The gravel–sand transition in a disturbed catchment

Abstract

More than 40 million cubic metres of mining waste were supplied to the Ringarooma River between 1875 and 1984, leading to successive phases of aggradation and degradation. The natural bed material is gravel but, given the volume of introduced load and the fact that much of the input was less than 5 mm in diameter, the size composition of the bed changed from gravel to sand during the phase of downstream progressive aggradation. A very sharp gravel–sand transition developed in which median grain size decreased from over 30 mm to under 3 mm in less than 500 m. With upstream supplies of mining debris becoming depleted first, degradation followed the same downstream progressive pattern as aggradation, causing the transition to migrate downstream. By 1984, the river could be regarded as a series of zones, each characterized by a particular bed condition: a natural cobble–gravel bed, unaffected by mining inputs (0–32 km); pre-disturbance bed re-exposed by degradation over 35–40 years (32–53 km); sandy substrate with a gravel armour produced by differential transport during degradation (53–65 km); sand dominated but with developing surface patches of coarser material (65–75 km); sandy bed reflecting the size composition of the original mining input (75–118 km). Although the gravel–sand transition itself is sharp, the transitional zone is lengthy (53–75 km). As degradation continues, the gravel–sand transition is expected to progress downstream but it has remained in a stable position for 12 years. Indeed, two major floods during the period released large quantities of sand from the sub-armour layer and newly-formed banks of mine tailings, causing fining both above and below the transition. Surface grain size is an adjustable component in the transitional zone as the river strives to recover from a major anthropogenic disturbance.