Anastomosing rivers: a review of their classification, origin and sedimentary products

Anastomosing rivers constitute an important category of multi-channel river s on alluvial plains. Most often they seem to form under relatively low-ene rgetic 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 geomorp hology and channel pattern, is proposed in this paper: an anastomosing rive r is composed of two or more interconnected channels that enclose floodbasi ns. 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 channel s is commonly coupled with their multi-channel character. Here, it is sugge sted that these two properties be uncoupled. At the scale of channel belts, the terms 'straight', 'meandering' and 'braided' apply, whereas at a large r scale, a river can be called anastomosing if it meets the definition give n 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 th e 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 o ccurrence 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., now diversions t hat cause the formation of new channels on the floodplain. As a product of avulsion, anastomosing rivers essentially form in two ways: (1) by formatio n of bypasses, while bypassed older channel-belt segments remain active for some period; and (2) by splitting of the diverted avulsive flow, leading t o contemporaneous scour of multiple channels on the floodplain. Both geneti c types of anastomosis may coexist in one river system, but whereas the fir st may be a long-lived floodplain-wide phenomenon, the latter only represen ts a stage in the avulsion process on a restricted part of the floodplain. Long-lived anastomosis is caused by frequent avulsions and/or dow abandonme nt of old channels. Avulsions are primarily driven by aggradation of the ch annel belt and/or loss of channel capacity by in-channel deposition. Both p rocesses 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 wi th a specific climate, the occurrence of anastomosis is not. A rapid rise o f base level is conductive to anastomosis, but is not a necessary condition . Anastomosing rivers can be considered an example of equifinality, since a nastomosis 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 c hannel sand bodies. Laterally extensive, thick lenses of lithologically het erogeneous, fine-grained avulsion deposits can be an important element of t he 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 upw ard-fining trends, as well as abrupt flat tops. The overbank deposits commo nly comprise abundant crevasse splay deposits and thick natural levee depos its. Lacustrine deposits and coal are common in association with anastomosi ng river deposits. None of these characteristics is unique to anastomosing river deposits, and in most cases, anastomosis (coexistence of channels) ca nnot be demonstrated in the stratigraphic record. (C) 2001 Elsevier Science B.V. All rights reserved.