Sediment organisation along the upper Hunter River, Australia: a multivariate statistical approach

Abstract

Freely adjusting gravel-bed rivers are subject to recurrent input of material and frequent, but irregular, reworking of bedload materials. In contrast, many Australian rivers are characterised by low-bedload input and infrequent mobilisation of bedload materials. In this study, the influence of these boundary conditions upon surface bedmaterial organisation are analysed through application of extensive field sampling and multivariate statistics based on a range of bed-material parameters along an 8 km reach of the Upper Hunter River in New South Wales. The Hunter River is a mixedload, gravel-bed river that has been subjected to significant disturbance since European settlement. Since European settlement, the channel in the study reach has been transformed from a passive meandering river to a configuration in which the low-flow channel locally adjusts around various bar and bench features. Channel capacity has enlarged locally by a factor of 3, channel alignment has changed via cut-offs (both natural and human-induced), and channel-floodplain relationships have been altered. The multivariate statistical analyses performed in this study provided a very useful tool for identifying surface facies and the way in which sediment is organised. Bedload surface facies do not demonstrate an equivalent degree of organisation to that documented for fully self-adjusting rivers. A conceptual model is developed that relates the variability and spatial arrangement of differing classes of bed material to the type of geomorphic unit, elevation above the low-flow channel and ease of reworking (i.e., frequency of potential mobilisation). Spatial variability in the sediment mix of the study reach, appraised in light of long-term changes to sediment flux, prompts the need for refinement of bedload transport models for this kind of river.