Mondo shtuff from around the internet, all about ANASTOMOSIS!
Anastomosing river deposits, sedimentation rates and basin subsidence, Magdalena River, northwestern Colombia, South America: Situated in a tectonically active foreland basin, the Magdalena River consists of vertically accreting, levee-confined channels and adjacent extensive wetlands, which are interpreted as an anastomosing river sedimentary system. Equivalent rates of basin filling and subsidence average 3.8 mm yr -1 based on 18 14C dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement. Located in a savanna-tropical climate, anastomosing river deposits of the Magdalena are remarkably similar to the anastomosing deposits of the upper Columbia River in a temperate-cold climate in western Canada, suggesting that climate is not a controlling factor of anastomosis. The geometry of anastomosing channel-fills in the Magdalena consists of stratigraphically non-uniform, low sinuous, narrow stringers of sand up to 30 m thick by 600 m wide, a width-depth ratio of 20. Thin (1-2 m) off-channel crevasse-splay sand sheets extend laterally up to 10 km distance. When buried, both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps. While there are few modern anastomosing river systems as compared to braiding and meandering, there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion. Such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided-river deposition models.
Anastomosing rivers: a review of their classification, origin and sedimentary products: Anastomosing rivers constitute an important category of multi-channel rivers on alluvial plains. Most often they seem to form under relatively low-energetic conditions near a (local) base level. It appears to be impossible to define anastomosing rivers unambiguously on the basis of channel planform only. Therefore, the following definition, which couples floodplain geomorphology and channel pattern, is proposed in this paper: an anastomosing river is composed of two or more interconnected channels that enclose floodbasins. This definition explicitly excludes the phenomenon of channel splitting by convex-up bar-like forms that characterize braided channels. In present definitions of anastomosing rivers, lateral stability of channels is commonly coupled with their multi-channel character. Here, it is suggested that these two properties be uncoupled. At the scale of channel belts, the terms ‘straight’, ‘meandering’ and ‘braided’ apply, whereas at a larger scale, a river can be called anastomosing if it meets the definition given above. This means that, straight, meandering and braided channels may all be part of an anastomosing river system. Straight channels are defined by a sinuosity index; i.e., the ratio of the distance along the channel and the distance along the channel-belt axis is less than 1.3. They are the type of channel that most commonly occurs in combination with anastomosis. The occurrence of straight channels is favoured by low stream power, basically a product of discharge and gradient, and erosion-resistant banks. Anastomosing rivers are usually formed by avulsions, i.e., flow diversions that cause the formation of new channels on the floodplain. As a product of avulsion, anastomosing rivers essentially form in two ways: (1) by formation of bypasses, while bypassed older channel-belt segments remain active for some period; and (2) by splitting of the diverted avulsive flow, leading to contemporaneous scour of multiple channels on the floodplain. Both genetic types of anastomosis may coexist in one river system, but whereas the first may be a long-lived floodplain-wide phenomenon, the latter only represents a stage in the avulsion process on a restricted part of the floodplain. Long-lived anastomosis is caused by frequent avulsions and/or slow abandonment of old channels. Avulsions are primarily driven by aggradation of the channel belt and/or loss of channel capacity by in-channel deposition. Both processes are favoured by a low floodplain gradient. Also of influence are a number of avulsion triggers such as extreme floods, log and ice jams, and in-channel aeolian dunes. Although some of these triggers are associated with a specific climate, the occurrence of anastomosis is not. A rapid rise of base level is conductive to anastomosis, but is not a necessary condition. Anastomosing rivers can be considered an example of equifinality, since anastomosis may result from different combinations of processes or causes. Anastomosing river deposits have an alluvial architecture characterized by a large proportion of overbank deposits, which encase laterally connected channel sand bodies. Laterally extensive, thick lenses of lithologically heterogeneous, fine-grained avulsion deposits can be an important element of the overbank deposits of anastomosing rivers. These deposits may also fully surround anastomosing channel sandstones. Anastomosing channel sand bodies frequently have ribbon-like geometries and may possess poorly developed upward-fining trends, as well as abrupt flat tops. The overbank deposits commonly comprise abundant crevasse splay deposits and thick natural levee deposits. Lacustrine deposits and coal are common in association with anastomosing river deposits. None of these characteristics is unique to anastomosing river deposits, and in most cases, anastomosis (coexistence of channels) cannot be demonstrated in the stratigraphic record.
My botty best at summarizing from Wikipedia: an anastomosis is a connection or opening between two things normally diverging or branching . it may be normal or abnormal, or between blood vessels, leaf veins, or streams . the term is used in the -stom- syllable refers to connections between blood vessels or between other tubular structures such as loops of intestine . the circulatory anastomosis is further divided into arterial anastomoses form alternative routes around capillary beds in areas that don’t need a large blood supply . Surgical anastamosis techniques include linear stapled, hand sewn, in the cases of veins or arteries, traumatic fistulas usually occur between artery and vein . portacaval anastomosis allows blood to bypass liver in patients with portal hypertension . under anastomosis, species might recombine after initial branching out . the concept also applies to the theory of symbiogenesis . new species emerge from the formation of novel relationships . in some fungi, two different haploid mating types merge . somatically, they form morphologically similar mycelial wave front . each septated unit is binucleate, containing two “anastomosing” is used for mushroom gills which interlink and separate to form a network . braided rivers are much shallower and more dynamic than anastmosing rivers .
