Anastomosing Fluvial Systems

Geol 370: Sedimentology and Stratigraphy
Topic 10d: Anastomosing Fluvial Systems

1. 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.

2. • 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

3. 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.

4. 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.

5. 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.

6. 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.

7. Large-scale Architecture
Anastomosed fluvial systems are characterized by:
• Low sand:mud ratio
• Low sand body connectivity
• Lens geometry
• Very high accommodation development

8. Photo by W. W. Little

9. Photo by W. W. Little

16. Photo by W. W. Little