Properties of a cellular braided-stream model

We have shown in a previous paper that many of the main features of braided streams can be captured in a relatively simple cellular computer model. Here we examine some of the detailed characteristics of this model. We show the qualitative form of the braiding produced by the model is generally insensitive to changes in most of the numerical parameters used in the model. The most crucial parameter choice is the use of a non-linear exponent (>1) to describe the relation between sediment flux and local stream power. Use of water discharge instead of stream power to parameterize sediment flux produces braiding, but also unrealistically high-amplitude topography variations in the long term. Introduction of a threshold transport condition causes no noticeable change in the model's behaviour. Inclusion of lateral sediment transport due to gravitational effects on lateral slopes is not crucial to produce braiding, but is needed to provide reasonable lateral channel shifting, and to maintain a continuing dynamic behaviour. As long as lateral sediment transport is included, altering the initial topography for a run has no effect, other than a transient period of regrading. In addition, we show that there is a simple and apparently fundamental connection between braided-stream channel networks and crosional (dendritic) networks that has not been previously recognized. All that is needed to switch the model from braided to dendritic patterns is either to remove redeposition from the rules, simulating entrainment of cohesive sediment, or to add a cliff to the initial topography, making local redeposition unimportant. This result suggests that the presence or absence of significant local redeposition. which causes bar formation, channel division, and avulsion, determines whether a braided or dendritic pattern will form.