Summer rain bad for rivers

Wet weather was to blame for unswimmable waterways around Manawatu¯ last summer as almost all rivers in the region were deemed unsafe at some point.

After a damp spring, people will need to have a careful look at the weather before swimming in rivers this summer, too.

Horizons Regional Council measured 80 swim spots in and around Manawatu¯ from November 2016 to April 2017 to determine their swimmability, including beaches and lakes.

Sixty-three of 65 river spots tested were unsafe to enter at some point last summer, the exceptions being the Rangitı¯kei River and Pukeokahu.

Horizons uses a traffic-light system to highlight how safe a swim spot is. Green is good to go, amber means there are potential health risks and red is unsafe. This can be found on its website over summer.

The Manawatu¯ River near the gorge was safe for swimming about 30 per cent of the time, and another 30 per cent of the time swimmers were warned it could be a health risk. The rest of the time it was considered unswimmable.

Niwa principal scientist Chris Brandolino said there were no predictions yet for how this summer would be.

Horizons science and innovation manager Abby Mathews said if it was another wet summer people should wait to swim when it was clean and clear.

‘‘If it’s three days after rain and the water is not dirty or fastflowing, it should be safe to swim. Look for potential hazards such as unstable cliffs and banks, sunken rocks and logs, and rips at beaches.’’

People wanting to cool off at Ashhurst Domain last summer had more luck than at other sites as it was deemed safe 53 per cent of the time. Eleven per cent of the time it posed health risks and 37 per cent of the time it was unsafe.

Seventy-four of the 80 swim spots monitored posed a health risk at some point during the summer.

Health risks could include the presence of faeces orcyanobacteria.

When entering cyanobacteriacontaminated water, people are at risk of getting diarrhoea, nausea or gastroenteritis, which can lead to liver damage or even death in some cases, a council report says.

Other sites monitored include the Hokowhitu Lagoon, which despite not having many bathers is swimmable about 20 per cent of the time. About 55 per cent of the time it was unswimmable. There were possible health risks 25 per cent of the time.

The estuary at Akitio River is the worst spot to swim in as it was unswimmable 67 per cent of last summer.

For people looking for a consistently safe swim spot, the beach sites are the most reliable, with Castlecliff at Whanganui and Foxton Beach among the cleanest places to swim.

Horizons natural resources and partnerships manager Jon Roygard said last summer was wetter than usual. More rain meant river beds were stirred up and more sediment ended up in waterways, he said.

‘‘When the weather conditions were suitable for swimming, the majority of sites were suitable for recreational use.’’

Roygard said that between 2006 and 2015 swimmability improved by 22 per cent in 36 monitored river sites in Horizons’ region and none of the sites’ water quality declined.

The council will continue monitoring the 80 swim spots this summer at an estimated cost of about $40,000.

Waterways will be monitored from the end of November 2017 to April 2018.