Impact of the Opuha dam on the turbidity and health of the Opuha and Opihi Rivers

Abstract

Since the completion and commissioning of the Opuha dam in South Canterbury, New Zealand, in December 1998, a notable decline in the health of the Opuha River below the dam, and Ōpihi River further downstream, has been observed by expert fishers and local river users. The rivers have become visibly more turbid and ecological assessments confirm their poor health. However, the reasons for these changes remain unclear. Analysis of black disc clarity, turbidity and macroinvertebrate data from the National River Water Quality Network 30-year monitoring record suggests that the installation of the Opuha dam has resulted in significant increases in suspended fine sediment being discharged routinely down the Opuha River in lower flows. Prolonged releases of sediment-laden water and increased fine sediment deposition are most likely responsible for declines in the health of the Opuha and Ōpihi Rivers. The increase in fine sediment is due to a combination of factors including the Lake Opuha infrastructure design, the fixed water offtake behind the dam intersecting turbid water, and the dam altering the hydrology and continuity of sediment transport through the Opuha River system. The declines in ecological health reflect a hitherto unrecognised consequence of this water storage scheme. If other schemes are constructed in a similar fashion, then there will be a real risk that they too will release turbid water that could degrade the ecological health of rivers downstream. The impacts may be prevented by not using rivers as conduits for turbid irrigation water and instead piping water directly to irrigation schemes. In addition, allowing only settled sediment-free water to be discharged from storage lakes, ensuring storage lakes are of the right design to provide for sufficient sediment removal, and taking water only from zones free of suspended sediment, may help alleviate this issue.