Once widespread, shifting and full of life, Aotearoa New Zealand’s braided rivers are now among our most endangered ecosystems. At Te Whare Wānanga o Waitaha | University of Canterbury (UC), researchers from the Faculty of Science are working to uncover their hidden connections and show how we might help them endure and thrive.
Formed by floods and shifting gravel, braided rivers weave wide, ever-changing channels that create unique habitats for a diverse range of species. During her PhD at UC’s School of Biological Science, Dr Holly Harris took a whole-ecosystem approach to understanding how braided rivers function and how we can restore their resilience.
Globally uncommon ecosystems, many braided rivers in New Zealand are now under threat. While some have been altered for hydroelectric schemes, the lowland rivers now face gravel and water extraction, land encroachment and invasive weeds. Instead of sprawling, multi-channel systems, these rivers are being squeezed into narrower channels with far fewer braids. The loss of this natural variability strips away resilience linked to habitat diversity, which leaves native plants, fish and birds that depend on these habitats increasingly vulnerable.
“The connections between different parts of the river are very important,” Dr Harris says. “While one part of the river is flooded, others, especially springs, offer refuges. The variety of habitats collectively provide a productive larder for birds like the black-fronted tern, banded dotterel and wrybill that depend on the fish and insects, but the refuges ensure a resilient food supply.”
To trace these complex interactions, Dr Harris used isotopic analysis of species across the river, tracking who eats whom. She also undertook the painstaking task of catching, measuring and identifying fish and invertebrates, while recording bird counts along the riverbed. By combining this detail with measurements of total biomass across different channels, she revealed how all these components contribute to a river’s stability over time.
The research highlights a bigger issue: the resilience of braided rivers comes from processes acting on a landscape scale. Dr Harris says, "Despite appearing like a wide plain of bare gravel, braided rivers are actually teeming with life. Each different channel, and even different parts of the gravel plain, support unique groups of species. That variability is where the resilience comes from.
“Local fixes like flood protection barriers may solve short-term problems but they also simplify the ecosystem, and that reduces habitat diversity and weakens resilience.
“A national approach to braided rivers that reflects their variability, and importance would guide more inclusive and effective policies that support conservation efforts and potentially complement better long-term community flood outcomes. This would allow for natural processes like braiding and flooding to occur more freely, promoting habitat diversity, ecological resilience and the capacity of these systems to adapt to climate change and other stressors.”
For Dr Harris, the work is about more than science. It’s about using this knowledge to find solutions that carry over into an uncertain future. “Braided rivers are beautiful and cherished landscapes for many Kiwis. By understanding how they work as a whole, we have a chance to keep them thriving, along with the birds and biodiversity that call them home.”
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