Classification of Australian tropical rivers to predict climate change impacts
9th International River Symposium, Brisbane
Wayne Erskine, M.J. Saynor and J Lowry
Based on their experience with tropical rivers in the Northern Territory, Erskine et al. (2005) proposed a preliminary typology of Australian tropical rivers for subsequent application to river reach mapping and classification. A geomorphological approach to river characterization and classification was adopted because it is the only one possible based on the limited existing information for Australian tropical rivers. River reaches are homogeneous lengths of stream within which hydrological, geological, and adjacent catchment surface conditions are sufficiently constant so that a uniform river morphology or a consistent pattern of alternating river morphologies is produced (Erskine, 2005a). Channel reaches consist of relatively homogeneous associations of landforms and habitat types, which distinguish them from adjoining reaches and are typically 10 km to greater than 100 km in length (Bisson and Montgomery, 1996). While the core length of a reach is easy to identify, it is more difficult to define precisely the transitional boundaries from one river type or reach to another (Erskine, 2005a).
Channel reaches are the appropriate spatial scale to map specific river types, such as those defined by Rosgen (1994; 1996), Brierley and Fryirs (2000; 2005; Brierley et al., 2002) and Erskine et al. (2005). The Erskine et al. (2005) typology applies to reaches at least 5 km but usually greater than 10 km long. Previous assessments of stream condition in the Daly and Roper catchments in the Northern Territory were based on stream order as shown on 1:250,000 topographic maps (Faulks, 1998; 2001) which are of an appropriate scale for application of the present typology.
The approach adopted here is that a simple descriptive name based on the distinguishing geomorphic characteristics of the reach is used, such as 'Meandering River'. Greater terminological precision can be used when the unique characteristics of each type are determined (Erskine, 1999). This will allow subdivision of major types into more homogeneous categories as more information is obtained and as mapping progresses to larger scales. This work forms part of the Tropical Rivers Inventory and Assessment Project funded by Land and Water Australia and the Natural Heritage Trust.
It is essential to know what river types currently exist in catchments if they are to be managed effectively and if appropriate actions are to be proposed and implemented to address current and future threats, such as land development and climate change. The work reported below can be completed in about 2 days for a 110,000 km² catchment if topographic and geological maps and remotely sensed data are readily available.