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Globally, braided rivers are rare. They exist 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, with 59% of our braided rivers 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 (see aerial footage of some of our braided rivers here).
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.
Click on the image of the Tasman River above, and then use your mouse to scroll 360 degrees to see Aoraki Mt Cook, the Tasman Glacier just to the right, and Lake Pukaki (composite 47 photos taken from an elevation of 120m using a Mavic Pro drone).
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, flooding 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. 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.
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 bastions of biodiversity in the Canterbury Plains, forming a vital ecological link from the mountains to the sea. However, in spite of evolving strategies to live in an ever-changing and volatile environment, many braided river species are declining at an alarming rate. Several are at risk of extinction. The reasons for this are discussed here.
Most river beds fall under the responsibility of regional councils, the Department of Conservation and Land Information NZ. However, some river beds are in private ownership where adjacent landowners have property rights to the middle of the bed, Ad medium filium or more commonly known as AMF rights. This uncertainty over jurisdictional boundaries complicates the management of braided rivers because stakeholders often have conflicting priorities.
Land Information New Zealand (LINZ) has responsibilities for managing the control of pest plants (weeds) on Unallocated Crown-owned Lands (Crown riverbed), on behalf of the Crown, in braided rivers.
Use of water from the rivers, and pollutants going into the river, are under the jurisdiction of the regional territorial authority. In Canterbury, this authority is Environment Canterbury, commonly known as ECan. ECan manages water as a resource through the Resource Management Act 1991.
Following a drought in 1998, the Ministry of Agriculture and Forestry, Ministry for the Environment, and ECan developed the Land & Water Regional Plan for Canterbury, with leadership from the Canterbury Mayoral Forum. Under this Plan, the Canterbury Water Management Strategy (CWMS) is being developed to address water related issues in the region through district Zone Committees, each with their own Zone Implementation Programme (ZIP). The Hurunui-Waiau Zone, for example, now 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.
Land, Air, Water Aotearoa (LAWA) was established by like-minded organisations 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 now a partnership between the councils, Cawthron Institute, Ministry for the Environment and Massey University.
In 2015, ECan released a report showing that between 1990-2012, some 11,300ha of formerly forested or undeveloped berm land was converted to intensive agricultural use.
The encroachment of ‘dairy’ farm development into the flood plains of Canterbury’s major braided rivers is an increasing problem that seriously threatens their ecological ‘integrity’. These undeveloped floodplains, despite being typically weedy, are integral to the long term ecological health and functioning of braided rivers. They also are important buffer zones and provide habitats for native flora and fauna generally. They are also valuable for their future restoration potential being the only substantial areas remaining in lowland Canterbury that have not been developed.
The impacts of encroachment into braided river flood plains can be seen clearly overtime in the smaller Canterbury braided rivers – Opihi, Pareora, Hinds , etc. These were once proper braided rivers but their natural flood plains have been reduced and confined to a relatively narrow channel, and their habitat values for braided rivers birds has been much reduced as a result. Alas the same encroachments and loss of habitats is happening to our major braided rivers.
* ‘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.