Assessing morphologic complexity and diversity in river systems using three-dimensional asymmetry indices for bed elements, bedforms and bar units.

Geomorphologists are becoming increasingly interested in assessing morphologic structure and the diversity and/or complexity in morphologic structure across multiple scales within river systems. Unfortunately, many of our existing tools/variables are unsuitable for this task because they do not work across multiple scales or with changing discharges. Asymmetry is one variable that can be used to either include or exclude variations in flow stage and that can be assessed across multiple scales. Existing asymmetry indices, however, are limited in scope and largely focus on only cross-sectional form. This study examines three existing asymmetry indices in the cross-stream and downstream planes (for cross-sections and riffle or pools, respectively) and develops nine new asymmetry indices that incorporate vertical, cross-stream and downstream asymmetry for bed elements (e.g. riffle crests, pool troughs, riffle entrance slope), bedforms (pools or riffles) and bar units (pool-riffle sequences) to investigate the utility of asymmetry as a measure of morphologic structure and diversity in fluvial systems. These 12 indices are field tested on the Embarras River in East Central Illinois, USA. The results of this study indicate that there is considerable morphologic diversity in bed elements, bedforms and bar units both at bankfull and also with varying flow stage. This multi-scale, multidimensional, multistage variability in morphologic structure highlights the complexity of natural river systems. The highly variable nature of fluvial form within a reach has important implications for river restoration and/or assessments of physical habitat or river health especially in instances where pools, riffles or pool-riffle sequences are the focus of study. In general the most robust and useful combination of asymmetry indices for most applications includes A* and A(L1) for bed elements and bedforms and A(L3), A(w) and A(H) for bar units.