In physical geography, a channel is a type of landform consisting of the outline of a path of relatively shallow and narrow body of fluid specifically its bed and banks, most commonly the confine of a river, river delta or strait.
Natural channels are formed by fluvial process and are found across the Earth. These are mostly formed by flowing water from the hydrological cycle, though can also be formed by other fluids such as flowing lava. Channels also describe the deeper course through a reef, sand bar, bay, or any shallow body of water. These can be either natural or human-made.
Extraterrestrial natural channels are found elsewhere in the Solar System, the longest and widest of which are the outflow channels on Mars and the channels of Venus many of which are tens of kilometres wide (the network of channels flowing from Argyre Planitia on Mars for example is 8000 km in length and the Baltis Vallis Venus is 7000 km compared to the 6,650 km Nile, the largest active channel on Earth). The exact formation of these large ancient channels is unknown although it is theorised that those on Mars may have been formed due to catastrophic flooding and on Venus by lava flow. In planetary science the term "rille" is sometimes used for similar formations found on The Moon and Mercury that are of inconclusive origin. Channels have also been recently discovered on Titan. The Saturnian moon has the only known permanently active channels in the Solar System other than Earth, the largest known of which is 400 km in length. These are believed to be formed from flowing hydrocarbons form the hypothesized methanological cycle.
Nautical channels 
It is especially used as a Nautical term to mean the dredged and marked (See: Buoy) lane of safe travel which a cognizant governmental entity guarantees to have a minimum depth across its specified minimum width to all vessels transiting a body of water. The term not only includes the deep-dredged ship-navigable parts of an estuary or river leading to port facilities, but also to lesser channels accessing boat port-facilities such as marinas. When dredged channels traverse bay mud or sandy bottoms, repeated dredging is often necessary because of the unstable subsequent movement of benthic soils.
Responsibility for monitoring navigability conditions of navigation 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 affect navigability. In the U.S., navigation channels are monitored and maintained by the United 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 the Rivers and Harbors Act of 1899 and modified under acts of 1913, 1935, and 1938, which are contained in chapter 33 of the US Code, "Navigation and Navigable Waters." For example, the USACE developed the Intracoastal Waterway, and has the Mississippi Valley Division responsible for the Mississippi River from the Gulf to Cairo, Illinois, the North Atlantic Division for New York Harbor and Port of Boston, and the South Pacific Division for Port of Los Angeles and Port of Long Beach. Waterways policing as well as some emergency spill response falls under United States Coast Guard jurisdiction, including inland channels serving ports like Saint Louis hundreds of miles from any coast. The various state or local governments maintain lesser channels, for example former Erie Canal.
In a larger nautical context, as a geographical place name, the term channel is another word for strait, which is defined as a relatively narrow body of water that connects two larger bodies of water. In this nautical context, the terms strait, channel, sound, and passage are synonymous and usually interchangeable. For example, in an archipelago, the water between islands is typically called a channel or passage. The English Channel is the strait between England and France.
See also 
- Hydrology transport model
- Ship canal
- Rivers and Harbors Act of 1899
- Rivers and Harbors Acts
- Surge channel
- Channel pattern
- O'Neill, Ian. Titan's 'Nile River' Discovered Dec 12, 2012
- pg 71. Large Rivers: Geomorphology and Management. Avijit Gupta. John Wiley & Sons, 2007
- History of the Waterways of the Atlantic Coast of the United States, USACE, January 1983