Chapter 13 Effects of Mine-Derived River Bed Aggradation on Fish Habitat of the Fly River, Papua New Guinea

Abstract

Abstract Monitoring of the effects of the Ok Tedi Mine on the Fly River system has shown that the discharge of waste rock and tailings into the river has resulted in elevated sediment loads, increased particulate and dissolved copper levels, aggradation of the river bed, increased frequency and duration of floodplain inundation, and dieback of floodplain forest. Biological monitoring has reported significant and continuing declines in fish catch in the Fly River downstream of the mine. Given that previous studies ruled out acute toxic effects from metals, it was hypothesized that major cause of the declines in fish catch and diversity was loss of fish habitat due to river-bed aggradation. In November 2002, field measurement and hydrographic survey were used to quantify fish habitat at impacted and control reaches to assess habitat loss in relation to aggradation. Fish habitat ‘availability’ was quantified using 55 measured and derived parameters at 19 reaches, ranging in aggradation for 0–4.5 m. Habitat availability was found to decline with increasing bed aggradation. Attributes of fish habitat that consistently demonstrated a negative relationship to aggradation included number and size of deep backwaters, amount and complexity of large woody debris (snags), amount of cover for fish close to the banks (i.e., trailing vegetation, root mats, undercuts, riparian canopy), heterogeneity of the banks and river bed, and combinations of these variables when weighted by depth and water velocity. Results support the hypothesis that aggradation reduces fish habitat availability, which is likely a cause of the declines in fish catch in the lower Ok Tedi and upper Middle Fly River. The impact on fish habitat appears to be a physical process, driven by the load of waste rock and tailings deposited in the river system. As such, it may be possible to predict future effects on fish habitat by modeling sediment delivery, sediment transport, and associated aggradation/degradation of the river bed.