A morpho-statistical classification of mountain stream reach types in southeastern Australia

Abstract

The concepts of sediment transport capacity ( Q c) and sediment supply ( Q s) have shown promise in broadly differentiating mountain streams. The important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks. This study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0.01 and 0.20 m/m to describe mountain stream morphology in southeastern Australia. Reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology. Spatial autocorrelation analysis based on Moran's I is used to detect the frequency and occurrence of in-stream features, such as bars and pools. These data are combined with the grain size data to describe eight dominant reach morphologies. Most channel morphologies display broad characteristics similar to previous accounts of mountain streams, although a number of intermediate morphologies affecting subtle variations in form and process where evident. There was no significant longitudinal arrangement of the observed morphologies, particularly with respect to catchment area and slope. This is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope. Five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape. Regime diagrams applied to quantify the physical controls on the different reach morphologies identified different Q c– Q s domains for most morphologies. However, some lithology-dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape (or sphericity) in assigning dimensionless critical shear stress values in determining Q s. Overall, the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material.