Channel
A channel is a passageway, duct, or tube allowing the flow of liquids, gases, or signals. The term has numerous applications across various fields. In geography, it often refers to a natural or artificial waterway, such as the English Channel, a body of water separating southern England from northern France. Other common uses include: For detailed information on these and other meanings, see the relevant sections below.Geography
Natural waterways
A natural waterway channel is a body of water, either navigable or non-navigable, that separates land areas or directs the flow of water, often formed through natural geological processes and serving as vital connectors in global hydrology. These channels include straits, passages, and river courses that facilitate water movement between oceans, seas, or inland bodies, influencing navigation, migration, and ecological connectivity. Prominent examples of such channels include the English Channel, an arm of the Atlantic Ocean separating southern England from northern France, stretching approximately 560 kilometers in length and historically serving as a key route for human migration, trade, and military movements since prehistoric times. The Strait of Dover, the narrowest section of the English Channel at about 34 kilometers wide, connects the North Sea to the Atlantic and is renowned for its strong tidal currents that shape regional maritime activity. Another significant example is the Torres Strait, a shallow waterway approximately 150 kilometers wide between northeastern Australia and the island of New Guinea, which supports diverse indigenous cultures and acts as a barrier to deep ocean currents while allowing limited water exchange. Geologically, natural waterway channels typically form through long-term erosion processes driven by tides, ocean currents, and river flows, which carve deeper paths into softer sedimentary rock or glacial deposits over millennia. For instance, repeated tidal scouring and sediment transport in coastal areas deepen these channels, while post-glacial rebound and sea-level changes further refine their shapes, as seen in many temperate-zone straits. These channels play a crucial role in marine ecosystems by creating biodiversity hotspots where nutrient-rich waters mix, supporting diverse species from phytoplankton to migratory fish and seabirds. They also influence global climate patterns; the English Channel, for example, extends the warm Gulf Stream's effects, moderating temperatures in northwestern Europe and preventing harsher winters. In Australia, the Bass Strait—separating Tasmania from the mainland and spanning about 240 kilometers—harbors unique marine habitats but is notorious for its turbulent waters, contributing to over 200 recorded shipwrecks due to fierce winds and swells. Human modifications, such as dredging for shipping, occasionally alter these natural formations to enhance navigability.Regional landforms
Regional landforms associated with channels primarily consist of dry or seasonal riverbeds, known as wadis in North Africa and the Middle East or arroyos in the southwestern United States and northern Mexico, which are intermittent watercourses that remain dry except during heavy rainfall or flash floods.[1][2] These features often occur alongside alluvial plains, broad flat areas formed by the deposition of sediments carried by rivers in arid or semi-arid environments, creating expansive, shallow pathways that facilitate episodic water flow.[3] The formation of these landform channels results from a combination of erosion and sedimentation processes in arid and semi-arid zones, where infrequent but intense rainfall leads to flash flooding that incises valleys and deposits fine sediments, resulting in wide, shallow channels prone to braiding.[4][5] Erosion dominates during sudden storms, carving out the dry beds, while sedimentation occurs as floodwaters slow and drop silt across floodplains, stabilizing the landscape until the next event.[6] In Australia, the Channel Country in southwestern Queensland exemplifies these features, encompassing a vast inland area of approximately 200,000 square kilometers where ephemeral rivers like the Cooper Creek and Diamantina River form intricate braided channels during rare floods, spreading water across expansive floodplains before evaporating or infiltrating the soil.[7][8] These braided systems, which intertwine into hundreds of rivulets, support unique desert ecosystems but remain mostly dry, highlighting the region's low annual rainfall of under 250 millimeters.[9] North American examples include the Rio Grande's channel along the United States-Mexico border, stretching about 3,050 kilometers from Colorado to the Gulf of Mexico, where it serves dual roles as an international boundary and a critical irrigation source for arid farmlands in Texas and northern Mexico.[10][11] In the Lower Rio Grande Valley, the channel's waters sustain agriculture across over 300,000 hectares through diversion systems, though overuse and drought have led to intermittent flows in its lower reaches.[12] Human impacts on these landforms are evident in regions like the Mississippi River delta in Louisiana, where channels have been modified for agricultural use and flood control; levees and diversions channel floodwaters to protect farmlands covering millions of acres, while irrigation draws from the delta's distributary channels to support rice and sugarcane production amid subsidence and sea-level rise.[13][14] These interventions, including sediment diversions proposed for restoration, aim to mimic natural sedimentation to counteract land loss, which has exceeded 4,900 square kilometers since the early 20th century.[15] In Europe, the Po Valley in northern Italy features historical channel systems developed for irrigation since the Renaissance in the 15th century, where ancient and medieval canals derived from the Po River and its tributaries transformed marshy lowlands into fertile plains supporting intensive agriculture.[16] These networks, including the Naviglio Grande dating to the 12th century, distribute water across over 1.5 million hectares, exemplifying early engineered adaptations of natural river channels for crop production in a semi-arid Mediterranean climate.[17][18]Infrastructure and regions
Infrastructural channels refer to engineered waterways, tunnels, and transport routes designed to facilitate navigation, trade, or connectivity across geographical barriers. These structures often modify or parallel natural features to enable efficient human transport, contrasting with purely natural formations by incorporating locks, rail systems, or paved roadways.[19] A prominent example is the Channel Tunnel, also known as Eurotunnel, a 50-kilometer rail link beneath the English Channel that connects Folkestone in the United Kingdom to Coquelles in France. Completed in 1994 after six years of construction involving over 13,000 workers, it serves as the longest undersea tunnel in the world, with 37.9 kilometers lying underwater, and primarily transports passengers via high-speed Eurostar trains and vehicle shuttles.[20][21][19] Another key infrastructural channel is the Panama Canal, an 80-kilometer artificial waterway in Panama that connects the Atlantic Ocean via the Caribbean Sea to the Pacific Ocean. Built between 1904 and 1914 under U.S. supervision following earlier French attempts, it revolutionized global shipping by eliminating the need to navigate around South America, with its three sets of locks raising vessels up to 26 meters above sea level to traverse the continental divide.[22][23] The Channel Islands, an archipelago comprising five principal islands and numerous smaller islets in the English Channel approximately 20-30 kilometers west of Normandy, France, represent a regionally defined group of infrastructural and political channels as British Crown dependencies. Comprising primarily Jersey, Guernsey, Alderney, and Sark, these self-governing territories maintain close ties to the UK while handling their own affairs, and they feature enhanced ferry and air links as vital transport channels to the mainland. During World War II, from June 1940 to May 1945, the islands endured the only German occupation of British soil, involving fortifications, forced labor, and eventual liberation by Allied forces, which shaped their modern infrastructure resilience.[24] Engineering challenges in these infrastructural channels often center on underwater tunneling and lock systems to manage elevation and water flow. For the Channel Tunnel, key difficulties included navigating unstable chalk marl strata prone to water ingress, employing massive tunnel boring machines to excavate three parallel tunnels (two for rail and one service) while maintaining pressure balance to prevent flooding, and coordinating a precise breakthrough in 1990 after drilling from both shores. In the Panama Canal, constructing the locks involved pouring over 2 million cubic meters of concrete to form watertight chambers, overcoming landslides in the Culebra Cut excavation and tropical diseases that claimed thousands of lives, with the system using gravity-fed water from Gatun Lake to lift ships efficiently without pumps.[19][25][26] In Tasmania, Australia, the Channel Highway (designated B68) exemplifies a scenic coastal transport route spanning 98 kilometers from Hobart to Huonville, paralleling the D'Entrecasteaux Channel with winding paths through eucalyptus forests and beaches, offering views of modified estuarine channels while serving as a key link for local agriculture and tourism.[27][28]Media and entertainment
Traditional broadcasting
In traditional broadcasting, a channel denotes a designated frequency band allocated by regulatory authorities for the transmission of television or radio signals over the airwaves. For television in the United States, each channel spans 6 MHz of bandwidth, encompassing video and audio carriers within very high frequency (VHF) bands (54-216 MHz, channels 2-13) or ultra high frequency (UHF) bands (470-806 MHz, channels 14-69). For radio, AM broadcast channels occupy 10 kHz bands in the medium frequency range (535-1705 kHz), while FM channels use 200 kHz bands in the VHF range (88-108 MHz). The concept of broadcast channels emerged in the 1930s amid experimental television transmissions, with the Federal Communications Commission (FCC) allocating initial VHF channels for non-commercial use starting in 1937. Regular high-definition television service began internationally with the BBC's launch on November 2, 1936, from Alexandra Palace in London, marking the world's first scheduled public broadcasts on a dedicated channel. In the United States, the FCC formalized the NTSC standard in 1953, establishing a framework of 82 channels (12 VHF and 70 UHF) to standardize analog transmission and accommodate growing demand, though UHF channels were not widely adopted until the 1950s due to technical limitations in early receivers. This system relied on amplitude modulation for video signals, enabling black-and-white broadcasts before the integration of color. Key milestones include the BBC One channel, which initiated regular programming in 1936 and evolved into a cornerstone of public broadcasting. In the United States, NBC pioneered compatible color television under the NTSC standard, with the FCC approving the system on December 17, 1953, allowing the first nationwide color broadcasts, such as the 1954 Tournament of Roses Parade. Technically, television channels in the U.S. allocate 4.2 MHz for luminance and color video, 0.5 MHz for audio via frequency modulation, and guard bands to prevent interference. Radio broadcasting employs amplitude modulation (AM) for long-distance propagation in the AM band, where signal strength varies to encode audio, and frequency modulation (FM) in the FM band for higher fidelity, where the carrier frequency deviates proportionally to the audio signal, reducing noise susceptibility. Cable television expanded traditional broadcasting by delivering multiple channels via coaxial wires, bypassing over-the-air limitations. The 1980s marked significant growth, driven by deregulation under the Cable Communications Policy Act of 1984, with average channels per system rising from about 20 in 1980 to 33 by 1989 as providers like those offering HBO and emerging networks increased capacity. Today, many U.S. cable providers offer over 100 channels, reflecting ongoing infrastructure upgrades. This analog era laid the groundwork for broadcasting, though it has largely transitioned to digital formats for enhanced efficiency.Digital and online platforms
In the context of digital and online platforms, a channel refers to a user-specific or algorithmic feed that delivers personalized content through internet-based services, enabling creators to distribute videos, streams, or messages to audiences via online tools and communication vehicles.[29][30] These channels emerged prominently after 2005 with the rise of Web 2.0, which emphasized user-generated content, interactive participation, and collaborative platforms like early social networks and video-sharing sites.[31] This evolution incorporated recommendation algorithms to curate feeds based on user behavior, watch history, and engagement signals, fostering seamless integration with broader social media ecosystems. Building briefly on roots in traditional broadcasting, digital channels shifted from fixed schedules to on-demand, interactive experiences.[32] A prominent example is the YouTube channel, a user account where individuals or organizations upload and manage video content, serving over 2.7 billion monthly active users as of mid-2025 and allowing monetization primarily through ad revenue shared via Google's AdSense program.[33][34] Another key platform is Twitch, where channels facilitate live streaming focused on gaming and interactive entertainment; launched in June 2011 as a spin-off from Justin.tv, it has grown into a central hub for real-time viewer-creator engagement. Additional examples include Discord channels, which function as dedicated text, voice, or video rooms within customizable servers, gaining popularity for community building since the platform's public release in 2015.[35] On TikTok, channels manifest as personalized algorithmic feeds, particularly the #ForYou page, which recommends short-form videos based on user interactions like likes, comments, and completion rates to maximize relevance and retention.[36][37] Common features across these platforms include subscription models for ad-free access and exclusive perks, such as YouTube Premium, which offers offline downloads, background play, and original content for $13.99 per month for individuals.[38] Creators benefit from integrated analytics tools to track viewer metrics and optimize content, while robust content moderation policies enforce community guidelines against harmful material, including violent or misleading content, through human reviewers and AI detection.[39][40] Social media integrations further enhance channels, as seen in Instagram's broadcast channels, which allow one-to-many updates shared via stories to foster direct audience connections.[41]Other artistic uses
In the arts, channels serve as mediums or pathways through which creative expression and narrative flow are conveyed, enabling artists to direct emotional, conceptual, or sensory elements toward audiences. This concept extends beyond technical transmission to encompass the structured routes for ideas and experiences in various disciplines, fostering immersion and interpretation.[42] In music, multi-channel audio represents a key evolution from monaural to spatial soundscapes, with stereo systems emerging in the 1950s and surround sound formats like quadraphonic audio experimented with in the early 1970s to create immersive listening environments.[43] Dolby Laboratories advanced this during the 1970s by developing Dolby Stereo for cinema, which encoded four channels (left, center, right, and surround) into a two-track format, laying the groundwork for later home systems like 5.1-channel surround sound introduced in the 1980s and 1990s.[44] In film editing, montage techniques function as narrative channels by sequencing disparate shots to condense time, build rhythm, or parallel actions across multiple storylines, enhancing psychological depth and viewer engagement.[45] Similarly, in literature, writers channel characters through immersive psychological processes, simulating their mental states to evoke empathy and self-reflection in readers, as supported by studies on narrative transportation where identification with fictional figures alters personal attitudes.[46] Performance art has incorporated "channeling" as an artistic device, where performers adopt altered states to embody or convey spiritual entities, blending creative dissociation with expressive ritual in a non-literal, interpretive context.[47] This approach draws from 19th-century spiritualist influences, as seen in works by mediums like Georgiana Houghton, who used drawing to channel communications from spirits, pioneering abstract forms of visionary art.[48] Video art installations often employ multi-channel formats to expand spatial and temporal narratives, with artist Bill Viola pioneering such works from the 1980s onward; his pieces, like The Crossing (1996), use dual projections and synchronized audio to immerse viewers in themes of human emotion and transcendence.[49] Viola's multi-channel videos, developed through collaborations with institutions like MoMA, integrate high-definition technology to create enveloping environments that challenge linear storytelling.[50]Science and technology
Communications systems
In telecommunications, a communication channel is defined as a pathway or medium that conveys information signals from a sender (transmitter) to a receiver, potentially subject to noise or distortion during transmission.[51] This concept forms the foundation of signal transmission in engineered systems, where the channel acts as the physical or logical conduit for data, such as electrical signals, electromagnetic waves, or optical pulses. The theoretical framework for communication channels was established by Claude Shannon in his seminal 1948 paper, "A Mathematical Theory of Communication," which introduced the noisy channel coding theorem and the concept of channel capacity.[52] Building on earlier work by Ralph Hartley and Harry Nyquist on bandwidth and signaling rates, Shannon modeled channels as either discrete (handling finite symbol sets) or continuous (handling analog signals over time). For a continuous channel with additive white Gaussian noise, the maximum reliable transmission rate, or channel capacity C (in bits per second), is given by the Shannon-Hartley theorem: C = B \log_2 \left(1 + \frac{S}{N}\right) Here, B represents the channel bandwidth in hertz, S is the signal power, and N is the noise power, yielding the signal-to-noise ratio (SNR) S/N. The derivation involves maximizing the mutual information between input and output signals, using entropy measures: the capacity equals the difference in output entropy H(y) and conditional entropy H(y|x), optimized over input distributions. For Gaussian noise, this simplifies to the formula above, assuming the input signal is also Gaussian to achieve the bound. This model demonstrated that reliable communication is possible up to capacity by using error-correcting codes, fundamentally influencing modern digital communications.[52] Communication channels are broadly classified into wired and wireless types based on the transmission medium. Wired channels, such as coaxial cables, use guided media like conductive wires or optical fibers to propagate signals with low interference, commonly employed in cable television and broadband internet. Wireless channels rely on unguided electromagnetic propagation, such as radio frequency (RF) waves; for instance, 5G networks utilize millimeter-wave (mmWave) bands in the 24–52.6 GHz range (Frequency Range 2) to enable high-speed, low-latency connections in dense urban areas, supporting data rates exceeding 1 Gbps under ideal conditions.[53] Representative examples include the Public Switched Telephone Network (PSTN) channels, which allocate a nominal 3 kHz bandwidth (typically 300–3400 Hz) for voice signals to ensure intelligible speech transmission over twisted-pair copper lines.[54] In satellite communications, channels are implemented via transponders that relay RF signals in frequency bands like C-band (4–8 GHz) or Ku-band (12–18 GHz), facilitating global broadcasting and remote connectivity by amplifying and frequency-shifting uplink signals for downlink.[55] To efficiently share limited channel resources among multiple users, multiplexing techniques divide the channel into sub-channels. Frequency Division Multiple Access (FDMA) allocates distinct frequency bands to users, as in analog cellular systems; Time Division Multiple Access (TDMA) assigns time slots within a shared frequency, enabling digital voice in GSM networks; and Code Division Multiple Access (CDMA) uses unique orthogonal codes to allow simultaneous transmission over the same band, improving capacity in 3G systems like CDMA2000.[56] These methods enhance spectral efficiency while adhering to channel capacity limits.Biological and medical concepts
Biological channels are integral membrane proteins that form selective pores, allowing the passive transport of ions, water, or other small molecules across cell membranes in response to specific stimuli.[57] These structures maintain cellular homeostasis, enable signaling, and regulate physiological processes by controlling the flux of charged or polar solutes down their electrochemical gradients.[58] Ion channels represent a major class of biological channels, categorized by their gating mechanisms. Voltage-gated ion channels, such as sodium channels in neurons, activate upon membrane depolarization, permitting rapid sodium influx that initiates the rising phase of action potentials essential for nerve impulse propagation.[59] Ligand-gated ion channels, exemplified by nicotinic acetylcholine receptors at neuromuscular junctions, open when bound to neurotransmitters like acetylcholine, facilitating cation flow that triggers synaptic transmission and muscle activation.[60] The electrochemical driving force for ion movement through these channels is quantified by the Nernst equation, which calculates the equilibrium potential E for a given ion: E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}} \right) where R is the gas constant, T is the absolute temperature, z is the ion's valence, F is Faraday's constant, and [\text{ion}] denotes extracellular and intracellular concentrations, respectively; this equation underpins the resting and action potentials in neuronal signaling.[61] Beyond ion channels, aquaporins serve as specialized water channels, forming hourglass-shaped pores that selectively permit water molecules to traverse membranes at rates up to 3 billion per second while excluding protons and other ions.[62] Voltage-gated calcium channels play a pivotal role in excitation-contraction coupling in muscle cells, where their opening allows calcium entry that binds to troponin, initiating actin-myosin interactions for contraction.[63] Dysfunction in biological channels leads to channelopathies, a class of disorders arising from genetic mutations that impair channel function. Cystic fibrosis, for example, results from defects in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel critical for epithelial ion and fluid transport; the responsible gene was cloned and characterized in 1989, revealing over 2,000 associated mutations, with the most common being a deletion of phenylalanine at position 508.[64]Computing and engineering
In computing and engineering, a channel serves as a data pathway or virtual conduit that facilitates the transfer of information between system components, enabling efficient communication in both hardware and software contexts. This concept encompasses physical or logical routes designed to handle data flow, often optimizing for speed, concurrency, or bandwidth in processing systems.[65] A prominent hardware example is Channel I/O, pioneered by IBM in the late 1950s and widely adopted in mainframes during the 1960s, such as the System/360 series, to enable high-speed, asynchronous communication between the central processing unit and peripherals like tape drives or printers. This architecture offloaded I/O operations from the CPU, improving overall system throughput by allowing direct memory access through dedicated control units. Similarly, memory channels in modern architectures, such as dual-channel RAM configurations, pair memory modules across two pathways to effectively double the data bandwidth compared to single-channel setups, enhancing performance in memory-intensive tasks like graphics rendering or multitasking.[66][67] In semiconductor engineering, transistor channels form the core of devices like the n-channel metal-oxide-semiconductor field-effect transistor (MOSFET), where the channel is an inverted region beneath the gate that conducts electrons from source to drain when biased. The drain current in the linear operating region is approximated by the equation I_d = \mu C_{ox} \frac{W}{L} (V_{gs} - V_{th}) V_{ds}, where \mu is the electron mobility, C_{ox} the gate oxide capacitance per unit area, W/L the aspect ratio of the channel, V_{gs} the gate-source voltage, V_{th} the threshold voltage, and V_{ds} the drain-source voltage; this model, rooted in semiconductor physics, underpins the scaling and efficiency of integrated circuits.[68] On the software side, channels appear in concurrent programming paradigms, notably in the Go programming language, released by Google in 2009, where they act as typed conduits for safe communication and synchronization between lightweight threads called goroutines. A basic channel declaration uses syntax likech := make(chan int), allowing producers to send values and consumers to receive them without shared memory risks, promoting scalable concurrency in applications like web servers. In cloud-native environments, Kubernetes— an open-source container orchestration platform launched in 2014—leverages similar channel mechanisms in its Go-based codebase for internal coordination, while pod-to-pod communication relies on cluster network pathways that abstract IP addressing for reliable inter-container data exchange. Additionally, in digital image processing, channels decompose color images into independent components, such as the red, green, and blue (RGB) channels, each representing an 8-bit grayscale intensity map that combines additively to form the full-color pixel values essential for tasks like filtering or compression.[69][70][71]