A rocky ride for freshwater species

Fish in the lower reaches of the Hekeao/Hinds River will find a slightly more interesting environment in the coming months.

A cluster of huge boulders have been installed in the riverbed near Longbeach. They may look out of place – and they are, coming from South Canterbury – but they will help create a better river.

The boulders were placed in the river as part of a pilot programme that aims to improve diversity in river conditions – and create better habitats for a range of species.

Funding for the operation was provided by Fonterra for improving waterways in mid and south-Canterbury.

Environment Canterbury Land Management Advisor Lachie Ashton says the point of installing boulders is to create different flows of water with a pool, run, riffle effect.

Pool, run, riffle effect

Unlike most manmade drains and canals, natural waterways are not uniform – they vary in water flow, speed and depth. These variations create different habitats for a greater range of species.

Immediately downstream of the new boulders in the Hekeao, deeper and slower-flowing water forms a pool, perfect habitat for trout. Towards the north bank, the water flows faster over a shallow channel, known as a riffle. Other sections of river nearby form runs – smooth, steady flowing sections of river.

Together, the diversity of water flows create a habitat for a greater range of species.

Motorists crossing the state highways at Mayfield and Hinds over summer may question whether there is even a habitat for freshwater species, with most of the Hekeao / Hinds River running dry.

But while the water that seeps into the ground upstream, much of that same groundwater reappears in springs that feed the river’s lower reaches near the coast.

Waiting for results

During heavy rains, the lower reaches of the river experience periodic floods. When that happens over the wetter winter months, the full effect of the boulder planting will be seen.

Flooding is all part of the project,” Ashton says. “We need to let the river do its natural process and just wait and monitor the effects.”

If the pool, run, riffle effect holds over a several seasons, the operation will be deemed a success, and we may see more boulders going into the river over the coming