Downcutting 4895120 165927478 2007-10-20T22:04:18Z Sam Blacketer 3011647 Disambiguating link to [[Channel]] - [[Wikipedia:Disambiguation pages with links|You can help!]] using [[Project:AutoWikiBrowser|AWB]] [[Image:San Juan River Utah 1927.jpg|thumb|300px|right|Several stages of downcutting by the [[San Juan River (Utah)|San Juan River]] in [[Utah]] can be identified in this [[1927]] photo. Remnants of former [[floodplain]]s stand as [[stream terrace|terraces]] above the river's modern level.]] '''Downcutting''', also called '''erosional downcutting''' or '''downward erosion''' or '''vertical erosion''' is a [[geology|geological]] process that deepens the [[Channel (geography)|channel]] of a [[stream]] or [[valley]] by removing material from the stream's bed or the valley's floor. How fast downcutting occurs depends on the stream's ''[[base level]]'', which is the lowest point to which the stream can [[erosion|erode]]. [[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]] that resists erosion. A concurrent process called [[lateral erosion]] refers to 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|mountainous]] areas tend to be narrow and swift, forming [[V-shaped valley]]s, while streams in [[lowland]] areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed. The term ''[[stream gradient|gradient]]'' refers to the elevation of a stream relative to its base level. The steeper the gradient, the faster the stream flows. Sometimes geological [[Tectonic uplift|uplift]] will increase the gradient of a stream even while the stream downcuts toward its base level, a process called "[[rejuvenation (river)|rejuvenation]]." This happened in the case of the [[Colorado River]] in the western [[United States]], resulting in the process that created the [[Grand Canyon]]. ==Lake bed downcutting== [[Lake]] bed downcutting is the erosion of cohesive material such as [[clay]] or [[glacier|glacial]] [[till]] from a shoreline by [[ocean wave|wave]] action. When the [[sand]] cover is stripped away and the cohesive layer is exposed, cohesive material is lost to the [[water column]]. Unlike sand, cohesive material cannot be replenished by natural events such as [[Hill|bluff]] 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)] *[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 [[Category:Geological processes]] [[Category:Hydrology]]