The modern-day Taieri River heads northeast along the Rock and Pillar Range, before making a hard hairpin turn around the end of the range and heading out to the coast. Scientists from the University of Otago have found the reason for this hairpin and pieced together clues to discover how and when it happened.
Previously, the Kye Burn stream existed where the upper Taieri is now, and flowed southward to the Clutha River. But over time, the flow was blocked by the uplift of the South Rough Ridge, Lammermoor Range and Rock and Pillar Range, ultimately reversing the flow until the river joined the Taieri.
Evidence of the change of course was found in ancient river gravels at the saddle that now separates the Taieri from the Teviot River—quartz-rich argillite and conglomerate that could only have come from the Hawkdun and Kakanui Ranges far to the north. “Their presence at that saddle shows that what is now the upper Taieri used to flow south, not north as it now does,” says study author Jon Waters.
It was analyses of local fish DNA that put a date on the event. When the mountains rose and disrupted the river, this cut one fish population in two, leaving some fish stranded in the Teviot and the others in the new upper Taieri. Furthermore, downstream in the lower Taieri, there’s yet another group of unrelated fish that were once entirely isolated. Despite now being part of the same watercourse, they are yet to remix genetically with their long-lost cousins.
The rising South Rough Ridge Lammermoor Range and Rock and Pillar Rnge blocked the southward flow of a major Clutha River tributary, the ancestral Kye Burn, causing its flow to reverse, and run out the Taieri resulting in the unusual biographical range of fish in the river system today.