Downcutting, also called erosional downcutting, downward erosion or vertical erosion, is a

geological Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...

process by hydraulic action that deepens the channel of a

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...

valley A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...

by removing material from the

stream bed A streambed or stream bed is the bottom of a stream or river and is confined within a Stream channel, channel or the Bank (geography), banks of the waterway. Usually, the bed does not contain terrestrial (land) vegetation and instead supports d ...

or the valley floor. The speed of downcutting depends on the stream's ''

base level In geology and geomorphology a base level is the lower limit for the vertical position of an erosion, erosion process. The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis wh ...

'', the lowest point to which the stream can erode.

Sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

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'' 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 A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher t ...

ous areas tend to be narrow and swift, forming V-shaped valleys, while streams in

lowland Upland and lowland are conditional descriptions of a plain based on elevation above sea level. In studies of the ecology of freshwater rivers, habitats are classified as upland or lowland. Definitions Upland and lowland are portions of a ...

areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed. The '' 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 will increase the gradient of a stream even while the stream downcuts toward its base level, a process called '' rejuvenation''. This happened in the case of the

Colorado River The Colorado River () is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and in northern Mexico. The river, the List of longest rivers of the United States (by main stem), 5th longest in the United St ...

in the western

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, resulting in the process that created the

Grand Canyon The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a mile (). The canyon and adjacent rim are contained within Grand Canyon Nati ...

Lake bed downcutting

Lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

bed downcutting is the erosion of cohesive material such as

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

glacial A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

from a shoreline by

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

action. When the

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

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 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

Black Hawk College: Fluvial Processes II (Physical Geography 102)

U.S. Army Corps of Engineers: Lake Bed Downcutting
*Leet, L. Don. ''Physical Geology'', 6th Edition. Englewood Cliffs, NJ: Prentice-Hall, 1982. {{Rivers, streams and springs Geological processes Hydrology Erosion