River channels

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, most commonly the confine of a river, river delta or strait. The word is cognate to canal, and sometimes shows in this form, e.g. the Hood Canal. Most examples of this are fjords in the Pacific Northwest of North America; a notable exception is the Casiquiare canal. All likely share borrowing from Spanish, Portuguese or French.

Channels can be either natural or human-made. A channel is typically outlined in terms of its bed and banks.

Channel initiation refers to the site on a mountain slope where water begins to flow between identifiable banks. This site is referred to as the channel head and it marks an important boundary between hillslope processes and fluvial processes. The channel head is the most upslope part of a channel network and is defined by flowing water between defined identifiable banks. 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. Channel heads are often associated with colluvium, hollows and landslides.

Overland flow is a primary factor in channel initiation where saturation overland flow deepens to increase shear stress and begin channel incision. 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. Vegetation slows infiltration rates during precipitation events and plant roots anchor soil on hillslopes.

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.Channel heads associated with hollows in steep terrain frequently migrate up and down hillslopes depending on sediment supply and precipitation.

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. One example is the Columbia River, a river located in the US states of Washington and Oregon, and which empties into the Pacific Ocean in Astoria, Oregon.

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