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

Abstract

© 1999 The International Association of Sedimentologists and published for them by Blackwell Science Ltd. All rights reserved. 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.