Water

Research shows how
braided rivers work

New Zealand’s braided rivers dominate our eastern coasts. But how do they affect the amount of water in our aquifers available for human activity and nature?

Until recently, no one knew how braided rivers worked beneath the surface and interacted with regional aquifers. But through significant advances in how we use technology, we are finding answers.

Lincoln Agritech’s Scott Wilson is leading a five-year project, funded by the Ministry of Business, Innovation & Employment to find out how braided rivers and regional aquifers interact to store and transport water.

The team has made some important discoveries, using a lot of technology in novel ways. This includes the novel use of fibre optic cables beneath the ground, horizontally under rivers and also vertically to analyse river-groundwater exchange under different flow conditions. “This is giving us really beautiful profile through the aquifer system, helping us to learn about aquifer structure,” says Scott.

It also includes equipment never used before in New Zealand such as an APSU surface nuclear magnetic resonance system. This measures the porosity and water content of sediment non-invasively (and so doesn’t alter the observation through the very act of measuring). It was imported into New Zealand, along with its specialist operator Mathias Vang from Denmark’s Aarhus University.

The team also used tTEM (an electromagnetic system that is dragged along the ground to produce detailed 3D geophysical and geological maps of the sub-surface), satellite imagery combined with flow recorders to accurately measure river losses, and radon tracing to measure river water loss.

What we now know

“We now know how braided rivers work in the sub-surface. We know specifically what controls how much our rivers recharge the regional aquifers,” says Scott. “And we now have a conceptual model of how they work, where we know that braided rivers create their own aquifer system.”

Comprised of gravel, reworked through floods, this “braid plain aquifer” extends far beyond where we can see water. The river uses this reservoir to regulate its flow and temperature. But if human activity diminishes this aquifer too much, it affects how much the river can recharge the regional aquifer and sustain river flows and temperatures during dry seasons.

This research will help New Zealand councils quantify the environmental and economic benefits of different river management strategies.

“This will be an important step in helping councils meet the Government’s National Policy Statement on Freshwater Management, and ensuring that competing needs for water can be met in the best way possible,” says Scott.