Flooding is the most common natural hazard in Waitaha/Canterbury. Find out how we manage floods. Flooding is the most common natural hazard in our region.
Don’t be fooled by the sunshine: El Niño could cause Canterbury rivers to rise rapidly
A hot, sunny day might seem like the perfect time to head to one of Waitaha Canterbury's rivers. But while Canterbury enjoys hot, dry weather, heavy rain could be falling in the headwaters of alpine rivers.
This summer, what's happening upstream could matter more than the weather above your head.
El Niño conditions have strengthened considerably since mid-winter and are expected to influence Canterbury’s weather through spring and summer. For the region, this is likely to bring stronger westerly and nor’west winds, drier conditions east of the Main Divide and periods of heavy rain in the Southern Alps.
That combination creates the risk of what is sometimes called “sunny day flooding”. While it can be hot and fine across the Canterbury Plains, heavy rain may be falling in the headwaters of our alpine rivers, causing water levels downstream to rise rapidly.
It means a riverbed that looks dry and safe when you arrive may look very different a few hours later – even if the sun is shining.
Why El Niño increases the risk
The strong, hot, dry wind of Canterbury’s infamous nor'westers usually means heavy rain in the upper reaches of our alpine rivers such as Waiau-uha, Hurunui, Waimakariri, Rakaia, Rangitata, Waitaki (and Waitaki Lakes) and Ahuriri.
This kind of rain has a significant impact on what’s happening in our local waterways, flood controller Nick Griffiths says.
"The northwest conditions that generate rain in the alpine parts of these large catchments catch many people out – they cause hot and sunny weather on the plains, coupled with rapidly rising river levels and the potential for these waterways to flood with very little warning," Nick says.
Canterbury’s wide, braided rivers can make the risk harder to spot. People may cross a shallow or dry channel to reach part of the riverbed, only for rising water to cut off their route out later in the day.
A risk that's caught people out before
The risk of sunny day flooding isn't just theoretical.
Regional Park Field Ranger Quentin Garlick has seen vehicles become stranded and even washed downstream when people have been caught out by river levels rising unexpectedly, despite fine weather where they are.
In one past incident, five people had to be rescued from the roof of their vehicle in the Waimakariri River by the Westpac Rescue Helicopter, after becoming trapped by floodwaters. On another day, three vehicles became stuck in the river.
"It can be a perfectly sunny day and there's no water at all coming down the river when suddenly it floods," says Quentin Garlick.
A Four Wheel Drive that got caught out by sunny-day flooding
Check the weather before you go
"It can happen so quickly. The problem is that people aren’t checking weather conditions in the mountains before they rock on down to the river.”
If there is a nor'west blowing or even a small amount of flooding, Quentin’s advice is simple: stay away.
"Sometimes we get several groups of people going onto riverbeds in flood. That’s a dangerous idea – if it’s a fast flood, there’s no way you could get everyone off the riverbed before it all turns to chaos," he says.
"Cars can be washed right off the riverbed and putting yourself in harm’s way by entering flood waters puts others at risk who might try to help you when you get into trouble."
Rainfall patterns during previous strong El Niño summers show much wetter conditions west of the Southern Alps and drier conditions across eastern areas. Similar patterns could occur again during the 2026/27 El Niño. Graphic: Earth Sciences New Zealand.
Don’t let the sunshine fool you
Conditions in the mountains matter just as much as what you can see at the river. Before heading out:
- Check the weather forecast for the Southern Alps and river headwaters, not only your destination.
- Check current Canterbury river flows.
- Check for alpine riverbed alerts. If an alert is in place, it will appear on the Environment Canterbury homepage and our Facebook page.
If a nor’wester is blowing, rain is forecast in the mountains or river levels are rising, stay off the riverbed. If in doubt, keep out.