The morphology and facies of sandy braided rivers: some considerations of scale invariance

A fundamental and unresolved question in fluvial sedimentology concerns the nature of scale invariance and whether it is appropriate to apply data from a single river or outcrop of alluvial sediments to others of a different size. This issue is addressed herein by i) examining the similarity in aspects of the morphology of modern bars in braided rivers and ii) comparing the subsurface facies of three sandy braided rivers of differing scale (30-2000m channel width), as revealed by ground penetrating radar (GPR). Measurement of braid bar shape in 15 rivers, covering four orders of magnitude in spatial scale, demonstrates that a simple index of bar planform shape, the width:length ratio, is scale invariant. Additionally, scour depths at channel confluences are similar in their relative scale across channels of greatly differing size. Comparison of the subsurface sedimentary facies of three sandy braided rivers using GPR demonstrates that sandy braided rivers exhibit a degree of scale invariance with the ubiquitous occurrence of trough cross-stratification associated with migrating dunes. However, significant differences exist in the occurrence of other facies both between rivers and between bars within the same river, most notably in the predominance of either high-angle planar cross-stratification or low-angle stratification. These differences are controlled by a wide range of factors that may include the discharge regime, local bar and channel topography, anabranch width:depth ratio and the abundance of vegetation. Hence although rivers and individual bars within the same river may have similar surface planform shapes, their subsurface facies may be very different. A single, universal facies model for sandy braided rivers is thus probably inappropriate and will remain elusive.