# Downcutting

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{{short description|Process of deepening a stream channel by erosion of the bottom material}}
{{in-line citations|date=November 2017}}
[[Image:San Juan River Utah 1927.jpg|thumb|300px|right|Several stages of downcutting by the [San Juan River](/source/San_Juan_River_(Colorado_River)) in [Utah](/source/Utah) can be identified in this 1927 photo.  Remnants of former [floodplain](/source/floodplain)s stand as [terraces](/source/fluvial_terrace) above the river's modern level.]]

'''Downcutting''', also called '''erosional downcutting''', '''downward erosion''' or '''vertical erosion''', is a [geological](/source/geology) process by [hydraulic action](/source/hydraulic_action) that deepens the [channel](/source/Channel_(geography)) of a [stream](/source/stream) or [valley](/source/valley) by removing material from the [stream bed](/source/stream_bed) or the valley floor. The speed of downcutting depends on the stream's ''[base level](/source/base_level)'', the lowest point to which the stream can [erode](/source/erosion). [Sea level](/source/Sea_level) is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter [bedrock](/source/bedrock) that resists erosion.

A concurrent process called ''[lateral erosion](/source/lateral_erosion)'' is the widening of a stream channel or valley. When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its base level, the rate of lateral erosion increases. This is why streams in [mountain](/source/mountain)ous areas tend to be narrow and swift, forming [V-shaped valley](/source/V-shaped_valley)s, while streams in [lowland](/source/lowland) areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed. 

The ''[stream gradient](/source/stream_gradient)'' is the ratio between the elevation drop of a stream and the horizontal distance considered. The steeper the gradient, the faster the stream flows. Sometimes geological [uplift](/source/Tectonic_uplift) will increase the gradient of a stream even while the stream downcuts toward its base level, a process called ''[rejuvenation](/source/rejuvenation_(river))''. This happened in the case of the [Colorado River](/source/Colorado_River) in the western [United States](/source/United_States), resulting in the process that created the [Grand Canyon](/source/Grand_Canyon).

==Lake bed downcutting==

[Lake](/source/Lake) bed downcutting is the erosion of cohesive material such as [clay](/source/clay) or [glacial](/source/glacier) [till](/source/till) from a shoreline by [wave](/source/ocean_wave) action. When the [sand](/source/sand) cover is stripped away and the cohesive layer is exposed, cohesive material is lost to the [water column](/source/water_column). Unlike sand, cohesive material cannot be replenished by natural events such as [bluff](/source/Hill) erosion. This can result in a process called "bluff recession," in which waves erode and carry away the material at the toe of the bluff and cause it to become steeper. When the slope reaches a certain angle, the bluff becomes unstable and fails, causing it to recede inland.

==References==
*[http://facweb.bhc.edu/academics/science/harwoodr/Geog102/study/river2.htm Black Hawk College: Fluvial Processes II (Physical Geography 102)] {{Webarchive|url=https://web.archive.org/web/20160303191532/http://facweb.bhc.edu/academics/science/harwoodr/Geog102/study/river2.htm |date=2016-03-03 }}
*[https://archive.today/20121212203107/http://www.lre.usace.army.mil/greatlakes/hh/greatlakestudies/greatlakesregionalsedimentmanagement/lakebeddowncutting U.S. Army Corps of Engineers: Lake Bed Downcutting]
*Leet, L. Don. ''Physical Geology'', 6th Edition. Englewood Cliffs, NJ: Prentice-Hall, 1982.  {{ISBN|0-13-669762-3}}

{{Rivers, streams and springs}}

Category:Geological processes
Category:Hydrology
Category:Erosion

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