In
geomorphology
Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...
, a knickpoint or nickpoint is part of a
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
or
channel where there is a sharp change in channel
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
, such as a
waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in several ...
or
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
. Knickpoints reflect different conditions and processes on the river, often caused by previous
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
due to
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate bet ...
or variance in
lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Li ...
. In the
cycle of erosion model, knickpoints advance one cycle upstream, or inland, replacing an older cycle.
A knickpoint that occurs at the head (furthest upstream extent) of a channel is called a
headcut.
[ ] Headcuts resulting in
headward erosion
Headward erosion is erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, lengthening the stream channel.Essentials of Geology, 3rd Ed, Stephen Marshak It can also refer to ...
are hallmarks of unstable expanding drainage features such as actively eroding gullies.
[ ]
Knickpoints also occur on other planetary bodies that previously had or currently have surface liquids, namely
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
and
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
.
On Mars, the knickpoints have a common elevation that suggest a common sea level for a
former Martian ocean.
On Titan, mountain valleys adjacent to the present-day
hydrocarbon seas show evidence of knickpoints and recent
sea-level change.
Formation
Knickpoints are formed by the influence of tectonics, climate history, and/or lithology. For example, uplift along a fault over which a river is flowing will often result in an unusually steep reach along a channel, known as a ''knickzone''. Glaciation resulting in a
hanging valley are often prime spots for knickpoints. If lithology of the rock varies, such as shale amongst igneous rock, erosion will occur more steadily in the softer rock than the surrounding, tougher rock.
Base level is the elevation of the surface of the water body into which a river ultimately drains, usually the ocean. A drop in base level causes a response by the river system to carve into the landscape. This incision begins at the formation of a knickpoint, and its upstream migration depends heavily upon the drainage area (and so the discharge of the river), material through which it cuts, and how large the drop in base level was.
Modern examples
Knickpoints include both waterfalls and some lakes. These features are common in rivers with a sufficient
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
, i.e. enough change in
elevation above sea level over their length to encourage
degradation.
Influenced by lithology
Variations in stability of the underlying rock influence development of a bedrock-channeled river, as the waters erode different rock types at different rates.
Victoria Falls
Victoria Falls ( Lozi: ''Mosi-oa-Tunya'', "The Smoke That Thunders"; Tonga: ''Shungu Namutitima'', "Boiling Water") is a waterfall on the Zambezi River in southern Africa, which provides habitat for several unique species of plants and anim ...
, on the
Zambezi River
The Zambezi River (also spelled Zambeze and Zambesi) is the fourth-longest river in Africa, the longest east-flowing river in Africa and the largest flowing into the Indian Ocean from Africa. Its drainage basin covers , slightly less than hal ...
, is a spectacular example of this. The gorges visible by satellite imagery illustrate the
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
al processes behind the formation of the falls. Here, much of the surface rock is a massive
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
sill
Sill may refer to:
* Sill (dock), a weir at the low water mark retaining water within a dock
* Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma
* Sill (geostatistics)
* Sill (river), a river in Austria
* Sill plate, ...
, with large cracks filled with easily weathered
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
made visible by the Zambezi's course across the land. The
gorges downstream of the falls through which it flows were eroded over time by the action of the water.
Influenced by tectonic activity
Throughout
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...
, tectonic uplift and faulting are actively contributing to knickpoint initiation and recession. The
Waipoua River system, on the North island, has been studied and used to create mathematical models to predict the behavior of knickpoints. The study showed a direct correlation between upstream
drainage area
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
and rate of migration, producing modeled data closely approximating the collected data. The Waipoua River system incises through
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
s, for the most part, as opposed to
bedrock
In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet.
Definition
Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...
.
Influenced by glacial activity
Sharp changes in slope are common in rivers flowing through the heavily carved landscape left behind when
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
s retreat.
Glacial valley
U-shaped valleys, also called trough valleys or glacial troughs, are formed by the process of glaciation. They are characteristic of mountain glaciation in particular. They have a characteristic U shape in cross-section, with steep, straight s ...
s, as well as
isostatic rebound
Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...
resulting from the removal of the mass of glacial ice contribute to this.
Niagara Falls
Niagara Falls () is a group of three waterfalls at the southern end of Niagara Gorge, spanning the Canada–United States border, border between the Provinces and territories of Canada, province of Ontario in Canada and the U.S. state, state ...
, on the border of the United States and Canada, is a characteristic example of knickpoint. The falls have slowed in migration from approximately 1m per year as of 1900 to their modern 10 cm per year. The falls, particularly
Horseshoe Falls
Horseshoe Falls is the largest of the three waterfalls that collectively form Niagara Falls on the Niagara River along the Canada–United States border. Approximately 90% of the Niagara River, after diversions for hydropower generation, flows ...
, are dramatically steep and caused by
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate bet ...
. The
Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
themselves lie in the depressions left behind by glaciers, as the crust is still
rebounding.
Bridalveil Fall
Bridalveil Fall is one of the most prominent waterfalls in the Yosemite Valley in California.
The waterfall is in height and flows year round.
Geology
The glaciers that carved Yosemite Valley left many hanging valleys that spawned the wate ...
, in
Yosemite Valley
Yosemite Valley ( ; ''Yosemite'', Miwok for "killer") is a U-shaped valley, glacial valley in Yosemite National Park in the western Sierra Nevada (U.S.), Sierra Nevada mountains of Central California. The valley is about long and deep, surroun ...
, California, pours over the lip of a
hanging valley.
Paleomorphology
Evidence of a knickpoint in the geologic past can be preserved in the shape of the bedrock below any subsequent depositions, as well as within sedimentary depositions left unchanged by human or other activity. Lakes characteristically fill in with sediment over time, but waterfalls often erode away. There are few obvious, dry examples still visible today of prehistoric knickpoints.
Evidence of massive prehistoric flooding
Dry Falls
Dry Falls is a scalloped precipice with four major alcoves, in central Washington scablands. This cataract complex is on the opposite side of the Upper Grand Coulee from the Columbia River, and at the head of the Lower Grand Coulee, northern ...
, a 3.5 mi long precipice in central
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
, is an example of an ancient knickpoint. Geologic evidence strongly suggests that the water which formed this feature flowed over the
Channeled Scablands
The Channeled Scablands are a relatively barren and soil-free region of interconnected relict and dry flood channels, coulees and cataracts eroded into Palouse loess and the typically flat-lying basalt flows that remain after cataclysmic floods ...
,
bursting
Bursting, or burst firing, is an extremely diverse general phenomenon of the activation patterns of neurons in the central nervous system and spinal cord where periods of rapid action potential spiking are followed by quiescent periods much longe ...
from the
glacial lake
A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier.
Formation
Near the end of the last glacial period, roughly 10, ...
Missoula
Missoula ( ; fla, label=Salish language, Séliš, Nłʔay, lit=Place of the Small Bull Trout, script=Latn; kut, Tuhuⱡnana, script=Latn) is a city in the U.S. state of Montana; it is the county seat of Missoula County, Montana, Missoula Cou ...
during an event known as the
Missoula Floods and into the
Columbia River Gorge
The Columbia River Gorge is a canyon of the Columbia River in the Pacific Northwest of the United States. Up to deep, the canyon stretches for over as the river winds westward through the Cascade Range, forming the boundary between the st ...
.
Evidence within karst topography
On the
Upper Cumberland River,
Tennessee
Tennessee ( , ), officially the State of Tennessee, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. Tennessee is the List of U.S. states and territories by area, 36th-largest by ...
, there exist a series of hydrologically abandoned
cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...
s which still hold river-deposited sediments. These caves were the subject of an effort to measure the rate of knickpoint migration along the river, as well as to approximate the
discharge
Discharge may refer to
Expel or let go
* Discharge, the act of firing a gun
* Discharge, or termination of employment, the end of an employee's duration with an employer
* Military discharge, the release of a member of the armed forces from ser ...
of the river over time. In
karst topography
Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...
, a river dropping in level influences more than just its
channel; as there is no longer water flowing at a certain level, caves and
water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
s will drop locally to the new level.
Evidence of large-scale base level drop
Large drainages into the oceans the world over can be seen to have continued over land which was once exposed, whether due to tectonic subsidence,
sea level rise
Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...
, or other factors.
Bathymetric imagery is available for much of the United States' western coast, and in particular the
ocean floor just offshore of rivers in the Pacific Northwest exhibit such underwater features.
In certain locations there are still knickpoints preserved in these drowned river channels and valleys. A study conducted within the
Mediterranean basin
In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and wa ...
focused on such features. Here, incision was caused by the
closing of the Mediterranean at the end of the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
. This sudden lack of ocean water influx allowed the basin to decrease in volume and increase in
salinity
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
, and as a result of the drop in surface level many of the rivers which flow still today into the Mediterranean began to incise.
Movement
As is observed for many major waterfalls, knickpoints migrate upstream due to bedrock erosion leaving in their wake deep channels and abandoned
floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s, which then become
terraces. Knickpoint retreat is easily demonstrated in some locations affected by
postglacial isostatic response and relative sea-level drop such as in
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
. In other areas, dating of exposed bedrock terraces is more consistent with spatially uniform incision and persistence of the knickzone at about the same location.
A river, having gained or lost
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potenti ...
with its changed
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
, will then proceed to work the knickpoints out of its system by either erosion (in the case of waterfalls; gained potential energy) or deposition (in the case of lakes; lost potential energy) in order for the river to reattain its smooth concave graded profile.
The rates of knickpoint migration, in the case of waterfalls, generally range between 1mm and 10 cm per year, with some exceptional values.
Mathematical modeling
Knickpoint propagation is typically modelled with the semi empirical
stream power law where the
drainage basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
size is used as a proxy for
discharge
Discharge may refer to
Expel or let go
* Discharge, the act of firing a gun
* Discharge, or termination of employment, the end of an employee's duration with an employer
* Military discharge, the release of a member of the armed forces from ser ...
, which in turn has a
positive nonlinear correlation to the rate of knickpoint migration.
Both analytical
and numerical solutions
have been proposed to solve the
stream power law.
Automated extraction in GIS
Knickpoints and knickzones can be semiautomatically extracted from
Digital Elevation Models in
Geographic Information System
A geographic information system (GIS) is a type of database containing geographic data (that is, descriptions of phenomena for which location is relevant), combined with software tools for managing, analyzing, and visualizing those data. In a ...
software (i.e.
ArcGIS
ArcGIS is a family of client, server and online geographic information system (GIS) software developed and maintained by Esri. ArcGIS was first released in 1999 and originally was released as ARC/INFO, a command line based GIS system for manipul ...
). The problem with most of existing methods is that they are frequently subjective and require time-consuming data processing. A solution for these problems is a tool designed for ArcGIS, called Knickzone Extraction Tool (KET) which vastly automates the extraction process.
See also
*
Hydrology
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...
References
{{river morphology
Geomorphology
Fluvial landforms
Erosion landforms