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The Origin of Anastomosis on the Columbia River, British Columbia

Dimitri Abbado & Rudy Slingerland Department of Geosciences, Penn State University Norman D. Smith Department of Geosciences, University of Nebraska with help from Manuel Filgueira-Rivera and Matt Machusick. The Origin of Anastomosis on the Columbia River, British Columbia.

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The Origin of Anastomosis on the Columbia River, British Columbia

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  1. Dimitri Abbado & Rudy Slingerland Department of Geosciences, Penn State University Norman D. Smith Department of Geosciences, University of Nebraska with help from Manuel Filgueira-Rivera and Matt Machusick The Origin of Anastomosis on the Columbia River, British Columbia

  2. Background • Anastomosed rivers consist of two or more interconnected, co-existing channel belts that enclose flood basins. • Anastomosed rivers typically possess: • low flow strengths, episodic flows, frequent overbank flows • tough banks • excess sediment supply

  3. Background (cont.) Columbia River, B.C.

  4. What Causes Anastomosis? • Two classes of explanation: • Anastomosis is a consequence of frequent avulsions and slow abandonment of earlier channel belts • A chance by-product of the competition between channel creation and abandonment • Immediate causes of avulsions: rise in base level or high rates of aggradation • Immediate causes of slow abandonment: low sediment levels, low stream power, and tough consolidated floodplain mud, although few data exist

  5. What Causes Anastomosis? • Two classes of explanation: • Anastomosis is an equilibrium fluvial form where channels are adjusted in geometry, number, and hydraulic friction to just transmit imposed water and sediment discharges • Where gradient cannot easily be increased to carry an imposed sediment load, a switch from one to many channels leads to an increase in sediment transport rate per unit water discharge

  6. Purpose of this Research • To better understand the hydraulic and sedimentologic factors responsible for forming the anastomosed reach of the upper Columbia River in British Columbia, Canada • To test the hypothesis that anastomosis increases sediment transport rates through a reach and therefore may be an equilibrium channel pattern

  7. Study Reach • Upper Columbia River near Golden, British Columbia, Canada

  8. Study Reach (cont.)

  9. Study Reach (cont.) • Morphology of typical floodbasin

  10. Study Reach (cont.) Active inlet to floodbasin early In flood season

  11. Study Reach (cont.) Interior of floodbasin

  12. Study Reach (cont.) Exit of floodbasin

  13. Study Reach (cont.) • Makaske’s (1998) cross section showing stability of channels over 1000’s of years.

  14. Longitudinal Variations in Anastomosis & Related Features

  15. Longitudinal Variations in Anastomosis & Related Features

  16. Longitudinal Variations in Anastomosis & Related Features • Water Surface Slope & Active Channels

  17. Longitudinal Variations in Anastomosis & Related Features • Crevasse Splay Activity

  18. Longitudinal Variations in Anastomosis & Related Features • Valley Width, Alluvial Fan Area, Bed Material Size

  19. Longitudinal Variations in Anastomosis & Related Features • Aggradation Rates in Steep Reach • Sediment budget for one floodbasin during rising limb in year 2000: 4 mm/yr. • Is this higher than: • Locking (1983) 3.7 mm/yr? • Smith (1983) 1.5 to 6mm/yr? • Makaske (1998) 1.7 mm/yr?

  20. Longitudinal Variations in Anastomosis: Conclusions • the Columbia River can be divided into two sub-reaches: 1) a 20 km long, highly anastomosed reach with 3-5 channels, and 2) a 50 km long, weakly anastomosed reach containing 1-3 channels • The highly anastomosed reach begins immediately downstream from the Spillimacheen tributary, a major source of sediments in the valley • This reach is steeper, contains a higher number of crevasse splays, possibly sits in a wider valley, and contains a coarser bed material

  21. Why? • Nanson, Knighton, and Huang argue that anabranching rivers maximize bed sediment transport • Is this what the Columbia is doing? • To answer this question, we compute the theoretical total sediment flux for two reaches of Columbia using modified Bagnold and Rouse • Reach A has one channel of W/D = 40; reach B has three channels, each with W/D = 10, consistent with Columbia channels. For both cases, Qtot = 125 m3/s, S = 0.0001, and n = 0.026

  22. Are Multiple Channels More Efficient at Transporting Sediment? • Results • For 1 channel: Qstot = 0.0077 m3/s • For 3 channels: Qstot = 0.0075 m3/s • Results similar for van Rijn transport laws

  23. Conclusions: A Conjecture • Large sediment input of Spillimacheen tributary over last few millennia could not be transported by Columbia River, given its regional gradient • Aggradation of Columbia channel led to increased levee overtopping • An increased number of crevasses promoted the formation of new channels through floodbasins

  24. A Conjecture (cont.) • Higher valley slope from local alluviation increased sediment transport enough to stablize the system, allowing multiple channels and intervening floodbasins to aggrade in-place for thousands of years • Increased efficiency of multiple channels not proven

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