Rivers & lakes
Rakaia River & Lake Coleridge (Image: Google Earth June 2021)
Rivers & lakes
- Our freshwater ecosystems are under continuing threats from agriculture, habitat loss due to introduced trees and weeds including willows, lupins and broom, and demands for water storage to irrigate land and generate electricity as the climate warms.
- Because of climate change, lakes are becoming more acidic and warmer, leading to less oxygen (3 to 9 times worse than the ocean) and combined with agricultural pollution, more toxic algae blooms (Fig. 3).
- Braided rivers have been confined so much that many are no longer truly ‘braided’ (Fig. 1). This confinement exacerbates the costs of floods (Fig. 2) and the loss of critical life-supporting ecosystem services including clean water and mahinga kai. Climate change is increasing these risks (Fig. 5)
- New Zealand is ranked amongst the worst in the world for lowland river and lake quality and greenhouse gas emissions.
- Management strategies must be enacted and enforced for the benefit of all not just the agricultural sector.
“Lakes and rivers occupy just 1% of the Earth’s surface but are incredible hotspots for biodiversity, sheltering 10% of all species globally. Particularly in older and deeper lakes, life has had millions of years to evolve and adapt to the peculiarities of that habitat, giving rise to unique forms. But since 1970, numbers of freshwater vertebrates, including birds, fish, amphibians, reptiles and mammals, have declined by a staggering 83% through the extraction of lake water, pollution, invasive species and disease. Now, climate change threatens to drive even deeper losses.” – Antonio Law, Keele University
“Hydrologists have recognized for some time that climate change has undermined stationarity* in water management—indeed, they have declared that stationarity is dead. But economists have by and large not recognized that this applies to climate effects. They approach climate damages as minor perturbations around an underlying path of economic growth, and take little account of the fundamental destruction that we might be facing because it is so outside humanity’s experience… In effect, economists have assigned them a value of zero, when the risks are decidedly not.” – Profs. Naomi Oreskes & Nicholas Stern
Right: Aerial image Google Earth 2018
Fig. 1: The Okuku River (top centre left) merging with the Ashley River Rakuri, which then flows downstream to the town of Rangiora. Red arrows signify the width of the braided river braidplain (scroll down). Inside the yellow line is the approximate extent of the braidplain based on the geomorphology, i.e., indentations in the land showing where the braided channels were active prior to European settlement. Back then, the braidplains were largely free of vegetation other than low-lying native herbs and cushion plants, with smaller native shrubs interspersed with wetland vegetation around springs.
The scars of old channel are evidence that these rivers periodically flowed well outside the yellow lines (Fig. 5 is a good representation of this on a small scale.) Reducing the width of the braidplain so that just a few channels are allowed to flow in a confined space has already destroyed most of these ecosystems.
Climate change is leading to more droughts, exacerbating existing ecological problems. It’s also leading to higher bursts of rainfall that exceed their artificially restricted carrying capacity rivers, leading to extreme flooding. We are now facing a vicious cycle, with property owners blaming councils for failing to protect them and demanding greater protection by further confining the rivers. Digging deeper channels by removing shingle and/or building higher stopbanks does not dampen the energy of floodwaters, which are set to increase in frequency and intensity as the climate continues to warm.
Environment Canterbury manages 59 river and drainage rating districts (i.e. areas where ratepayers contribute to the cost of flood protection). This map shows the extent of each rating district around rivers. Areas outside these zones are not protected from floods.
Under the Canterbury Water Management Strategy, the catchment of each waterway—their lakes, wetlands, springs, groundwater, and rivers that flow down to estuaries—is considered together. Managing catchments in this integrated way means problems can be considered based on each catchment’s unique attributes. These maps will help you identify which zone you are in.
Squabbles over water have been happening for thousands of years. The word ‘river’ comes from the Latin noun rivalis: rivals who use the same stream as a source of water. This saga continues in Canterbury, where an outdated approach to managing freshwater is being exacerbated by climate change. Here, rivers are seen as water sources to be mined for hydroelectricity and irrigation, flood risks to be defended against, braidplains to be converted to agriculture and forestry (Fig. 1), and drains to carry pollution to the coast. Economics drivers have turned our rivers into some of the most polluted rivers in the world (Video 1), where any drop that reaches the ocean is considered wasted:
“If things are going to get hotter, then we need to store water. It gives options and allows us to survive and thrive. New Zealand has been backward over water storage because we waste so much of it, it just flows out to sea.” – Conor English, Federated Farmers CEO 2008-2014; Agribusiness CEO 2014-
Along with many other problems, this prevents braided rivers from delivering sediment to the coast to keep pace with rising sea levels:
“The conventional wisdom is that you harvest flood water in the winter and store it until it’s needed (for agriculture) in the summer. However, floods are required to carry gravels to the coastal zone and if there’s not enough gravel, the waves get hungry and start eroding the land.” – Dr Scott Lanard, NIWA
Polluted from agriculture and urban runoff, rivers and lakes are warming, becoming more acidic, unable to produce oxygen, and frequently beset by toxic algae blooms. This threatens both freshwater and the terrestrial ecosystems, as well as us. The algae also produces greenhouse gasses when it decays (Fig. 3) and is a danger to stock and human health (Video 1). Ratepayers are increasingly required to pay councils to clean water to ‘drinkable’ standards that fall well below standards recommended by the WHO, and for flood protection (for those within river ratings districts).
Glaciers: The changing climate is causing our frozen rivers—our glaciers—to rapidly shink. Lakes levels are no longer reliable, threatening hydro-electricity generation as well as water supplies for people and stock.
Droughts and floods: As Canterbury’s climate grows ever warmer, smaller rivers and springs are likely to dry up more often. Some, permanently. Perversely, they are also likely to flood more often with potentially more catastrophic avulsions as levees and stock banks built to hold back past floods fail to meet the challenges of increasing rainfall (Fig. 5) from atmospheric rivers (see the explainer: Video 3).
“New Zealand has a potential FLHA [flood hazard area] land area of over 20,000km2, occupied by a usually-resident population of approximately 675,000. The FLHA has over 411,000 buildings with a NZD$135 billion replacement value (2016 replacement values). FLHA infrastructure network components include more than 19,000 km of roads, over 1,500km of railway, 20 airports, 3,397 km of electricity transmission lines and more than 21,000 km of three-waters pipelines.” – NIWA
Lakes: absorb CO2 from the atmosphere in the same way as oceans, becoming increasingly acidic. However, lakes also absorb CO2 from decomposing organic and inorganic matter, including pollutant runoff from agriculture. As they are becoming warmer, they can carry less dissolved oxygen. Together, this is leading to more toxic algae blooms (Fig. 3).
Dams: When a river is dammed, the blocked water that builds up behind it creates an unnatural, stagnant lake that eventually kills most of the original ecosystem that once existed there. Everything—plants, forests, the insects, microorganisms and fungi in the soils crucial for sequestering carbon—are all drowned. Bacteria and methanogens in the lake decompose the dead, emitting CO2 and methane. This bubbles to the surface adding more greenhouse gasses into the atmosphere. Damming lakes for hydroelectricity is not as ‘green’ as you might think.
“Rivers and their floodplains have the potential to act as shock absorbers to climate change, and are powerful agents for keeping wildlife and communities healthy and resilient. The most effective climate action plans will account for this and incorporate rivers into their plans for a climate-resilient future.” – Michele Thieme, freshwater scientist, World Wildlife Fund
“The Commission recommends that the Government creates a climate-resilience agency…. the new entity should also assist regional councils and communities consider the best way to lessen future risk of flooding from rivers. This would include, where appropriate, the potential for using the best-practice model of giving rivers room and developing multiple innovative uses of the wider river corridors.” – New Zealand Productivity Commission 2019 draft report
Globally, as well as here in Aotearoa, there is a growing recognition that rivers of all types must be ‘given room to move’, that is, allowed to reclaim floodplains and braidplains so that they can act as shock-absorbers to mitigate the rapidly increasing risks of climate-driven mega-floods, and to recharge wetlands, which act as nature’s kidneys.
But choices will need to be made: what to save and what to sacrifice.
Cantabrians are currently exposed to the greatest flood risk in all of Aotearoa (Fig. 2). Most of our cities and towns are built on braidplains and coastal wetlands, damaging or destroyed the ecosystems we need to help protect us in a warming world. While the pace of urban and rural river restoration programmes is accelerating, big choices still need to be made. To protect the relatively small spacial footprint of towns and cities and critical infrastructure (road, bridges, power supply, water supplies etc.) braided rivers need to be restored so that they can provide one of their key ecosystem services: dampening the effects of flooding by sacrificing less populated lands. Similarly questions must also be asked of all the other essential ecosystem services provide by biodiversity.
“River managers….cannot look to tools of old: conventional management techniques that aim to restore ecosystems to their original state. Ongoing human development and climate change mean that this is no longer possible. And models based on past correlations do a poor job of predicting how species might respond to unprecedented changes in future. A different approach is called for.” – Tonkin et al. (2019)
Planning and implementing a managed retreat from the edges of rivers, and restoring braidplains and coastal wetlands are critical preventative measure against such losses will be costly, but far less so than doing nothing, especially given the unavoidable impacts that rising sea levels will have on our coastal settlements beside these rivers, and the increasing frequency of devastating floods.
Choosing to do nothing takes away our ability to protect what’s really important to us.
Protect what remains of natural river and lake ecosystems. Refrain from building any more dams and irrigation systems. Under the RMA, braidplains are not recognised as integral components of braided rivers but agriculture continues to expand across them (Fig. 1). A change in the RMA is needed to protect them.
Remove introduced weeds and immediately stop planting more; this includes willows that have infested waterways. This would enable water flows to behave more naturally and have major benefits for wildlife.
Re-instate native plants along waterways to would enhance both aquatic and terrestrial ecosystems and help filter out pollutants, especially where waterways include wetlands, which act as the lungs and kidneys of ecosystems. In places where stopbanks have been put in place to protect towns, cities, and critical infrastructure, planting with natives links aquatic and terrestrial ecosystems.
Stop agricultural pollutants from entering waterways to prevent high nitrogen and phosphate loads downstream, including lowland lakes such as Tai Waihora Lake Ellesmere and coastal wetlands, estuaries, and oceanic ecosystems. This will help restore water quality, replenish mahinga kai, and reduce greenhouse gas emissions from the land, waterways, and ocean (particularly blue carbon).
Release strangled rivers to reduce the financial impact of floods and restore ecosystem services. This will require land currently used for agriculture on braidplains to be retired.
Enforce compliance. The ‘Essential Freshwater Package‘ aims to prevent further degradation and improve water quality. It requires regional councils to implement policies (that Fonterra and some elected officials object to, even before the additional requirement to consider climate change . See Video 1 and Mike Joy’s article):
“I remain concerned about council capacity to implement these new policies, especially given most councils were struggling to implement the freshwater reform package passed by the previous National government.” – Dr Nicholas Kirk, Environmental Social Researcher, Manaaki Whenua – Landcare Research
Braided rivers are different to ‘normal’ rivers
Braided rivers originate in Southern Alps Kä Tiritiri o te Moana and most flow all the way to the ocean. Most of the time, only a few channels or ‘braids’ of water flow between temporary shingle bars and islands (see the Rakaia River image at the top of this page). Following storms and snowmelt, water levels rise rapidly and some or all of the braids join together to become a single wide channel across the area known as the braidplain. When water levels drop, new channels and islands will appear in different places across the braidplain (Video 2).
The origin of these globally rare ecosystems began around 40 million years ago, when tectonic forces pushed Kä Tiritiri o te Moana out of the sea. Over millions of years, a staggering 20 kilometres of the Earth’s crust was uplifted. Heavy rain and snowfall (as much as 12 metres annually) and glaciers eroded the mountains, creating huge volumes of moraine gravels and sediment. Fast-flowing rivers carried the sediment down through valleys and deposited it into the shallow seafloor surrounding the mountains. So much sediment accumulated that it eventually built up to become land. The rivers flowed over this new lands, spreading out into their distinctive braids and creating deltas, depositing some gravels here, and eroding them elsewhere until they built so much and that joined into a giant ‘megafan’ that we now call the Canterbury Plains.
Figure 4 is a good example of what that would have looked like, albeit at a much smaller scale.
Flowing from the mountains to the sea, braided rivers supported some of the most biologically rich and diverse landscapes on Earth. The sudden rise of water levels in them is not only normal, it’s an essential and defining feature that gives braided rivers their name.
Without these abrupt changes in river flows, these incredibly diverse ecosystems and the services they provide—clean water, plentiful kai, and bringing sediment to the coast to keep pace with rising sea levels—would vanish along with the rich biodiversity they support.
When Europeans settled Aoteaora, they viewed these high water events as dangerous floods rather than life-giving services. They systematically forced the braids into narrow spaces by building stopbanks and levees to contain them. Willows were planted to suck up the water and to stop the floods from eroding what was rapidly being converted into ecologically barren agricultural lands. Huge volumes of water was a siphoned off to irrigate these green deserts.
Today, these past actions are still ongoing. Willows and invasive weeds like lupins and broom are adding to the destruction of these rivers, by turning temporary shingle islands into permanent ones, forcing the braids to cut deep channels that accelerate during high water flows instead of drawing off energy as they once did. Every aspect of what it once was to be a braided river ecosystem is being taken away.
But heavy bursts of rainfall over Kä Tiritiri o te Moana will intensify as the climate changes (Fig 6). The rivers will find chinks and force their way through man-made barricades, dragging fences, uprooted trees and other debris along with them, smashing them into bridges and buildings as they reach for the sea.
It’s time to give these rivers room to move. If we don’t, as the climate warms, the cost of them reclaiming their place as braided rivers will be staggering (Fig. 2).
Stationarity means that statistically, how rivers behave in the future will be similar to how they behaved in the past. For example, most of the time a river level will go up and down due to droughts or floods, to a known maximum or minimum level over long periods of time. This idea of a predictable changes over time is how river engineers, hydrologists, and insurance companies once assessed the risks of flooding or the opposite, extreme drought.
For every 1°C of warming the hydrology cycle—precipitation and evaporation—is 7% stronger. That doesn’t seem like much, but that’s just an average. In places where warming exceeds 1°C, the hydrology cycle is that much more intense: longer and more intense droughts and larger floods. ‘One-in-100-year-floods‘ are now happening more frequently, and worse, floods are becoming far more powerful than previously experienced. So the past is no longer a useful guide to what will happen in the future. Hence ‘stationarity’—the once predictable patterns of rainfall and floods—is dead.
“Avulsions are often called the ‘earthquakes of rivers’, because they are so sudden and catastrophic…. Upstream deforestation and development is adding silt to rivers in unpredictable ways. Levees and dams are altering flows and paths, sometimes worsening the threat. But levees force a river to drop silt within a constrained channel, hastening the next avulsion. Climate change is another wild card: Rising seas are shifting avulsion hot spots that occur on coastal deltas, another place where rivers slow down, drop silt, and raise riverbeds,
For all their destructiveness, avulsions bring benefits to both nature and society. They unleash regular floods that nourish many of the world’s great wetlands… And by smearing fresh sediment across flood plains and deltas, avulsions fertilize lands that have nurtured some of the planet’s great civilizations… Meanwhile, coastal deltas are facing rising seas. By eroding the coastline and pushing tides farther upstream, they cause rivers to dump more sediment farther inland, shifting the entire backwater reach upstream, along with the avulsion hot spot. We are turning the knob the wrong way. We should brace ourselves for more avulsions.” – Pierce 2021
Resources and contacts
- Environment Canterbury
- Workstreams to restore some sections of braided rivers
- Flood management strategies
- Immediate Steps Biodiversity Funding
- Ashburton Hakatere River
- Ashburton Hakatere River mouth and hapua
- Weed control projects to enhance Waihao River tributaries
- Rebuilding Whakaora Te Ahuriri wetlands
- River flow data (updated frequently along with alerts)
- Rainfall data (updated frequently along with alerts)
- BRaid: braided river aid
- Working Waters Trust
- The Styx Living Laboratory Trust
- NIWA: New Zealand Fluvial and Pluvial Flood Exposure (part of the Deep South Challenge New Zealand)
- LAWA: interactive website river flows; water quantity & quality
- (PDF) Guidelines for native planting in Canterbury
- LERNZ: Lake Ecosystem Research New Zealand
- Te Ara Kakariki/Greenway Canterbury Trust: ecosourcing native plants and seedlings
- Tonkin Lab: University of Canterbury
- Student Volunteer Army
- Canterbury Water Management Strategy – find out what catchment you’re in and which zone committee you can speak to about helping to protect and restore wetlands and waterways in your area, by clicking on the catchment maps below:
References and further reading
- Ministry for the Environment: First national climate change risk assessment for New Zealand
- NIWA/ECan: Climate change projections for Canterbury
- NIWA: Increasing flood resilience across Aotearoa
- NIWA: New Zealand Fluvial and Pluvial Flood Exposure (part of the Deep South Challenge New Zealand)
- Marine and freshwater heatwaves: modelling and prediction
- Ministry for the Environment: Issue 4: Climate change is affecting freshwater in Aotearoa New Zealand
- 2021: Reid et al; Extreme rainfall in New Zealand and its association with Atmospheric Rivers, ,
- 2021: IPBES-IPCC- Biodiversity and Climate Change Workshop
- 2021: Law; Climate change: world’s lakes are in hot water – threatening rare wildlife, The Conversation, June 08
- 2021: Harrison et al; Year-2020 Global Distribution and Pathways of Reservoir Methane and Carbon Dioxide Emissions According to the Greenhouse Gas from Reservoirs (G-res) Model, AGU:Global Biogeochemical cycles (in print)
- 2021: Jane et al; Widespread deoxygenation of temperate lakes, Nature 594, pp66–70
- 2021: Pascolinni-Campbell et al; A 10 per cent increase in global land evapotranspiration from 2003 to 2019, Nature 593, pp543–547
- 2021: Brierley et al; Why we should release New Zealand’s strangled rivers to lessen the impact of future floods, The Conversation
- 2021: Joy; Vested Interests in Big Agriculture a freshwater scientist’s personal experience, Policy Quarterly Victoria Wellington University 17|2 pp 51-55
- 2021: Pierce; When the levees break, Science 14 | 372, Issue 6543 pp676-679
- 2021: Valenza et al; Downstream changes in river avulsion style are related to channel morphology, Nature Communications 11 | 2116
- 2021: Lanard; The Future Shape of Water, Water And Atmosphere, NIWA
- 2021: Musselman et al, Winter melt trends portend widespread declines in snow water resources, Nature Climate Change 11, pp418–424
- 2021: Thieme, Rivers can be climate change solutions, too, Stuff April 12.
- 2020: Chadwick et al; Accelerated river avulsion frequency on lowland deltas due to sea-level rise, July 28, 117 (30)
- 2020: National Policy Statement For Freshwater Management 2020
- 2020: Wannan: Climate change: lakes and rivers will become drier, increasingly infectious and toxic, Stuff April 30
- 2019: Oreskes & Stern, Climate Change Will Cost Us Even More Than We Think, NY Times, Oct 23
- 2019: Productivity Commission: Local government funding and financing (financing and management of rivers given the climate change risks)
- 2019: Joy & Talbot-Jones (eds.); A Potted History of Freshwater Management in New Zealand, Policy Quarterly, Victoria Wellington University Vol 15:3 August 2019
- 2019: Paulik et al (NIWA); Coastal Flooding Exposure Under Future Sea-level Rise for New Zealand
- 2019: Paulik et al (NIWA); New Zealand Fluvial and Pluvial Flood Exposure
- 2019: Deep South National Science Challenges
- Conference: water in our changing climate (Youtube video)
- 2019: Tonkin et al; Prepare river ecosystems for an uncertain future, Nature 570, pp301-303
- 2018: Hassler et al; Biological consequences of weak acidification caused by elevated carbon dioxide in freshwater ecosystems, Hydrobiologia 806, pp1–12
- 2016: Deemer et al; Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis, BioScience, 66|11 pp949–964
- 2015: Zarfi et al; A global boom in hydropower dam construction, Aquatic Sciences 77, pp161–170
- 2008: Milly et al; Stationarity Is Dead: Whither Water Management? Science 319 | 5863, pp573-574
- Merriam -Webster dictionary: It Takes Two: The History of ‘Rival’
- (undated): Hearnshaw et al; Ecosystem Services Review of Water Storage Projects in Canterbury: The Opihi River Case
- 2000: St. Louis et al; Reservoir Surfaces as Sources of Greenhouse Gases to the Atmosphere: A Global Estimate: Reservoirs are sources of greenhouse gases to the atmosphere, and their surface areas have increased to the point where they should be included in global inventories of anthropogenic emissions of greenhouse gases BioScience 50 (9) 766–775