Quick Dips

Channelling in Ancient Days

Scienctists from Topoclimate, Sam Carrick, Lauren Eyre and Peter McIntosh, have confirmed that gravelly deposits stretching from near Lumsden to Glenure, southwest of Balfour, in the Waimea Plains, were deposited by the Oreti River that flowed eastwards to Gore about 15,000 years ago.

"Farmers have long suspected that the relatively fresh gravels in the Waimea Plains near Balfour probably have an Oreti origin, even though the present day course of the Oreti River is southward," says Topoclimate South chief pedologist, Peter McIntosh.

What the scientists have shown is that these deposits form a distinct channel that is continuous from Lumsden to Glenure. The soil mapping indicates that the channel and the deposits in it were deposited by the Oreti River, probably during the closing stages of the last glaciation about 15,000 years ago. There was a lot of erosion in the high country at that time. The rivers were loaded with gravel and the Oreti river channel was probably higher, causing it to spill over into the Waimea Plains. The flood channel water then joined the Waimea Stream and eventually flowed into the Mataura River near Mandeville.

"Our mapping shows that the Oreti River has been flicking to and fro between catchments at least four times," says McIntosh. "We know that the Waimea Plains were formed by a precursor of the Oreti River hundreds of thousands of years ago, but up until now it was thought that the Oreti switched to its southward course via Dipton about 300,000 years ago, and had stayed on that course ever since.

"This new evidence indicates that it switched to an eastward course again about 15,000 years ago, followed that course for some time, and then went south yet again."

What caused the last change of course is unclear, but the Topoclimate team believe that active fault movements along the foothills of the Hokonui Hills and geological tilting of the Waimea Plains may have been responsible.

One bit of information the topoclimate team has picked up from farmers is that springs and groundwater in the "Oreti" gravels in the last glaciation channel never run dry.

"We suspect that the Oreti River water may be still seeping down this channel, into the Mataura catchment, even though there is no surface flow," says McIntosh. "Chemical tests on the water might prove whether this conclusion is correct."

Groundwater specialists from the Southland Regional Council are intrigued by the findings and possible implications for the groundwater resources.

Peter McIntosh is a scientist with Topoclimate South, Mataura.