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. Thalweg- An imaginary line, drawn in a flow-parallel orientation, which joins the deepest points of a stream channel. 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…). Terraces stand at elevations higher than the modern floodplain and can be formed through the processes of channel degradation or entrenchment. Terrace- A generally flat valley-floor surface that represents an abandoned floodplain. Right bank- By standard convention, left and right are always expressed from the perspective of an individual facing downstream. As the abandoned meander channels are now dry, agricultural use has reclaimed them to the extent that they are difficult to distinguish on the ground. 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. Some rivers shift in this way on a time scale from a few years to several decades whereas others are essentially static.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.Īctive channel- The natural waterway that contains all streamflows at and below the bankfull discharge.īankfull stage- The elevation of the water surface when rising water completely fills the active channel and first begins to spill onto the local floodplain.įloodplain- 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.įluvial plain- The relatively flat area outside the active channel that makes up the valley floor (or valley flat). When deposition finally seals off the cut-off from the river channel, an oxbow lake is formed. The present study focuses on identifying the meanders of River Manu and their cutoffs. Point bar deposits and ox-bow lakes are the products of lateral bend migration and meander cutoff.
When this happens, a new straighter river channel is created and an abandoned meander loop, called a cut-off, is formed. A common phenomenon associated with alluvial rivers is their meander evolution, eventually forming cutoffs. The narrow neck of land between the two neighbouring concave banks is finally cut through, either by lateral erosion of the two concave banks or by the strong currents of a flood. Abandonment occurred when the erosive power of the stream increased and it began to rapidly downcut to a lower elevation. A stream terrace is simply an old floodplain that is now abandoned. Another common feature of alluvial systems is the stream terrace. Continuous erosion of a concave bank and deposition on the convex bank of a meandering river cause the formation of a very pronounced meander with two concave banks getting closer. This abandoned meander then forms a lake known as an oxbow. In contrast, both lateral erosion and undercutting occur on the concave bank where the stream’s velocity is the highest. Deposition occurs on the convex bank because of the ‘slack water’, or water at low velocity. The distance between meanders in related to factors such as width but typically the average meander wavelength. When a river reaches a low-lying plain in its final course to the sea or a lake, it meanders widely. The abandoned meander now becomes an ox-bow lake. The bulges in the border reflect changes in the course of the river when the river shifted its course and cut off the former channel, the border did not change. A Horseshoe or oxbow lake near Hughes, Arkansas, USA.
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