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About our braided rivers

[Quick link: What birds are found on which rivers].

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.

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 (see aerial drone footage of some of our braided rivers here).

Distribution of braided rivers in New Zealand

How braided rivers formed the Canterbury Plains

As glaciers came and went over the past 3 million years, they ground through the Southern Alps, creating huge volumes of moraine gravels and sediment. With precipitation as much as 12 metres a year at high altitudes thanks to the powerful north-westerly winds, fast flowing rivers carried the gravels and sediment to the coast east of the Main Divide, eventually depositing so much into the ocean that deltas formed and the coastline grew eastward. Thanks to plate tectonics, the relatively young mountains were uplifted at some 20mm/year, about the same rate as they were eroded, providing an endless supply of sediment. 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 rainfall stripping an estimated 10,000 tonnes/km2 annually from the mountains.

Why do the rivers ‘braid’?

Braided rivers both erode and deposit gravel, depending on the gradient of the river and speed and volume of the water, which is dictated by the amount rainfall and/or snow melt. 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 the water ‘braids’ into smaller channels around temporary gravel islands. Following storms and snow melt, the rivers rise (high cumecs) and some or all of the braids coalesce into single flow, covering and sometimes washing 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 know as the braidplain.  Periodically however, the flow is so great (very high cumecs) that rivers overtop their banks and flood the surrounding low-lying land. Here they deposit gravel and sediment, continuing the natural processes that built the Canterbury Plains.

Why are Canterbury’s braided rivers so 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 here.

Who is responsible for managing our braided rivers? A complex and contentious history that’s still unfolding.

Conflicting priorities, uncertainty over jurisdiction, and confusion over the definition of what exactly is a braided river has led to in their degradation. The following is a brief outline of the key points.

1991: Water resources in New Zealand are managed through the Resource Management Act 1991 (RMA). Under the RMA, any activity that may effect rivers requires an application for a Resource Consent outlining those activities and how any potential damage will be prevented. However, the RMA defines riverbeds as ‘the space of land which the waters of the river cover at its fullest flow without overtopping its bank’.  But by definition, braided riverbeds (braidplains) are largely 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). [Fast forward a few years and the Hurunui-Waiau Zone, for example, has an operational Regional Plan under the CWMS, called the Hurunui-Waiau River Regional Plan. Interested in attending, or making a submission to, the ZIP public meetings in your area? See our calendar ].

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.

Land use change on the margins of lowland Canterbury braided rivers, 1990 - 2012

Land use change on the margins of lowland Canterbury braided rivers, 1990 – 2012

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.

2017: The Braided River Action Group (BRAG) is convened in response to the above report on 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.

Further information

* ‘Main Divide’

Refers to the area of the Southern Alps that divides the water catchments of the eastern side of the island from those on the west coast. The Main Divide also forms the boundary between the Canterbury and West Coast Regions.

** ‘Foothill-fed and Alpine-fed’

‘Alpine or Foothill’ refers to the headwaters of the rivers. ‘Alpine’ means the waters come from rain and snow falling in the Southern Alps. ‘Foothill’ means it comes from rain in the eastern foothills. As these foothills receive considerably less rain and virtually no snow, their flow regimes are quite different from Alpine-fed rivers.

References

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