Inphysical geography andhydrology, achannel is alandform on which a relatively narrowbody of water is situated, such as ariver,river delta orstrait. Whilechannel typically refers to a natural formation, thecognate termcanal denotes a similar artificial structure.
Channels are important for the functionality ofports and other bodies of water used fornavigability for shipping. Naturally, channels will change their depth and capacity due toerosion anddeposition processes. Humans maintain navigable channels bydredging and other engineering processes.
By extension, the term also applies to fluids other than water, e.g.,lava channels.The term is also traditionally used to describe thewaterless surface features onVenus.
Channel initiation refers to the site on a mountain slope where water begins to flow between identifiable banks.[1] This site is referred to as the channel head and it marks an important boundary between hillslope processes and fluvial processes.[1] The channel head is the most upslope part of a channel network and is defined by flowing water between defined identifiable banks.[1] A channel head forms as overland flow and/or subsurface flow accumulate to a point where shear stress can overcome erosion resistance of the ground surface.[1] Channel heads are often associated withcolluvium,hollows andlandslides.[1]
Overland flow is a primary factor in channel initiation where saturation overland flow deepens to increase shear stress and begin channel incision.[1] Overland flows converge in topographical depressions where channel initiation begins. Soil composition, vegetation, precipitation, and topography dictate the amount and rate of overland flow. The composition of a soil determines how quickly saturation occurs and cohesive strength retards the entrainment of material from overland flows.[1] Vegetation slows infiltration rates during precipitation events and plant roots anchor soil on hillslopes.[1]
Subsurface flow destabilizes soil and resurfaces on hillslopes where channel heads are often formed. This often results in abrupt channel heads and landslides. Hollows form due to concentrated subsurface flows where concentrations of colluvium are in a constant flux.[1] Channel heads associated with hollows in steep terrain frequently migrate up and down hillslopes depending on sediment supply and precipitation.
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Natural channels are formed byfluvial process and are found across theEarth. These are mostly formed by flowingwater from thehydrological cycle, though can also be formed by other fluids such as flowinglava can formlava channels. Channels also describe the deeper course through areef,sand bar,bay, or any shallow body of water. An example of a river running through a sand bar is theColumbia Bar—the mouth of theColumbia River.
A stream channel is the physical confine of astream (river) consisting of abed andstream banks.Stream channels exist in a variety of geometries. Stream channel development is controlled by bothwater andsediment movement.[2] There is a difference between lowgradient streams (less than a couple of percent in gradient or slightly sloped) and high gradient streams (steeply sloped). A wide variety of streamchannel types can be distinguished (e.g.braided rivers, wandering rivers, single-threadsinuous rivers etc.). Duringfloods, water flow may exceed the capacity of the channel and flood waters will spill out of the channel and across thevalley bottom,floodplain ordrainage area.
Examples of rivers that are trapped in their channels:Grand Canyon andBlack Canyon of the Gunnison.
In a larger nautical context, as a geographical place name, the termchannel is another word forstrait, which is defined as a relatively narrow body of water that connects two larger bodies of water.[3] In thisnautical context, the termsstrait,channel, andpassage are synonymous and usually interchangeable. For example, in anarchipelago, the water betweenislands is typically called achannel orpassage.[citation needed] TheEnglish Channel is the strait between England and France.
The channel form is described in terms of geometry (plan, cross-sections, profile) enclosed by the materials of its bed and banks. This form is under influence of two major forces: water discharge and sediment supply. For erodible channels the mutual dependence of its parameters may be qualitatively described byLane's Principle (also known asLane's relationship):[4] the product of the sediment load and bed grain size is proportional to the product of discharge and channel slope.[5]
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A term "navigable channel" is used as anautical term to mean a lane for ship travel, frequently marked (cf.Buoy) and sometimesdredged. Thoresen distinguishes few categories of channels, from A (suitable for day and night navigation with guaranteedfairway depth) all the way to D with no navigational aids and only estimated depths provided to theshipmaster.[6] With regard to thedredging, channels can beunrestricted (wide enough to accommodate 10 to 15 widths of a largest ship used in this channel,semi-restricted with limited dredging in shallow waters, andfully restricted, where the entire channel is dredged.[6] The latter, entirely human-made, channel is frequently called acanal,[7] with thePanama Canal providing an example.[8]
The term not only includes thedeep-dredged ship-navigable parts of anestuary or river leading toport facilities, but also to lesser channels accessingboatport-facilities such asmarinas. When dredged channels traversebay mud or sandy bottoms, repeated dredging is often necessary because of the unstable subsequent movement of benthic soils.[9]
Responsibility for monitoring navigability conditions ofnavigation channels to various port facilities varies, and the actual maintenance work is frequently performed by a third party. Storms, sea-states, flooding, and seasonal sedimentation adversely affectnavigability. In the U.S., navigation channels are monitored and maintained by theUnited States Army Corps of Engineers (USACE), although dredging operations are often carried out by private contractors (under USACE supervision). USACE also monitors water quality and some remediation. This was first established under theRivers and Harbors Act of 1899 and modified under acts of 1913, 1935, and 1938. For example, the USACE developed theIntracoastal Waterway, and has theMississippi Valley Division responsible for theMississippi River from the Gulf toCairo, Illinois, theNorth Atlantic Division for New York Harbor andPort of Boston, and theSouth Pacific Division forPort of Los Angeles andPort of Long Beach. Waterways policing as well as some emergency spill response falls underUnited States Coast Guard jurisdiction, including inland channels servingports likeSaint Louis hundreds of miles from any coast. The various state or local governments maintain lesser channels, for example formerErie Canal.
