## 1. Introduction

[2] What processes form modern delta distributary networks and their orderly topological properties? The distributary network of the Mossy delta, Saskatchewan, Canada, for example (Figure 1), consists of a succession of bifurcations in which the channel widths and lengths to the next bifurcation systematically decrease. While the width trend will be shown to be an outcome of hydraulic geometry scaling relationships, the decrease in distributary channel lengths is not as easily explained. In this paper we seek to understand the origin of these and other topological trends of delta networks by focusing on the processes that give rise to them. Such a detailed study will aid in predicting future behavior of these depositional environments and, from an engineering standpoint, will help improve floodplain and delta management. Also, an understanding of the morphodynamics of river-dominated deltas will lead to better stratigraphic models that more accurately predict the evolution and form of these alluvial sand bodies.

[3] Early work on the topology of delta networks focused on channel vertices and links, allowing *Smart and Moruzzi* [1972] for example, to conclude that the frequency of channel link types can be explained by a model of random connections of vertices. *Morisawa* [1985] then extended this analysis to 20 deltas and found that the frequencies of channel junctions, forks, and links of natural deltas are not random. *Coleman and Wright* [1971] and *Wright et al.* [1974] used field observations and maps to analyze 34 major river systems. Cluster analyses revealed that distributary network patterns are highly variable and cannot be predicted from a single controlling variable. *Mukerji* [1976] analyzed five inland alluvial fans in India and found that the fan network has a consistent shape which is a function of the bifurcation angle at the head of the fan. A global analysis of the world's deltas [*Syvitski et al.*, 2005] revealed that the number of distributary channels within a network is a negative function of delta gradient and a positive function of river length. *Olairu and Bhattacharya* [2006] studied the terminal distributary channels of delta networks and found that within the Volga delta network, channel widths and lengths are log normally distributed, and channel widths decrease exponentially down delta. *Marciano et al.* [2005] showed that tidal channel networks are fractals and channel lengths in a tidal basin decrease by with each bifurcation. While *Marciano et al.*'s [2005] result is noteworthy, whether or not it applies to the case of river delta networks remains to be determined. These studies are valuable in quantifying some aspects of delta distributary topology, but they do not address global variation of channel-length scales within a delta, nor do they provide a morphodynamic model that explains the topologic relationships.

[4] The purpose of this paper is to present additional topologic and morphologic data on delta networks and show that these data can be explained using a physically based numerical model of channel elongation and bifurcation around river mouth bars.