Avulsion and Crevassing in the Sandy, Braided Niobrara River: Complex Response to Base-Level Rise and Aggradation

  1. N. D. Smith4 and
  2. J. Rogers5
  1. F. G. Ethridge1,
  2. R. L. Skelly1,† and
  3. C. S. Bristow2

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304213.ch14

Fluvial Sedimentology VI

Fluvial Sedimentology VI

How to Cite

Ethridge, F. G., Skelly, R. L. and Bristow, C. S. (1999) Avulsion and Crevassing in the Sandy, Braided Niobrara River: Complex Response to Base-Level Rise and Aggradation, in Fluvial Sedimentology VI (eds N. D. Smith and J. Rogers), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304213.ch14

Editor Information

  1. 4

    Department of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, USA

  2. 5

    Cape Town, South Africa

Author Information

  1. 1

    Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523-1482, USA

  2. 2

    Department of Geology, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK

  1. Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-4778, USA

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 7 OCT 1999

ISBN Information

Print ISBN: 9780632053544

Online ISBN: 9781444304213

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Keywords:

  • avulsion and crevassing in sandy, braided Niobrara river complex response to base-level rise and aggradation;
  • avulsion and crevassing in sandy, braided river;
  • history of lower Niobrara River;
  • crevassing and avulsion in lower Niobrara River valley;
  • processes and deposits in lower Niobrara River - distributary or anabranching system;
  • avulsions of lower Niobrara River - initiated and developing as crevasse splays

Summary

During a study of the alluvial architecture of the lower portion of the sandy, braided Niobrara River, north-eastern Nebraska, a series of crevasse splays formed and several avulsions occurred. The majority of these events happened between 1995 and 1997, and they can be related to a maximum 2.9 m base-level rise and aggradation of the main Niobrara channel belt, which began in the 1950s following damming of the Missouri River. Crevasses and avulsions, along with a rising groundwater table, have turned the lower 3.3 km of the Niobrara, above its confluence with the Missouri, into an extensive wetland with characteristics and processes similar to those found in some anabranching rivers.

Following 43 yr of aggradation, the Niobrara channel has become elevated above its floodplain, which has led to a series of avulsions, often initiated by the development of crevasse splays. Floodplain aggradation is occurring rapidly with up to 1.5 m of crevasse-splay deposition in a year. Crevasses have formed at low points in the river banks and levees and are locally constrained by floodplain topography and human-made structures. The timing of crevasse initiation may be linked to localized bank erosion or the presence of ice dams rather than increased discharge. The river appears to be evolving into an anabranching or distributary system, reactivating old channels and flowing across former islands and floodplains. The major changes have occurred very rapidly, over a 2-yr period (1995–1997), following more gradual systematic changes in channel-belt width over the previous 41 yr (1954–1995). This pattern of gradual change, followed by dramatic short-term change, is interpreted to indicate the crossing of a geomorphological threshold, beyond which the river behaviour changed from aggradational to avulsive. Our data support models for avulsion occurring when there is a decrease in channel-belt slope and/or an increase in cross-valley slope and indicate that aggradation and superelevation of the channel belt is a major factor in allowing avulsion to occur and persist. In the Niobrara, the major external factor forcing channel change has been a significant rise in base level.