Channel Versus Valley: Semantics or Significance?

William A. Wescott, Lesli J. Wood

Channels are the active courses of streams at any given point in time and can be of several different geometries. Valleys are the areas which confine the channels and their associated fluvial components. With the exception of bedrock incised channels, valleys are wider than individual channels and contain multiple channel deposits at the same stratigraphic horizon. Valleys are longer lived features and extend for greater distances in a dip direction than channels. In contrast, individual channels typically have a higher sinuosity than valleys. Channels not associated with valley-fill deposits generally are associated with deltaic and coastal-plain environments; including delta distributary, tidal and storm-flow return channels. In addition, fluvial channels can develop on emergent shelves during eustatic low stands as a result of drainage elongation, headward incision or avulsion.

Valley geometry is a function of local structure, lithology, slope and climate. Broad, shallow valleys develop on stable platforms through lateral incision of channels. Deep valleys develop in response to either extreme lowering of sea level or tectonic uplift. Channel type is a function of sediment supply, discharge, valley slope and vegetation. Channel response to change can take many forms and is, in part, a function of pre-change state, post-change climate and vegetation, and magnitude of change. Although channels may alter in response to baselevel change, valley geometries will remain relatively unchanged since they are dominantly a function of variables independent of sea level.

The differences in scale, geometry, and facies distributions are significant and important with regard to subsurface detection and exploitation of reservoirs in these and associated settings.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995