Morphologic Complexity and Diversity in Natural and Channelized Streams

Ecologists have long recognised the links between morphologic (or hydrologic) diversity and biologic diversity. In essence, highly diverse landform structures (and flow conditions) increase the potential range of available habitat types which, in turn, has positive impacts on both the abundance and diversity of plant and animal species. More recently, geomorphologists have become interested in notions of complexity and diversity of physical form as indicators of geomorphic health (or as indicators of geomorphic change) in human modified river systems. The purpose of this study is to explore how geomorphic complexity and the diversity of morphologic structure differ between natural and channelised (planed and straightened) reaches of the Embarras River, USA. This study incorporates both traditional geomorphic indicators (e.g., width, depth, velocity, etc.) and the asymmetry of the river's cross-sections, bedforms (e.g., pools and riffles) and bar units (e.g., pool-riffle sequences). The results reveal considerable differences between the complexity and diversity of form within the two study reaches with the natural reach having a much more complex and diverse geomorphic character than the channelised reach. This complexity and diversity in geomorphic character is most clearly detected using a combination of traditional geomorphic indicators and asymmetry parameters rather than the more traditional geomorphic indicators alone. The advantage of asymmetry as an indicator lies in its ability to discriminate the internal variability of morphologic structures at multiple scales (e.g., bed elements, bedforms and bar units) which means it is not limited to considering parameters in a single dimension that may not have been significantly altered by channelisation whereas the more traditional indicators are suitable for identifying changes in overall channel dimensions.