What Makes a River Bend?
A river always takes the easiest course through channels and soft ground to the sea, flowing swiftly down mountain sides but much more slowly on the level plains. On the plain the river takes advantage of every difference in gradient. This winding course is accentuated by the process of silting and erosion.
As a river flows round a curve, the water on the outer bend moves more swiftly to cover the greater distance in the same time as the water flowing past the inner bend. The more rapidly moving water will tend to wear away the banks of the channel, while the slower movement of the water on the inside wall will allow silting to take place.
When the curve becomes more pronounced it is known as a “meander”. Notable examples of meanders are to be found in the Wye in England, and the Meuse in France. Sometimes the river erodes the bank so fiercely that a new channel is formed, leaving an island of earth in the middle of the stream.
A meander is produced by a stream or river swinging from side to side as it flows across its floodplain or shifts its channel within a valley, it erodes the sediments comprising an outer, concave bank (cut bank) and deposits this and other sediment downstream on an inner, convex bank which is typically a point bar.
The result of sediments being eroded from the outside concave bank and their deposition on an inside convex bank is the formation of a sinuous course as a channel migrates back and forth across the down-valley axis of a floodplain.
The zone within which a meandering stream shifts its channel across either its floodplain or valley floor from time to time is known as a meander belt. It typically ranges from 15 to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.
The degree of meandering of the channel of a river, stream, or other watercourse is measured by its sinuosity. The sinuosity of a watercourse is the ratio of the length of the channel to the straight line down-valley distance. Streams or rivers with a single channel and sinuosities of 1.5 or more are defined as meandering streams or rivers.
Whereas a knee, or river knee, is a bend in a river changing its course significantly within a short distance to a different direction (in an angle of around 90 degrees), it is different from a riverbend which is a single isolated bend, and from a meander which consists of several bends in a sinuous course, both without changing the river’s main course. In European history, many river knees have proven to be strategically favorable locations to found cities.
Many rivers have significant bends due to geological reasons:
the Rhine knee in Basel, Switzerland, the river’s most significant knee – other knees appear in German cities Wiesbaden and Bingen
the Danube knee in Vác, Hungary
the Volga knee at Volgograd, Russia
the Rhône knee at Martigny, Switzerland
the Petitcodiac knee at Moncton, New Brunswick, Canada, locally known as Le Coude, in English erroneously called “the Bend”
Furthermore, some riverbends are called “knees” although they are actually single bends or a part of meanders:
the “Vltava knee” in Prague, Czech Republic
the “Elbe knee” at Königstein Fortress, Germany
the “Rhine knee” at Düsseldorf, Germany (see also de:Rheinkniebrücke)