Bankfull Terminology

To help understand some of the terms that are key in defining bankfull stage and bankfull determinations, the reader is directed to the text and figures below.

Active channel- The natural waterway that contains all streamflows at and below the bankfull discharge.

Bankfull stage- The elevation of the water surface when rising water completely fills the active channel and first begins to spill onto the local floodplain.

Floodplain- The relatively flat valley-floor surface that has been constructed, during the present hydrologic regime, by the natural processes of point-bar deposition, lateral channel migration, and vertical accretion of sediments that have been transported beyond the active channel boundaries during periods of flooding.

Fluvial plain- The relatively flat area outside the active channel that makes up the valley floor (or valley flat). The fluvial plain represents the combination of depositional and erosional geomorphic features resulting from streamflow processes and most often includes the modern floodplain, local stream terraces, and abandoned meander scars.

Left bank vs. Right bank- By standard convention, left and right are always expressed from the perspective of an individual facing downstream.

Terrace- A generally flat valley-floor surface that represents an abandoned floodplain.  Terraces stand at elevations higher than the modern floodplain and can be formed through the processes of channel degradation or entrenchment.  Where there are multiple terraces in a cross-valley profile, they can be named according to their relative position above the active channel (for example, low, middle, high, etc…).

Thalweg-  An imaginary line, drawn in a flow-parallel orientation, which joins the deepest points of a stream channel.

Valley Edge- For any given cross-valley profile, the two locations (left and right) where the fluvial plain meets the slopes that rise to the local uplands.


A line drawing of an idealized cross-valley profile illustrating key terms. (Click image to enlarge)
A small stream channel in Morgan County, Ind., where many of the bankfull terms can be viewed in a single photograph. (Click image to enlarge)

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