Our Braided Rivers

Our Braided Rivers

What's a 'braided' river?

Globally, braided rivers are rare. They occur only where a very specific combination of climate and geology allows rivers to form ever-changing and highly dynamic ‘braided’ channels across a wide gravelly riverbed (see ‘braidplains’).

New Zealand is a braided river hot-spot. Some 59% of our braided rivers are in Canterbury, with a catchment of over 140,000ha. Indeed, the entire Canterbury Plains was formed by sediment and gravel carried from the Southern Alps by braided rivers. Southland, the West Coast and Otago regions also feature spectacular braided rivers such as the Makarora River that flows from Mt Aspiring National Park into Lake Wanaka.

rivers_graph

Why do rivers 'braid'?

Braided rivers both erode and deposit gravel, depending on the gradient of the river and speed and volume of the water, which changes according to the amount of rainfall and/or snowmelt. The volume and speed of the water is measured in cubic metres per second or cumecs. During low flow periods (low cumecs), gravel is dropped and the water ‘braids’ into smaller channels around temporary gravel islands. Following storms and snowmelt, the rivers rise (high cumecs) and some or all of the braids coalesce into a wide single channel that covers and sometimes washes away the gravel islands.

While fresh flows or ‘freshes’ wash the river clean, the water generally stays confined within the wide, pre-formed braided river banks. This area is known as the braidplain. Periodically the flow is so great (very high cumecs) that in places where the rivers are not confined by mountains, they may shift across a wide area. As these waters can inundate land and destroy property, it’s called a flood. In fact, it’s the process that built the Canterbury Plains.

Why are they special?

Centuries of development have largely robbed the Canterbury Plains of its forest, wetland, and coastal habitats. The biologically rich braided rivers are one of the last remaining strongholds of biodiversity in the Canterbury Plains, forming a vital ecological link from the mountains to the sea. However, while braided river plants and animals evolved strategies to live in these dynamic and volatile environments, many species are declining at an alarming rate. Several are at risk of extinction. The reasons for this are discussed in the section on threats and the section below.

Creating the Canterbury Plains

Around 40 million years ago, tectonic forces pushed the edges of two plates together along the Alpine Fault. This mountain building or orogeny created the Southern Alps – Kä Tiritiri o te Moana. Over millions of years, a staggering 20 kilometres-thick section of the Earth’s crust was uplifted. Most of it was eroded by heavy rain and snowfall (as much as 12 metres annually) over the mountains. Glaciers came and went, grinding through the Southern Alps, creating huge volumes of moraine gravels and sediment. With so much precipitation, fast-flowing rivers carried the gravels and sediment to the coast east of the Main Divide, depositing the material into the ocean until it built into deltas.  Over time, the deltas joined into alluvial fans until they eventually created the giant ‘megafan’ that we now call the Canterbury Plains.

The process continues today, with the mountains continuing to rise around 20mm/year, about the same rate as they’re eroding, ensuring an endless supply of sediment as rainfall strips an estimated 10,000 tonnes/km2 annually from the mountains.

Losing the Canterbury coastline

In spite of this, now that many of our braided rivers are confined by engineering works (stop banks, levees, and dams) to stop flooding, and water is withdrawn for agriculture, the power of the rivers to deliver sediment to the coast is severely diminished. The sediment that once replenished coastal lands, protecting them against erosion and rising sea levels, is instead being carried out to sea.

Ashley-Rakahuri River

Who is responsible for managing our braided rivers?

Conflicting priorities, uncertainty over jurisdiction, and confusion over the definition of what exactly is a braided river have led to in their degradation. The following is a brief outline of a contentious history that’s still unfolding.

1991: Water resources in New Zealand are managed through the Resource Management Act 1991 (RMA). Under the RMA, any activity that may affect rivers requires an application for a Resource Consent outlining those activities and how any potential damage will be prevented. However, the RMA defines riverbeds much the same way they are defined elsewhere in the world, as ‘the space of land which the waters of the river cover at its fullest flow without overtopping its bank’.

But by definition, large sections of these globally rare types of rivers, ie, the riverbeds (braidplains) are dry most of the time. This has resulted in the ‘dry’ areas either side of the active channels being instead thought of as ‘land’ that could be converted to agriculture.

The problem is compounded because rivers are managed in different ways by:

  • Land Information New Zealand (LINZ), responsible for managing pest plants (weeds) on Unallocated Crown-owned Lands (‘Crown’ riverbed)
  • Department of Conservation (DOC), responsible for managing rivers on Conservation lands.
  • Regional Councils, responsible for managing water resources (including riverbeds) through RMA.
  • AMF rights; some landowners have historic property rights to the middle of the riverbed. These are known as Ad medium filium (AMF) rights.

1998: Following a drought, the Ministry of Agriculture and Forestry, Ministry for the Environment, and one of the Regional Councils, Environment Canterbury (ECan), develops the Land & Water Regional Plan. Under this Plan, the Canterbury Water Management Strategy (CWMS) is to address water-related issues in the region through district Zone Committees, each with their own Zone Implementation Programme (ZIP).

2010: While Canterbury farms are rapidly being converted to dairy farms courtesy of irrigation schemes, the democratically elected regional council, ECan, is sacked by then MP for the Environment Nick Smith, because it had, ‘Failed to introduce a water plan for the region, allowing it to make the most of its alpine water and reap the economic rewards of large scale irrigation.’ (Radio NZ). The Council is replaced by non-elected Commissioners.

2012: Land, Air, Water Aotearoa (LAWA) is established with a view to helping local communities find the balance between using natural resources and maintaining their quality and availability. Initially, a collaboration between New Zealand’s 16 regional and unitary councils, LAWA is a partnership between the councils, Cawthron Institute, Ministry for the Environment and Massey University.

2015: ECan, still led by Commissioners, releases a damning report showing that between 1990-2012, some 11,300ha of formerly forested or ‘undeveloped land’ on braidplains had been converted to intensive agricultural use.

2016: Dairy farming conversion, facilitated by irrigation, continues to grow at a rapid pace. Canterbury now has more dairy cows per hectare than any region in New Zealand.

dairycows
Waiau River and braidplain 1966
Waiau River and braidplain 1966
Waiau River and irrigated braidplain 2019

2017: The Braided River Action Group (BRAG) is convened in response to the above report on the loss of braided river habits to agriculture. Comprising representatives from ECan, Territorial Authorities, Te Rūnanga o Ngāi Tahu, DOC, LINZ, Forest & Bird, Fish & Game and Federated Farmers, the members have a range of roles, responsibilities, and interests in braided rivers. There is also a range of existing policy and regulation. The focus of the group is the seven large, alpine-fed braided rivers: Clarence/Waiau Toa, Waiau, Hurunui, Waimakariri, Rakaia, Rangitata, and Waitaki rivers); and three foothill-fed rivers: Ashley/Rakahuri, Selwyn/Waikirikiri (part), and the Ashburton/Hakatere.

2017-2018: The continued encroachment of dairy farm development into the braidplains of Canterbury’s major braided rivers seriously threatens their ecological integrity. ECan releases more reports (Braidplain Delineation Methodology and Braided Rivers: Natural characteristics, threats, and approaches to better management) to assist in defining the boundaries of braided rivers so that they can be better managed under the RMA. ECan also launches the Bridge project, to work with stakeholders to develop a common approach to managing braided rivers.

2019: The Bridge project is halted following a High Court decision in December 2018 that the RMA definition of a riverbed does not include braidplains. There are currently no statutory rules that prevent the continued conversion of these globally rare ecosystems into intensive agriculture. Environment Canterbury intends to appeal the decision.

Waiau River & irrigated braidplain 2019

Further information & references