 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Knickpoint" – news ·newspapers ·books ·scholar ·JSTOR(December 2015) (Learn how and when to remove this message) |
Ingeomorphology, aknickpoint ornickpoint (knick adopted fromGermanKnick, a sharp turn) is part of ariver orchannel where there is a sharp change inchannel bedslope, such as awaterfall orlake. Knickpoints reflect different conditions and processes on the river, often caused by previouserosion due toglaciation or variance inlithology. In thecycle of erosion model, knickpoints advance one cycle upstream, or inland, replacing an older cycle.[1] A knickpoint that occurs at the head (furthest upstream extent) of a channel is called aheadcut.[2]Headcuts resulting inheadward erosion are hallmarks of unstable expanding drainage features such as actively eroding gullies.[3]
Knickpoints also occur on other planetary bodies that previously had or currently have surface liquids, namelyMars[4] andTitan.[5] On Mars, the knickpoints have a common elevation that suggest a common sea level for aformer Martian ocean.[4] On Titan, mountain valleys adjacent to the present-dayhydrocarbon seas show evidence of knickpoints and recentsea-level change.[5]
Knickpoints are formed by the influence of tectonics, climate history, and/or lithology.[6] 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 aknickzone. Glaciation resulting in ahanging 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.[7]
Knickpoints include both waterfalls and some lakes. These features are common in rivers with a sufficientslope, i.e. enough change inelevation above sea level over their length to encouragedegradation.
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, on theZambezi River, is a spectacular example of this. The gorges visible by satellite imagery illustrate theerosional processes behind the formation of the falls. Here, much of the surface rock is a massivebasaltsill, with large cracks filled with easily weatheredsandstone made visible by the Zambezi's course across the land. Thegorges downstream of the falls through which it flows were eroded over time by the action of the water.
ThroughoutNew Zealand, tectonic uplift and faulting are actively contributing to knickpoint initiation and recession. TheWaipoua River system, on the North island, has been studied and used to create mathematical models to predict the behavior of knickpoints.[8] The study showed a direct correlation between upstreamdrainage area and rate of migration, producing modeled data closely approximating the collected data. The Waipoua River system incises throughsediments, for the most part, as opposed tobedrock.
Sharp changes in slope are common in rivers flowing through the heavily carved landscape left behind whenglaciers retreat.Glacial valleys, as well asisostatic rebound resulting from the removal of the mass of glacial ice contribute to this.
Niagara Falls, 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.[9] The falls, particularlyHorseshoe Falls, are dramatically steep and caused byglaciation. TheGreat Lakes themselves lie in the depressions left behind by glaciers, as the crust is stillrebounding.
Bridalveil Fall, inYosemite Valley, California, pours over the lip of ahanging valley.
.jpg)
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.
Dry Falls, a 3.5 mi long precipice in centralWashington, is an example of an ancient knickpoint. Geologic evidence strongly suggests that the water which formed this feature flowed over theChanneled Scablands,bursting from theglacial lakeMissoula during an event known as theMissoula Floods and into theColumbia River Gorge.
On theUpper Cumberland River,Tennessee, there exist a series of hydrologically abandonedcaves 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 thedischarge of the river over time.[10] Inkarst topography, a river dropping in level influences more than just itschannel; as there is no longer water flowing at a certain level, caves andwater tables will drop locally to the new level.
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, or other factors.Bathymetric imagery is available for much of the United States' western coast, and in particular theocean 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 theMediterranean basin[7] focused on such features. Here, incision was caused by theclosing of the Mediterranean at the end of theMiocene. This sudden lack of ocean water influx allowed the basin to decrease in volume and increase insalinity, and as a result of the drop in surface level many of the rivers which flow still today into the Mediterranean began to incise.[7]
As is observed for many major waterfalls, knickpoints migrate upstream due to bedrock erosion[11] leaving in their wake deep channels and abandonedfloodplains, which then becometerraces. Knickpoint retreat is easily demonstrated in some locations affected bypostglacial isostatic response and relative sea-level drop such as inScotland. 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 lostpotential energy with its changedslope, 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.[7]
Knickpoint propagation is typically modelled with the semi empiricalstream power law where thedrainage basin size is used as a proxy fordischarge, which in turn has apositive nonlinear correlation to the rate of knickpoint migration.Both analytical[12]and numerical solutions[13] have been proposed to solve thestream power law.
Knickpoints and knickzones can be semiautomatically extracted fromDigital Elevation Models inGeographic Information System software (i.e.ArcGIS). 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.[14]
| Large-scale features | |
|---|---|
| Alluvial rivers | |
| Bedrock river | |
| Bedforms | |
| Regional processes | |
| Mechanics | |
