Base Level Rise, Aggradation and Facies Architecture of a Sandy Braided River: the Niobrara River, Nebraska

Raymond L. Skelly and Frank G. Ethridge

The Niobrara River of northeastern Nebraska has experienced a significant base level rise at its confluence with the Missouri River. Dams on the Missouri River have reduced peak discharges and created a backwater, resulting in rapid aggradation along the lower 13 miles of the Niobrara River. Environments and fluvial processes characteristic of the aggrading portion of this sandy braided river are the result of the interplay between base level rise and sediment supplied by the river.

The river valley in the aggrading portion of the Niobrara River is a mosaic of channel belt and floodplain environments. Actively migrating macroforms (bars) and bedforms (dunes) are characteristic of the channel belt whereas relatively stable surfaces with soil development, dense vegetation, and wetlands are characteristic of the floodplain. Bar complexes within the channel belt are continually modified by variations in the river's discharge. Interaction between these channel and floodplain environments ultimately result in heterogeneity of the valley fill.

Avulsion appears to be an important process in the aggrading portion of the river. This process creates significant heterogeneity in the valley fill deposits by juxtaposing channel and floodplain environments. A recent avulsion has occurred in the aggrading reach which might evolve into a new channel course. Avulsions occur with greater frequency in the aggrading reach, as opposed to stable or degrading reaches of the river, because of the significant cross-valley gradient that exists between the channel and floodplain.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California