The planform of splitting and rejoining channels that enclose variously shaped and sized islands of remnant floodplain or non-alluvial landforms.
A type of channel pattern
Anastomosing channels occur on a wide range of spatial scales. Although the width of islands enclosed by anastomosing channels tends to be multiple times channel width, there is no fundamental minimum size. Considerable variability in the size and shape of islands within one anastomosing system seems a typical feature.
Anastomosing patterns of (1) fluvial/alluvial channels and (2) lava channels.
The controls of anastomosing patterns are still under investigation. In case of lava channels, anastomosing patterns indicate highly fluid flow (Mouginis-Mark and Christensen 2005). Formation of alluvial anastomosing patterns on Earth has been associated with occurrence of trees stabilizing the river banks (Davies and Gibling 2011). Indeed, anastomosing rivers commonly have erosion-resistant, but not necessarily tree-lined, banks hampering the scour of fluvial channels of sufficient capacity to convey the water and sediments imposed from upstream. High bedload supply in combination with restricted ability to enlarge the conveyance capacity of individual channels by bank scour seems a principal driving mechanism for the formation of new branches in anastomosing systems (Makaske et al. 2009). The process of channel formation by diversion of flow from an existing channel onto the floodplain is called avulsion. During the process of avulsion, or a different formative event, an anastomosing pattern can also result from the contemporaneous scour of multiple channels enclosing intact parts of the original floodplain.
The basic components of an anastomosing pattern are individual channel reaches (branches, anabranches), which are connected to each other through bifurcations and confluences. Alluvial and lava channels are often lined with (natural) levees. The neighboring lower parts of the plain are commonly termed flood basins in alluvial anastomosing systems. Crevasse channels, cutting through natural levees, and associated crevasse splays are also common elements in alluvial anastomosing river systems (Smith 1983; Makaske et al. 2002).
Anastomosing patterns of lava channels on Earth are associated low-viscosity basaltic lava flows over an irregular solidified surface. Alluvial anastomosing channels usually consist of sand-bed or gravel-bed channels that are inset into cohesive clayey or peaty floodplains. Another category of anastomosing patterns is scoured into bedrock.
Anastomosing lava channels have been described from many locations on Earth, Venus, and Mars. Well-studied alluvial anastomosing rivers include the upper Columbia River (British Columbia, Canada) (Fig. 1), Cooper Creek (Central Australia), and the Inland Niger Delta (Mali). More modern and ancient examples are described in Makaske (2001).
Lava channels of the Mauna Loa and Kilauea (Hawaii, USA); the alluvial anastomosing upper Columbia River (British Columbia, Canada); the bedrock channels of the Channeled Scabland (Washington, USA).
On Earth, fluvial anastomosing patterns occur in all climatic zones, most frequently in low-gradient sedimentary basins. Anastomosing lava channels on Earth are associated with eruptions of highly fluid lava of basaltic composition, which are strongly (but not exclusively) associated with spreading zones and hot spots.
Regional Variations on Earth
In arid and semiarid climatic zones on Earth, alluvial anastomosing river systems seem different from examples in humid climatic zones in composition and process.
History of Investigation
After redefinition of the anastomosing pattern by Schumm (1968), the evolution and geological implications of alluvial anastomosing channels were investigated by Smith (1983) for rivers in the temperate-humid climatic zone. Arid-zone anastomosing rivers, with different morphology and processes, were reported by Rust (1981) and Nanson et al. (1986). Makaske (2001) carried out a global inventory of alluvial anastomosing rivers and investigated universal controls of this channel pattern. Since then, many more examples of modern anastomosing rivers have been reported from various climatic zones on Earth. Investigation of extraterrestrial anastomosing patterns, especially on Mars, started in the 1970s (e.g., Baker and Kochel 1979) and has yielded many new examples in the 1990s and 2000s with the strongly increasing availability of high-resolution images (Kargel et al. 1994; Leverington 2004; Miyamoto et al. 2004; Mouginis-Mark and Christensen 2005).
Origin of Term
The history of the geomorphological usage of the term “anastomosing” was reviewed by Carling et al. (2014). The term stems from the Greek word anastomosis, which strictly means “opening.” In medical sciences, the word anastomosis is applied to indicate the lateral connection between two parallel hollow organs, like veins. In geomorphology, “braiding” and “anastomosing” have long been used as synonyms to describe the pattern of river channels dividing around bars (braided pattern; e.g., Leopold and Wolman 1957). Schumm (1968) was probably the first to apply anastomosing pattern to the phenomenon of channels dividing around large-scale, relatively stable, islands.
- Leopold LB, Wolman MG (1957) River channel patterns: braided, meandering and straight. U S Geol Surv Prof Pap 282B:39–85Google Scholar
- Miyamoto HJ, Dohm M, Beyer RA, Baker VR (2004) Fluid dynamical implications of anastomosing slope streaks on Mars. J Geophys Res 109, E06008. doi:10.1029/2003JE002234Google Scholar
- Mouginis-Mark PJ, Christensen PR (2005) New observations of volcanic features on Mars from the THEMIS instrument. J Geophys Res 110, E08007. doi:10.1029/2005JE002421Google Scholar
- Rust BR (1981) Sedimentation in an arid-zone anastomosing fluvial system: Cooper’s Creek, Central Australia. J Sediment Petrol 51:745–755Google Scholar
- Smith DG (1983) Anastomosed fluvial deposits: modern examples from Western Canada. In: Collinson J, Lewin J (eds) Modern and ancient fluvial systems. Special Publication of the International Association of Sedimentologists 6, Blackwell, Oxford, pp 55–168Google Scholar