The spatial pattern of large wood in a large low gradient river: the Barwon–Darling River

Abstract

It is hypothesized that the organization of wood within the Barwon-Darling River, Australia; a large low-gradient dryland river, differs to current models of wood in rivers. Current models are dominated by studies in high-energy systems where wood is highly mobile and non-uniform in their distribution. Large wood in this system is not highly mobile because of low-stream powers, high-wood densities and a low ratio of wood size to bankfull widths. This study analyses the organization of 7142 pieces along 210 km of the Barwon-Darling River, SE Australia. The majority of pieces (72%) were <10 m in length and of this >50% were complex in character. Most pieces (98%) were attached to the bank, distributed relatively uniformly and correlated to the riparian zone. Pieces were either completely (25.9%) or partially aligned (36.4%) to the flow suggesting passive realignment. The primary driver of wood alignment was related to its size, secondary influences were height above the river bed (a correlate of stream energy) and river channel geomorphology. Thus, large wood is not actively transported, rather pieces remain where they fall and then passively realign as a result of wood size, height in the channel and river channel geomorphology.