The Physical Diversity and Assessment of a Large River System: the Murray-Darling Basin, Australia

Abstract

This chapter is the outcome of a challenge by natural resource managers to river scientists to assess the physical condition of rivers within the Murray-Darling Basin, Australia. The essence of the management questions were; at the catchment scale what type of rivers do we have; what is their physical condition; and, what are the main drivers of this condition? In response, the approach taken highlights the importance of understanding the ecogeomorphology of river systems (Thorns and Parsons, 2002), integrating the disciplines of hydrology, geomorphology and ecology, and it makes a number of sequential points. First, natural resource managers required information on the condition of rivers at the catchment scale, thus providing a spatial domain or boundary, for data collection and its reporting. Second, the physical characterization of rivers, at the catchment scale, was deemed to be an important step in understanding the overall structure and behaviour of rivers within the basin for several reasons. The assessment of the physical condition of rivers was part of a larger audit on the sustainability of rivers in the Murray-Darling Basin, which also incorporates a series of chemical and biological components. The common link between the biological and physical components of a river system is the expression of physical habitat as a templet for biological communities (Southwood, 1977). From a biological perspective, physical habitat is considered as a templet upon which the ecological organization and dynamics of ecosystems are observed (Townsend and Hildrew, 1994). Audit protocols require a comparison of the physical, chemical and biological components from similar river types. In addition, physical diversity (a measure of the abundance and frequency of different river types) is also a useful indicator of the condition of river systems (Thoms et al., 2004). Third, many factors contribute to the physical condition of rivers and commonly a single index of condition is provided (Stevenson and Hauer, 2002). Whilst useful, interpretations would benefit from the addition of sub-indices that allow for more specific interpretation of environmental condition. In this study an aggregated index is provided along with four sub-indices that consider the influence of changes in habitat, hydrology, land use and sediment transport on the physical condition of rivers. In this chapter an overview of the geomorphology of the Murray-Darling Basin is provided, highlighting the influence of larger scale constraints on the contemporary character of river systems within the basin. The character of rivers is described followed by an assessment of their physical condition.