Life in the fast and slow lanes of braided rivers

In the ever-shifting streams and channels of a braided river, creatures must adapt to change. Claire Concannon joins a researcher on the spectacular Cass River near Tekapo for a spot of electrofishing and bird counting - part of a project seeking to understand this complex ecosystem and the threats it faces.

It's a blue-sky spring day in the Canterbury high country. The sun beams and the wind is whipping down the wide gravel plains of the braided Cass River.

To the left is the Gamack Range. To the right, the vivid turquoise of Lake Tekapo. Black-fronted terns call out as they drift above the weaving and winding water channels, on the hunt for fish to deliver as gifts as part of their courtship.

And standing in the middle of one of the streams, water up to her knees and a heart-shaped metal wand stuck into the fast-flowing water, is University of Canterbury PhD candidate Holly Harris. She is electrofishing.

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Changing channels

Braided rivers are rare worldwide. In New Zealand they are classed as naturally rare habitats, but we are lucky in comparison to other countries. There are a few in the south of the North Island, but most can be found in Te Waipounamu.

If you've flown over the South Island you'll have seen them in all their glory - silver snaking braids of water in wide gravel floodplains making their way from the Southern Alps to the ocean.

They are complex and dynamic ecosystems. There are springs that feed side streams, and water can flow through the gravel under the surface to pop up in other places. As the seasons change, floods wash out main braids and give life to side braids. The gravel shifts and the channels can change course. It's an intricate dance that the creatures that live here have learned to adapt to.

Life and death

As in every ecosystem, in braided rivers there is an interconnected web of things eating other things. The freshwater invertebrates eat the algae that grows on settled river stones. The fish eat the invertebrates. The birds eat the fish, lizards and terrestrial invertebrates.

As part of Holly's PhD she wants to better understand the elaborate interplay between life in the river: how it is interconnected, and how it can move and adapt to the changing channels. She uses isotopic analysis of different species to investigate this. …

Go to this episode on rnz.co.nz for more details