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

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