Integrate Q, caliber, load, & accommodation space • Limited future progradation Locke (2005) NC-GSA

    12. Terraces with time • Stochastic! • Represent control on alluvial slope by Q and load • More motion requires more Q • Affected by Hebgen Dam (upstream) • Representative? Locke (2005) NC-GSA

    13. Madison River: lessons learned • In a controlled environment: • Terrace formation • begins immediately. • is spatially predictable. • Terrace progradation • is episodic. • is temporally probabilistic. • is finite. • So what about the “real world”? Locke (2005) NC-GSA

    14. Shoshone River setting • Legendary • J. H. Mackin (1937) • J. H. Moss (1974) • Major outlet glacier • Base level = Bighorn/Wind River • Only one tributary (North Fork) in 125 km Locke (2005) NC-GSA

    15. Shoshone River terraces • All surfaces fit quadratic models • Younger (lastglacial) converges across 85 km • Older (Illinoisan?) is asymptotic/”parallel”. Locke (2005) NC-GSA

    16. Shoshone summary • The model works! • Lastglacial terraces • are finite/converge. • require >14,000 yr. • Are still prograding. • Older terraces • “parallel” the river. • denudation andisostasy? • What about more complex river systems? Modernanabranching Wisconsinan Illinoisan Locke (2005) NC-GSA

    17. Yellowstone River setting • Major outlet glacier (Pierce, 1979) • Many glaciated tributaries • Expected pattern? Locke (2005) NC-GSA

    18. Yellowstone River terraces • Lastglacial = finite, older = river-parallel? • Show evidence of local sediment supply • Anomalous reaches require other data (provenance, age) to decipher Locke (2005) NC-GSA

    19. Yellowstone River: lessons learned • Complex rivers have complex terraces reflecting a complex history! • Lastglacial signal can be minor • Lastglacial coarse bedload is still close to source. • The lowest terrace may not have resulted from the same stimulus at all points. Locke (2005) NC-GSA

    20. Synthesis: bedload distribution • Coarse bedload • controls channel morphology. • must be reworked locally. • Coarse bedload contribution • is volumetrically finite. • is reworked multiple times. • Proximal terraces • are immediate - “glacial outwash”. • appear to parallel the river, but converge. • Distal terraces • take forever – “interglacial”! • converge with the river, but end up parallel. Locke (2005) NC-GSA

    21. “Mississippi River”? GSA, 1959 • Headwaters coarse bedload isn’t coming! • There isn’t enough slope to carry the cobbles/gravel. • Outwash in the Mississippi itself probably behaved the same. Locke (2005) NC-GSA

    22. References Cited Chadwick, O. A, Hall, R. D, and Phillips, F. M., 1997, GSAB 109, 1443-52. Church, Michael and Ryder, J. M., 1992,GSAB 83, 3059-72. Church and Slaymaker, 1989, Nature 337, 452-4. Cui, Parker et al., 1997 ff Gilbert, G. K., 1917, USGS PP 105, 154 pp. James, L. Allan, 1989, Annals AAG 79, 570-592. Lisle, T. E., Cui, Yantao, and Parker, Gary, 2001, ESPL 26, 1409-20 Moss, J. H., 1974, Glacial Geomorphology (D. R. Coates, ed.), 293-314. Mackin, J. H., 1937, GSAB 48, 813-93. Pierce, K. L., 1979, USGS PP 729-F, 90 pp. Rice and Church, 2001, Water Resources Res. 37, 417-26. Snow and Slingerland, 1987, Journal of Geology 95, 15-33. Locke (2005) NC-GSA