Habitat loss

Human activities (weeds, agriculture, river engineering, and built structures) have reduced the physical area of braided river ecosystems and damaged their hydrology. These historic activities and mismanagement should not now be viewed as a license to exploit these (often weed-infested) damaged spaces in order to increase or protect personal or corporate profit margins. Allowing marginal/ephemeral spaces on braided rivers to be available for agricultural or other development will continue an ever-increasing cycle of contraction until braided rivers as ecosystems vanish, systematically squeezed into becoming ‘normal’ rivers.

BRaid’s response to the BRIDGE project to define a braided river

Braided rivers are an iconic and highly valued feature of the Canterbury landscape. They are also globally rare landscapes known as a braidplains (explained below), and as such, New Zealand has a special responsibility to protect them and the plants and animals that evolved to live here and nowhere else on Earth.

But the historic attitude towards rivers as water sources to be exploited and hazards to be controlled, and the perception that the braidplains these rivers inhabit were ‘wastelands’ to be converted into ‘productive’ forestry and agriculture, has forced these once-mighty rivers into ever-narrowing confined spaces.

Ecosystem services

Hydraulically-connected to the main river channels are the springs, wetlands, and small streams on the braidplain that act as lungs and kidneys of braided rivers. The biodiversity they support provides life-giving ecosystem services. They filter water and provide healthy habitats for plants, animals, and people. And they provide clean and plentiful mahinga kai.

It’s only relatively recently that the lifegiving ecosystem services these rivers provided have been recognised. But they’re now badly compromised due to over-exploitation. To reverse that, one of the targets in the Canterbury Water Management Strategy (CWMS) is to ‘protect the natural character of braided rivers’ and to ‘reverse the decline in braided rivers bird habitats’. These ecosystems and characteristics can only exist within braidplains.

To meet these goals, along with with with other research the regional council, Environment Canterbury (ECan) commissioned a report that ultimately revealed an astonishing  11,630ha of formerly undeveloped or forested river margin (braidplain) had been converted to intensive agricultural use between 1990 and 2012.

The dynamic nature of braided rivers is to change, primarily laterally over time, (what has been referred to as a ‘fourth dimension’). A defining feature of braided rivers is that during high water flows, their multiple channels often join into one single channel that fills the entire braidplain. In alpine areas, the braidplain is confined to alpine valleys.

Defining a braidedplain

On the plains, the braidplain is the area that has been occupied (indeed formed by) the active channels of the river at some time in the past—and this can be a very wide area (see the grey box below). When the water recedes, new channels may have migrated to different locations within the braidplain.

Thus, the braidplain is integral to the existence of braided rivers. Taking away braidplains kills braided rivers in the same way that taking away a kiwi’s habitat and placing it in a box would ultimately lead to the extinction of the species.

But the term ‘landscape’ and ‘natural character’ are not clearly defined under the Resource Management Act. Nor does the Act define braided rivers and braidplains. The current High Court ruling defines braided rivers in the same way that European rivers are defined: “...the space of land which the waters of the river cover at its fullest without overtopping its banks.”

This wildly inappropriate definition does not in any way describe the dynamic 4-dimensionality of braided rivers and the iconic braidplains they inhabit. Nor does it take into account the hydraulic connection between the active river channels, wetlands and springs that inhabit the braidplain. And so, the conversion of these globally rare ecosystems into agricultural lands continues and braided rivers head slowly towards extinction.

Riverbed, Floodplain or Braidplain: what's the difference? And why does it matter?

“To be effective, conservation efforts in glaciated mountain landscapes intended to benefit the widest variety of organisms need a paradigm shift that has gravel-bed rivers and their floodplains [ie, their braidplains] as the central focus and that prioritizes the maintenance or restoration of the intact structure and processes of these critically important systems throughout their length and breadth.”

– Hauet et al, Gravel-bed river floodplains are the ecological nexus of glaciated mountain landscapes Science Advances  24 Jun 2016: 2/6

The above image shows the Hurunui River braidplain on the Amuri Plains (formed by the same processes as the Canterbury Plains) facing west to the foothills of the Southern Alps. This illustrates the difficulty in defining ‘braidplains’ on lowland plains. Scroll down for an explanation of the terms ‘braidplain’ and ‘floodplain’. (To open this location in Google Earth, download this kmz file and open it from within the Google Earth ap.)

Riverbeds and floodplains

floodplain100

For planning and insurance purposes, flooding is said to occur when a river overtops its banks. Floodplains are defined as the area that a river is likely to flood. For example, a ‘1-in-10-year’ floodplain is a given area likely to flood once every 10 years. But terms like ‘1-in-10’ or ‘1-in-100’ are statistical probabilities based on historical data. In any given year, a ‘1-in-10’ year flood has a 10% chance of happening, while a ‘1-in-100’ year flood has a 1% chance of occurring.

Virtually all rivers in Europe have been (or are about to be) confined, channelled, redirected, harnessed for irrigation, water supply, and/or hydroelectricity production, and/or to prevent flooding.

When Europeans arrived in New Zealand, they regarded braided rivers in the same way as European rivers: resources to be exploited and potential hazards to control.

Braided river within a braidplain

braid-plain-over-time

Illustration adapted from: NIWA Braidplain Delineation Methodology (Fig 1.1 p7)

New Zealand’s braided rivers were formed when the glaciers that carved out the Southern Alps came and went over millions of years. When glaciers retreated during warmer periods* rivers reworked the gravelly glacial moraines left behind in the wide U-shaped valleys, creating temporary shingle islands separated by shallow interweaving channels or ‘braids’ across the valley.

Around three million years ago, the Canterbury coastline was located much further inland at the foothills of the Southern Alps. Where the braided rivers flowed down out of the mountains, they deposited so much gravel that giant fan-shaped deltas formed along the coast. Over millions of years these deltas grew eastwards, spread out, and ultimately joined one another to become the lands we now call the Canterbury and Amuri Plains.

In a process that continues today, when heavy rain or spring snowmelt flows down these rivers, the braids join, filling the valleys with fast-flowing water. When the water recedes it leaves behind a very different looking network of river braids and gravel islands.

This process is not a ‘flood’. It’s the very definition of a braided river’s dynamic behaviour. Its natural state is four-dimensional; that is, it’s not a waterway that generally stays confined within relatively stable banks over long periods of time. It’s a highly mobile system that rapidly changes the location and depth of its channels and islands over short periods of time.


*Interglacials: the current interglacial, the Holocene epoch, began ~11,500 years ago.

Different braidplain settings

braidplains

Illustration:  NIWA Braidplain Delineation Methodology (Fig 1.3 p9)

Not all braided rivers are the same. Defining ‘braidplains’ in the high country is relatively straightforward because they’re confined or partly confined by valleys formed by glaciers. But once the rivers leave the mountains and flow onto the Canterbury and Amuri Plains, they’re no longer confined. Instead, they wander across the plains. Working out exactly where a specific braidplain is for each river on the plains is much harder and may not, in fact, be possible in some cases.

The Waimakariri River, for example, has historically moved back and forth between its current location, to south of the Banks Peninsular at Lake Ellesmere, and then back again. The city of Christchurch is located on the Waimkarariri ‘braidplain’ delta.

To protect Christchurch from floods, the river has been confined by engineering works so that it now follows a much narrower ‘braidplain’. By the time the Waimakari reaches the ocean, it’s confined to just one channel, so that there is no more braiplain at the river mouth.

While this protects Christchurch from periodic river flooding, it also prevents the once-braided river from spreading sand across the coastal braidplain. Instead, much of the sand now flows into the ocean. Waves still carry some of this sand onshore to the beaches, but not enough to keep pace with rising sea levels. Protecting coastal cities and towns like Kaipoi from river flooding has, perversely, helped compromise the coast’s natural defences against inundation from the sea.

Can we protect what remains?

For legal and management purposes, lines do need to be drawn on maps to define ‘braidplain’. Without a clear definition, braided rivers cannot be protected, their unique fragile ecosystems restored, and the life-supporting ecosystem services they provide including as buffers against some of the impacts of climate change be re-instated.

They must also be managed so they do not destroy critical infrastructure.

In order to find a balance between these desired outcomes, one way of defining ‘braidplains’ is to consider them in a relatively modern context: that is, the space they have naturally occupied in the last 100 years.

This can be worked out by the location of terraces carved out by the rivers (see the Google Earth image above) and/or for sake of a commonly understood reference point, 1-in-100 year flood maps and/or aerial photos.

ECan’s approach to determining this necessary compromise has been twofold:

Braid’s responses to the BRIDGE project are here:

The process, and therefore the ability to protect what remains is in limbo due to the Dewhirst High Court ruling.

Further information & references

Additional information

  • New Zealand rivers carry a staggering 400 million tonnes of sediment from land to sea every year. Sediment yields from the Cropp River in the Southern Alps and Waiapu River near East Cape in the North Island are among the highest in the world. The Waiapu River, for instance, carries 35 million tonnes of sediment per year; the equivalent of a five-tonne truckload every 12 seconds.
  • The Canterbury Plains are primarily reworked glacial outwash up to 3 million years old, however around 2 million years ago, Mt Horrible near Timaru erupted, pouring out a 5m thick layer of basalt over some 130 sq km of the plains.
  • Section 6(e) of the Resource Management Act has provisions for “protection of the relationship of Maori and their culture and traditions with their ancestral land, water, sites, wahi tapu, and other taonga“. Maori regard braided rivers as taonga, and thus should be protected. But are braidplains ‘landscapes’? The Environment Court has suggested this definition: Landscape means the natural and physical attributes of land together with air and water which change over time and which is made known by people’s evolving perceptions and associations [such as beliefs, uses, values and relationships]”. 

  • Where braided rivers occur elsewhere in the world they are recognised as being largely ‘untamable’ and afforded different levels of protection. See for example Iceland Ministry for the Environment Iceland Protected Areas
  • The Tragedy of the Commons (simple explanation of Hardin’s treatise)

References