Palaeochannel

In the Earth sciences, a palaeochannel, also spelled paleochannel, is a significant length of a river or stream channel which no longer conveys fluvial discharge as part of an active fluvial system. The term palaeochannel is derived from the combination of two words, palaeo or old, and channel; i.e., a palaeochannel is an old channel. Palaeochannels may be preserved either as abandoned surface channels on the surface of river floodplains and terraces or infilled and partially or fully buried by younger sediments. The fill of a palaeochannel and its enclosing sedimentary deposits may consist of unconsolidated, semi-consolidated, or well-cemented sedimentary strata depending on the action of tectonics and diagenesis during their geologic history after deposition. The abandonment of an active fluvial channel and the resulting formation of a palaeochannel can be the result of tectonic processes, geomorphologic processes, anthropogenic activities, climatic changes, or a variable and interrelated combination of these factors.

Formation The avulsion of an active river or stream is the most common fluvial process resulting in the formation of palaeochannels. It is the process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. An avulsion can be either a full avulsion, in which all of the discharge is transferred out of the parent channel to a new one, or partial avulsion, in which only a portion of the discharge is transferred to a new one. Only the full avulsion results the formation of a palaeochannel. Partial avulsions result in the formation of anastomosing channels when the divided active channels rejoin downstream and distributary channels when the divided active channels do not rejoin downstream. At least three broadly different types of avulsions, (a) avulsion by annexation; (b) avulsion by incision; and (c) avulsion by progradation, are recognized. First, an avulsion by annexation is an avulsion in which an existing active channel is appropriated or if an existing abandoned channel is reoccupied. Second, an avulsion by incision is an avulsion in which a new channel is created by the scouring into the floodplain surface as a direct result of the avulsion. Finally, an avulsion by progradation is an avulsion that results in the formation of an extensive deposition and multi-channeled distributive network. Of these types of avulsions only the avulsion by incision results in the complete abandonment and preservation of a fluvial channel as a palaeochannel. The exact environmental conditions that favour incisional avulsions remain unsettled. However, it is generally agreed that they are promoted by a) rapid aggradation of the main channel and floodplain; b) wide unobstructed floodplain and down-valley drainage; and c) frequently recurring floods of high magnitude. In many floodplains, these conditions and frequent avulsions are correlated with superelevated alluvial ridges and river stages. The event or factor that can trigger a specific avulsion may be either external or internal to a river system and quite varied. Factors external to a river system that might cause an avulsion include fault activity, sea-level rise, or an increase in flood peak discharge. Factors internal to a river system that might cause an avulsion include sediment influx, breakout along animal pathways, and blockage by ice jams, plant growth, log jams, and beaver dams.

Recognition A variety of techniques have been used to recognize and map palaeochannels. At first, surficial data from aerial photography, soils maps, topographic maps, archaeological surveys and excavations, and field observations were integrated with subsurface data from geological and engineering borings and cores to recognize and map palaeochannels. As the importance of coarse-grained fluvial deposits associated with palaeochannels as sources of groundwater and favoured conveyance of subsurface water became appreciated, geophysical techniques sensing the physical properties of underlying ground and bedrock and groundwater and other fluids contained within them became more important and widely used. For example, palaeochannels can be identified using airborne electromagnetic surveys, as the coarse-grained sediments are more electrically resistive than surrounding materials. Also, lidar, more sophisticated remote sensing techniques, digital analysis, including computer modeling, of data were added to the various techniques used to detect and map palaeochannels.

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