Braided Rivers New Zealand

This programme investigates water loss from braided rivers into groundwater. Although globally rare, braided rivers are vital in New Zealand for habitat, agriculture, drinking water, and recreation. Currently, regional councils set water limits and management plans without knowing how much water is lost as rivers cross alluvial plains, nor how practices such as gravel extraction or flood protection affect groundwater recharge and river flow during dry periods.

© Thomas Wöhling

The project delivered new knowledge and models for the Selwyn/Waikirikiri (Canterbury), Wairau (Marlborough), and Ngaruroro (Hawke’s Bay) Rivers. These tools enable councils to estimate water loss from any section of a braided river and to quantify the environmental and economic benefits of different management strategies.

A multidisciplinary team from Lincoln Agritech Ltd, NIWA, Technische Universität Dresden (Germany), Aarhus University (Denmark), and Lincoln and Flinders Universities (Australia) made key discoveries using innovative technologies. Fibre optic cables were deployed both horizontally beneath rivers and vertically to analyse river–groundwater exchange under varying flow conditions. The team also introduced equipment new to New Zealand, including an APSU surface nuclear magnetic resonance system that non-invasively measures sediment porosity and water content. This instrument and its specialist operator, Mathias Vang of Aarhus University, were brought to New Zealand for the project. Other methods included tTEM (a mobile electromagnetic system producing detailed 3D sub-surface maps), satellite imagery with flow recorders to measure river losses, and radon tracing to quantify water loss.

The research revealed how braided rivers interact with groundwater and control recharge to regional aquifers. These rivers form their own “braid-plain aquifer,” composed of flood-reworked gravels extending beyond the visible channel. This aquifer regulates river flow and temperature, but human-induced depletion reduces the river’s capacity to recharge regional aquifers and sustain flow and temperature during dry periods.