Geomorphic issues associated with environmental flow assessment in alluvial non-tidal rivers

Abstract

This paper explores the link between flow variability and stream morphology in the context of the geomorphic component of environmental flow assessment. The paper also attempts to bring some methodological order to the contribution by geomorphologists to environmental flow assessment. The geomorphological extension of the natural flow paradigm is the concept of alluvial river attributes. These are basic hydro-geomorphic characteristics of natural streams thought to be necessary for maintaining ecosystem integrity in alluvial rivers. It appears that the more variable is the flow regime, the more variable are these attributes, although observations indicate that many channels conform to basic relationships between overall channel shape and size and two or more groups of flows definable in terms of frequency and magnitude. There is evidence that some Australian streams are in a continual state of adjustment to cyclical changes in flood regime, so they do not adjust to a fixed discharge. Also, most of the classical literature on channel forming processes does not consider the role of boundary roughness and strength in any detail, and it appears that extensive modification of this aspect of many Australian rivers (mainly through desnagging and riparian vegetation clearance) has caused a fundamental shift in their geomorphic character. The first stage of any environmental flow assessment would be to place the significance of flow regulation as an agent of geomorphic change within this wider context. This will provide a realistic perspective on the potential for environmental flows to either maintain or improve channel functioning. In disturbed streams, or streams subject to a highly variable flow regime, the classical geomorphic concepts of bankfull discharge, effective discharge and channel maintenance discharge may not apply in the same way as described for intact and less hydrologically variable North American and European streams. Sediment transport formulae may be used with caution in circumstances where the process is not sediment supply limited – an uncommon situation in many Australian streams.Natural flow regimes are composed of numerous facets, or components, occurring as a complex time series, not discrete, predictable and independent events. Scaled-down flow regimes, or regimes with certain components culled from the regime, should not be expected to produce the same morphology as a natural flow regime. Environmental (regulated) flows will nearly always be simpler and less variable than natural flow regimes, so the resulting geomorphology will probably be less diverse. Many geomorphic problems in regulated rivers cannot be solved simply by flow intervention, and in unregulated rivers, increased water diversions will not necessarily be the main cause of future degradation. In heavily modified rivers, it may be preferable (or necessary) to use environmental flows to enhance the current geomorphic attributes, rather than seeking a return to some previous, or ideal state.