Anastomosing Fluvial System

(Multichannel/mud-dominated)

Anastomosing fluvial systems have multiple, relatively shallow channels with a sinuosity of at least 2.0, a very low bed/suspended load ratio, and highly cohesive bank material. Anastomosing river deposits are typically dominated by mud and are characteristic of high-discharge perennial rivers flowing over very low gradients.

• Low gradient relative to discharge• High discharge relative to load• Very low bed load relative to suspended load• Very cohesive bank materials (commonly muddy & vegetated)• Perennial discharge with relatively low fluctuation

Conditions Favoring Anastomosing

Simplified Model

Anastomosed fluvial systems have facies associations similar to those of meandering systems (active channel, abandoned channel, overbank, splay) but in different proportions and with different geometries.

Contrast with Meandering

Channel processes are similar for both meandering and anastomosed stystems. The main difference is that subsidence rates are so high for anastomosed systems that channels become vertically stacked, rather than migrate laterally.

Common Facies

Anastomosed rivers are dominated by muddy facies representing thick floodplain successions. Swamp, peat bogg, splay, and other environments are common. Channel facies show point bar successions that are not as well developed as those of meandering systems.

Channel belt

Splay

Floodplain

Common Facies

Narrow, but thick deposits of trough cross-bedded sandstone dominate channel deposits. Thick overbank successions composed of laminated mud are common and often include significant deposits of peat. Thin beds of trough cross-bedded to ripple-bedded sandstone interbedded with mudstone represent splay deposits.

Large-scale Architecture

Anastomosed fluvial systems are characterized by:• Low sand:mud ratio• Low sand body connectivity• Lens geometry• Very high accommodation development

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little