RECONSIDERING THE CONCEPT OF BANKFULL FLOW: DO SINGLE-THREAD MEANDERING RIVERS OVERTOP THEIR BANKS AT A DISTINCT BANKFULL STAGE?

The concept of bankfull flow has occupied a prominent role in geomorphological theory about process dynamics in alluvial meandering river systems. This concept has formed the basis for associating equilibrium channels dimensions with a specific channel-forming discharge and for examining the relation between floodplain dynamics and lateral channel migration. It also has played a central role in practical efforts to restore degraded streams. According to the traditional geomorphological perspective, when the elevation of the water surface exceeds a well-defined elevation delimiting the top of the channel banks, or bankfull stage, the floodplain abruptly becomes inundated. Above this stage, water moving across the floodplain creates the potential for erosion of and deposition on the floodplain surface. The bankfull stage therefore represents a distinct threshold for geomorphological processes in river systems.

This paper reconsiders the traditional perspective in the light of recent evidence from several single-thread meandering river systems throughout the central and southern United States. This evidence indicates that in many cases at least partial inundation of floodplains occurs well below what normally is considered bankfull stage, or the average elevation at the top of the channel banks. Partial inundation occurs through connectivity of topographic lows on the floodplain with the main channel of the river system. This connectivity is characterized by local breaks in the channel banks that allow water within the main channel to progressively inundate greater proportions of the floodplain surface as stage within the main channel increases. The process of progressive inundation is revealed by two-dimensional hydraulic modeling of flow within the channel-floodplain system at different stages. The results of this study indicate that the traditional perspective on bankfull flow as a discrete threshold delimiting the stage above which fluvial processes begin to affect the floodplain surface may be the exception rather than the norm. Instead, connectivity between channels and floodplains, even in single-thread meandering rivers with well-defined bankfull channels, is more complex than has traditionally been conceived.