Chapter 3 The Rapid Spread of Mine-Derived Sediment across the Middle Fly River Floodplain

Abstract In 1990, 5 years after mining-related waste was introduced into the Fly River system by Ok Tedi Mining Limited, an annual monitoring program was instituted to detect the spatial extent of mine-derived sediment across the Middle Fly River floodplain. Initial sampling, which relied on elevated particulate copper detection as a mine signal, revealed a surprisingly rapid and extensive deposition rate across the floodplain. This led to more intensive sampling and sediment core analysis over subsequent years through 1995. Some 840 separate cores were taken across the 3,500 km 2 floodplain and about 4,000 separate particulate copper measurements were made. Mine-derived sediment was found to spread rapidly across tens of kilometers in just a few years due to injection of mainstem sediment laden flows out a 900 km long network of floodplain channels (tributary and tie channels). These flows entered off-river water bodies (ORWB) and spilled from their banks onto the adjacent plain, creating a depositional web across the floodplain. Consequently, even in the first annual samples, 75% of all off-river water body samples had elevated particulate copper levels. The overbank flows dropped sediment rapidly as it spread across the plain, leaving an exponential decline in deposition rate with distance from the nearest channel bank that fell to very low values within a kilometer. Using the defined exponential functions and measured channel lengths as well as estimates of deposition rates in the ORWB, we calculate that about 180 Mt of sediment (∼130,000 tonnes of copper) was deposited on the floodplain environment between 1985 and 1994, equaling about 40% of the total load entering the Middle Fly at D’Albertis Junction. Approximately, one-half of this load was deposited via floodplain channels and the other half by mainstem overbank deposition. This is consistent with the separate rough estimates of 20% of the total discharge being pushed out the floodplain channels and another 20% spilled overbank during flood events. This initial response to increased mine-derived loading may differ from subsequent years as bed aggradation (which was relatively minor at this time) would increase the frequency and duration of flooding. The rapid and extensive spread of sediment via floodplain channels has left a long-term legacy of mine-derived sediment across the full extent of the Middle Fly River floodplain environment.