DRYLAND ANASTOMOSING RIVERS SHOULD BE INVISIBLE IN THE ROCK RECORD

The application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions are to be reliable and useful. From modern observations, anastomosing rivers can be divided into humid and dryland kinds. Each class has different characteristics necessitating different criteria for their recognition in the rock record. Recognition of anastomosing rivers is currently imperfect because the criteria applied characterise only one of the two possible classes.

Crucial to the formation of the anastomosing pattern are both an irregular flow regime and high bank erosion resistance. The combination of these factors ensures that channels cannot easily respond to high flow events by expanding capacity. This forces discharge overbank which leads to the cutting of multiple channels across the floodplain. Erosion resistance is achieved by cohesion of bank sediments or armouring of the channel banks by vegetative root mats.

In dryland systems, vegetation densities are low, so bank resistance is achieved solely by cohesion of bank sediments. High resistance, and hence the anastomosing style, is only possible in drylands for suspended-load rivers transporting fine-grained sediment. The record of these rivers is uniformly fine-grained channel and floodplain deposits. Channel sandbodies are absent. The alluvial origin of such sediments can be difficult to determine, and there is much potential for misidentification of depositional environment. River style has to be inferred, it cannot be observed. Interpretations of deposits with prominent channel sandbodies as dryland anastomosing rivers are likely to be incorrect and need re-examining.