Anastomosing Pattern

  • Bart MakaskeEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-1-4614-9213-9_463-1

Definition

The planform of splitting and rejoining channels that enclose variously shaped and sized islands of remnant floodplain or non-alluvial landforms.

Category

A type of channel pattern

Description

On Earth, the term anastomosing is commonly applied to alluvial rivers that are composed of two or more interconnected channels that enclose flood basins (Makaske 2001; Figs. 1 and 2). Nevertheless, the term has also been applied to rivers that split and rejoin around bedrock islands that, obviously, lack flood basin morphology (Baker 2009). Alluvial rivers with a braided pattern roughly show a comparable planform of splitting and rejoining channels, but a fundamental difference is the cross-sectional morphology of the islands, which is bar-like and convex-up in braided rivers and typically saucer-shaped and concave-up in anastomosing rivers. These morphological differences reflect different modes of formation, i.e., in-channel bar accretion in braided rivers and channel avulsion (see avulsion channel), leading to floodplain excision, in anastomosing rivers. For this reason, Carling et al. (2014) proposed a process-based, rather than a morphology-based, definition and classification of the anastomosing channel pattern. The present description, however, is primarily based on morphology and scale. Islands in anastomosing rivers tend to be larger than braid-bars and the term anastomosing generally indicates a large-scale pattern. Occasionally, braided patterns and meanders can be recognized as morphology of individual channels that are interconnected in an anastomosing pattern. Typically, individual anastomosing channels are fairly straight or low sinuous (Makaske 2001). The term anabranching pattern has been proposed to describe “systems of multiple channels characterized by vegetated or otherwise stable alluvial islands that divide flows at discharges up to nearly bankfull” (Nanson and Knighton 1996). Defined as such, the category of anabranching rivers includes all anastomosing rivers but also systems with relatively large and high bar-like islands that lack flood basin morphology. Thus, as far as alluvial rivers are concerned, the term anabranching pattern has a wider meaning than anastomosing pattern. With respect to non-alluvial or non-fluvial patterns of conduits, or extraterrestrial channel patterns, of which detailed surface morphology is often unknown, the terms anabranching and anastomosing can be considered synonymous. The extraterrestrial channels shown in Figs. 3, 4, 5, and 6 are classified as anastomosing, because they have relatively large islands, with various shapes and sizes, and a surface morphology that resembles the neighboring plains. On Mars, anastomosing patterns could be observed for some unconfined outflow channels, fluvial and lava channels (Figs. 3, 4, and 5), and slope streaks (Miyamoto et al. 2004). On Venus, some lava channels that are classified as “complex” (Komatsu et al. 1993) display an anastomosing pattern (Fig. 6).
Fig. 1

The anastomosing upper Columbia River in southeastern British Columbia, Canada. Note the wet flood basins on the islands, which are enclosed by largely tree-covered, natural levees along the channels (From Makaske et al. 2002)

Fig. 2

Definition figure of an alluvial anastomosing river pattern (After Makaske (2001)

Fig. 3

Anastomosing channels on Mars on the flanks of Ascraeus Mons, which are believed to be of fluvial origin. They have streamlined islands in the channels and multiple terraces in the channel walls that are very rare for lava channels (Mouginis-Mark and Christensen 2005). Scale bar 2 km. THEMIS image V08155020 (NASA/JPL-Caltech/Arizona State University)

Fig. 4

Anastomosing channel as seen at THEMIS resolution on the northern rim of Ascraeus Mons caldera, Mars at 11.78°N, 255.56°E. These are interpreted as lava channels. This is the first identified anastomosing lava channel on Mars (Mouginis-Mark and Christensen 2005). Scale bar 2 km. THEMIS image V11612005. (NASA/JPL-Caltech/Arizona State University). HiRISE image (inset) reveals segmented nature of the channel. HiRISE PSP_005532_1915_RED (NASA/JPL/University of Arizona)

Fig. 5

Enipeus Vallis on Mars. Northeast Tharsis, 37.0°N, 267.2°E. Scale bar 10 km. THEMIS Day IR mosaic (NASA/JPL-Caltech/Arizona State University)

Fig. 6

Anastomosing channels on Venus (F-MIDRP 10S065;1 Magellan radar) at 12°S, 67°E (From Kargel et al. 1994). Scale bar 50 km (NASA/JPL)

Morphometry

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.

Subtypes

Anastomosing patterns of (1) fluvial/alluvial channels and (2) lava channels.

Formation

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.

Surface Units

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).

Composition

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.

Studied Locations

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).

Prominent Examples

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).

Distribution

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.

See Also

References

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Soil Geography and Landscape groupWageningen UniversityWageningenThe Netherlands
  2. 2.AlterraWageningen University and Research centreWageningenThe Netherlands