The Influence of Physical and Anthropogenic Factors on a Channel’s Geomorphic Diversity

Abstract

The geomorphic diversity (or the natural variability within and between geomorphic structures) of fluvial systems provides an indication of river health and biological activity as well as their resilience to change. Despite this, few studies have investigated the controls on geomorphic diversity and, as a result, our understanding of this fundamental aspect of rivers is incomplete. Similarly, investigations into the controlling factors on channel morphology tend to be limited in scope. For example, the influence of physical and anthropogenic external factors on the morphology of fluvial systems has typically been examined through the study of the effects of a single factor (e.g., woody debris) on either the cross-sectional form, the shape of the long-profile, the bed structure or the channel pattern of a river system. As rivers have been shown to adjust their channel morphologies to external controls (Knighton 2000) over all four of these degrees of freedom, isolating individual degrees of freedom may miss out on the complex interactions that occur between them. The aim of this study, therefore, is to examine the multi-scale and multifactor influences of physical and anthropogenic external factors (particularly confinement, riparian vegetation, woody debris, obstructions and anthropogenic impoundment) on the geomorphic structure and diversity of river systems at a range of scales, using the Turon River in Central West New South Wales as a case study. In this study, river channels were examined at four scales (i.e., cross-section, long-profile, bedform and bar unit) to assess the influence of five external factors (confinement, riparian vegetation, woody debris, obstructions (i.e., islands and in-channel bars) and anthropogenic impoundment (i.e., a causeway)) on the geomorphic diversity of the Turon River. To accomplish this, a total of 231 cross-sections were surveyed over a 600 m reach. These data were then used to calculate the size and variability of cross-sections, long-profiles, bedforms and bar units within the study reach. Morphology and diversity at each scale (and for each factor) were tested for statistical differences using non-parametric uni-variate approaches. The results presented in this study suggest that the presence of obstructions is the most influential external factor on channel size in the Turon River, affecting the size and shape of cross-sections, long-profiles and, to a lesser extent, bedforms and bar-units. That is, obstructed channels were found to be significantly different to channels devoid of obstructions insofar as they were smaller, shallower, contained steeper channel gradients had more vertical variation in their long-profiles, had longer pool-riffle sequence spacing and were of a different channel form to channel reaches devoid of obstructions. Obstructions, in association with the presence and type of woody debris, were also observed to be the most influential factors on the diversity of river channels. For example, the presence of either obstructions or woody debris increased the variability of crosssectional and bedform parameters, while the type of woody debris present influenced the variability of the long-profile’s vertical and angular variations (i.e., the vertical and angular variations in long-profiles containing in-channel woody debris were less variable than those with on-bank woody debris). Importantly, cross-sections are impacted upon more than long profiles, with their size and variability affected by both large-scale external factors (e.g., confinement and riparian vegetation) and small-scale influences (e.g., obstructions and impoundments). For example, cross-sections within confined reaches were found to be larger but less diverse than crosssections in unconfined channels, while the reverse is true for obstructed cross-sections (i.e., obstructed cross-sections were smaller and more diverse than unobstructed channels). Conversely, pool-riffle sequences were the least affected river components, only being influenced by obstructions and, to a lesser extent, woody debris. That is, bar-units within obstructed channels were smaller, longer and more asymmetric than bar-units within channels devoid of obstructions. The results presented in this study also indicate that the variability of channel characteristics was affected more by the influence of external factors than channel dimensions. Additionally, the findings of this study indicate a reversal in the influence external factors have on the size and shape of a channel and its diversity. That is, smaller channels were found to be more diverse than larger channels. This is the first study to examine the influence of multiple factors on multiple scales within a river reach. The results of this investigation illustrate that river systems have complex responses to a combination of different physical and anthropogenic external factors that are evident at multiple scales (from cross-sections through to bar units). Additionally, it has shown that interactions between the external factors in a reach can result in a highly geomorphically diverse environment.