The Ok Tedi copper-gold mine, located in the eastern part of the central mountain range of New Guinea, discharges daily approximately 80,000 tons of ore processing residues and a similar volume of waste rock and overburden into the headwaters of the Ok Tedi River. The Mount Fubilan orebody, which is the source of heavy metal-rich sediments de-posited along the Ok Tedi and on the Fly River floodplain, contains a suite of base metals, of which copper is of pri-mary environmental concern. The Ok Tedi flows into the Fly River 200 km downstream of the discharge point, where the mining wastes carried as suspended load are diluted. In the present study, the deposition of mine-derived sediments in the lower part of the Middle Fly River floodplain, and the hydrochemistry and potential mobilization of trace metals, particularly copper, in the alluvial plain was investigated. To this end, a total of 156 sediment cores and sur-face sediment samples and 117 water samples from the upper Ok Tedi and the Middle and Lower Fly River floodplain were taken. The suspended matter content in Middle Fly River water today is about 5-10 times higher than the natural background of about 60 mg/l.