Algal growth at Rakaia River mouth
Scientists from our Water and Land team have investigated reports of increased algal growth around the Rakaia River mouth.
A holiday season of easterly winds and few nor’westers may have left many Cantabrians feeling like summer’s only just arrived. Still, it’s also had the effect of lowering river flows in our large alpine rivers.
Increased algal growth due to low flows
Algae grow best in low-flow conditions. Recreational water users have reported substantial algal growth around the Rakaia River mouth this year.
Water and Land Science Manager Dr Elaine Moriarty says the lack of rain in the Alps means there hasn’t been a “fresh” of water through the river for some time.
“We’re seeing very low flows at the river mouth, with our modelling suggesting flows are getting close to 60 cubic metres per second. Without enough flow to flush the system, algae and scum can form,” she says.
Rakaia River mouth
Assessing effects of low flows on freshwater ecosystem
Our freshwater ecology team is visiting the alpine rivers to assess any effect these low flows are having on the freshwater ecosystem.
Dr Moriarty says it may take some time to analyse the algae, but the best solution is for a large amount of rain to fall in the mountains, creating a high flow that would flush much of the algae out.
“We acknowledge that low flows and algal growth can be a source of distress for recreational river users,” Dr Moriarty says.
“Our science team are currently working to understand the extent of these issues and to see what, if anything, we can do to mitigate the situation.”
Rain on the plains, but not in the alps
In recent months, Waitaha/Canterbury has experienced La Niña weather patterns.
These bring frequent easterlies and more rain to the plains. This is why much of the region has been cooler and wetter than recent summers.
However, this weather pattern results in fewer nor’westers and drier than usual conditions on the West Coast and Southern Alps – the catchments that feed our large braided alpine rivers.
Irrigation restrictions in place
The recent weather pattern is why the Waiau Uwha, Hurunui, Rakaia, Waimakariri, Rangitata, Ahuriri and other rivers are low and restrictions on water take are in place in these catchments.
Learn about water restrictions, irrigation schemes, and recommended irrigation techniques and how to comply with regulations for dams, irrigation ponds and fish screening facilities in Waitaha.
View current irrigation restrictions.
Soil moisture anomaly showing for 18 February 2025 illustrating relatively wet summer on the plains, but not in the alps - Image: NIWA
Complex water use scheme allows for water abstraction
The Rakaia is a modified catchment, with irrigation schemes and hydroelectricity generators using water.
While climate is the ultimate driver of river levels, these modifications have some influence on flow:
- The Rangitata Diversion Race usually discharges water into the Rakaia through the Highbank power station, adding to river flow. Highbank is currently not operating as it undergoes maintenance, removing what can be an additional source of water.
- Manawa Energy diverts water from the Rakaia’s headwaters into Lake Coleridge. It eventually discharges water back into the river through the Coleridge power station, but this can create a pattern of rapidly rising and falling water levels, especially when river levels are already low. Manawa Energy can take water at times when most other users are restricted, but water taken at these times is only able to be used for hydroelectricity generation – not for irrigation.
- Some irrigators have consent conditions and an agreement with Manawa Energy that allows them to purchase stored water from Lake Coleridge at times when they would otherwise be on restriction. This increases their access to water and means water can be taken during dry periods when the Rakaia River is in low flow.
More information
- Our Rakaia Water Balance Model calculates average daily flow, discharges, natural loss and abstraction to calculate flow at the river mouth.
- NIWA operates a river flow meter at Fighting Hill, which records water levels in the Rakaia at 10-minute intervals. View riverflow data for the Rakaia River at Fighting Hill.
- Understanding irrigation restrictions when it’s dry.