Editing Valley (section)

===River valleys {{anchor|River valleys}}=== <!-- {R to section} redirect target, "[[River valley]]" -->
[[File:Halikonjokilaakso.jpg|thumb|left|The valley of Halikko River in [[Halikko]], [[Finland]]]]
[[File:Palakaria-and-Belchin-3.jpg|thumb|right|220px|Valley of Palakaria river springing from [[Vitosha]] Mountain, seen in the background, in [[Geography of Bulgaria|Bulgaria]]]]
The development of a river valley is affected by the character of the bedrock over which the river or stream flows, the elevational difference between its top and bottom, and indeed the climate. Typically the flow will increase downstream and the gradient will decrease. In the upper valley, the stream will most effectively erode its bed through [[corrasion]] to produce a steep-sided V-shaped valley. The presence of more resistant rock bands, of [[fault (geology)|geological faults]], [[fracture (geology)|fractures]], and [[Fold (geology)|folds]] may determine the course of the stream and result in a twisting course with [[interlocking spur]]s.

In the middle valley, as numerous streams have coalesced, the valley is typically wider, the flow slower and both erosion and deposition may take place. More lateral erosion takes place in the middle section of a river's course, as strong currents on the outside of its curve erode the bank. Conversely, deposition may take place on the inside of curves where the current is much slacker, the process leading to the river assuming a [[meander]]ing character. In the lower valley, gradients are lowest, meanders may be much broader and a broader [[floodplain]] may result. Deposition dominates over erosion.<ref>{{cite book |last1=Monkhouse |first1=F.J. |title=Principles of Physical Geography |date=1971 |publisher=University of London Press Ltd |location=London |isbn=0340090227 |pages=152–157 |edition=Seventh}}</ref><ref>{{Cite book |last=Morisawa |first=Marie |url=https://www.worldcat.org/oclc/2968 |title=Classification of Rivers |publisher=Reinhold Book Corporation |year=1968 |editor-last=Rhodes W. Fairbridge |location=New York |pages=956–957 |oclc=2968 |author-link=Marie Morisawa}}</ref> A typical river basin or [[drainage basin]] will incorporate each of these different types of valleys.

Some sections of a stream or river valleys may have vertically incised their course to such an extent that the valley they occupy is best described as a [[gorge]], [[ravine]], or [[canyon]]. Rapid down-cutting may result from localized uplift of the land surface or rejuvenation of the watercourse as a result for example of a reduction in the base level to which the river is eroded, e.g. lowered global sea level during an [[ice age]]. Such [[river rejuvenation|rejuvenation]] may also result in the production of [[river terrace]]s.<ref>{{cite book |last1=Monkhouse |first1=F.J. |title=Principles of Physical Geography |date=1971 |publisher=University of London Press Ltd |location=London |isbn=0340090227 |pages=161–164 |edition=Seventh}}</ref>

{{for|various lists of river valleys|Category:River valleys}}