Partnership safeguards significant braided river ecosystem in the upper Waimakariri
Environment Canterbury media release: 17th September 2024
Two years into the ten year Upper Waimakariri River Weed Control Strategy, promising strides are being made towards realising a weed-free braided river network with flourishing native plants and wildlife.
The strategy is driven by the Waimakariri Environment and Recreation Trust. It is an integrated approach to managing invasive pest plants in a partnership between us and Land Information New Zealand, Selwyn District Council, Department of Conservation, University of Canterbury, NZ Transport Agency Waka Kotahi, KiwiRail, community groups and landowners.
Since its implementation, over 20 priority sites have received significant work to reduce weedy infestations, with two sites already meeting the criteria for surveillance status — a maintenance phase only achieved when weed presence has been nearly eradicated.
Values of the upper Waimakariri River catchment
The Waimakariri River is one of the largest braided river ecosystems in Waitaha/Canterbury — a dynamic web of channels that runs 150 kilometres from Kā Tiritiri o te Moana/the Southern Alps out to the sea.
A braided river flows in multiple, mobile channels across a gravel riverbed and wider braidplain over some parts of its length. Braided rivers are a naturally rare ecosystem in Aotearoa and worldwide.
The upper reaches of this riverine landscape are of outstanding biodiversity value. Supporting numerous native flora and fauna species, the catchment serves as a habitat for indigenous birds, lizards, fish and invertebrates.
Find out more about the importance of braided rivers.
Braided rivers like the Waimakariri also have significant cultural importance to mana whenua and support Ngāi Tahu (Kāi Tahu), tikanga Māori (customary practices) such as mahinga kai (mahika kai), the gathering of food and other natural resources.
Unfortunately, the wellbeing of these precious ecosystems is severely threatened by land conversion, water abstraction (removal of water for drinking, primary production, industry etc.) and invasive plant and animal species.
Pest plants threaten biodiversity and river morphology
Exotic and weedy pest-plants like gorse, scotch broom, lupins, and willows threaten the fragile balance of life in this catchment — reducing habitat for native fauna and providing shelter for predators.
Additionally, invasive plant life can even change the dynamic way that braided river channels move across a river plain. Dense weedy colonies growing in and along a braided river system can stabilise gravel, restricting water flow and river gravel deposit.
This restricted movement has major impacts on many of our native bird species, like the nationally endangered ngutu pare/wrybill, which rely on raised beds of bare river gravel to nest and breed.
"Many of the weedy invaders we have recorded in this area are garden-variety escapees from private land in the surrounding area,” says Will Todhunter, biodiversity and biosecurity advisor.
"To understand where the pest plants are coming from, we’ve had to take a catchment-wide approach, surveying the wider landscape including the surrounding forest, wetland and alpine ecosystems."
This work involved extensive ground and ariel surveys, which were used to inform a comprehensive map showing the weed density of the upper Waimakariri River basin.
From there, experts came together to finalise the strategy to attack problematic plant infestations to contain, or where possible, eradicate the unwanted species.
Weed interventions show progress
After just two years, promising initial gains have been made in tackling large seed sources of weeds and initiating control at new sites.
"The weed fighting game is a long-term one, meaning maintenance works will be critical over many years to keep the pest plants at bay.
"However, it’s been encouraging to see what can be accomplished when like-minded agencies, passionate community organisations and landowners come together," said Todhunter.
Looking ahead, work is planned to further protect the Craigieburn Forest and surrounding land from the invasion of ‘homestead weeds' like sycamore, holly, cotoneaster and cherry laurel.
The upper reaches of the Waimakariri will also see some interventions, as well as Arthurs Pass National Park to fight Russel lupin and Spanish heath.
Progress every year is measured and evaluated against the Upper Waimakariri River Weed Control Strategy [PDF].