Atourism operator in the upper reaches of the once-pristine Rangitīkei River says its water is increasingly dirty and filled with nutrients that grow nuisance weeds.

Brian Megaw owns River Valley Lodge, an adventure tourism business at Pukeokahu. He said the thunderstorms of early summer washed soil from winter crop cultivation into the river, turning it a dark chocolate brown.

With the precious topsoil came nutrients like phosphorous that feed weedy growths on the river stones.

Roger Dalrymple has started a Rangitīkei catchment group to improve environmental practices. Photo / file
Roger Dalrymple has started a Rangitīkei catchment group to improve environmental practices. Photo / file

"We get more weed growth earlier in the season."

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Thunderstorms and brief intensive rain have become common in early summer, just when hill country farmers are planting crops to feed stock in winter. There's a lot of bare ground close to the river.

Megaw fears topsoil will be stripped and fertility will decline, as well as the river losing its appeal.

He blames the increasing cropping happening on hill country.

"Farmers are adopting technologies and practices faster than they can write rules around them. We need to look more holistically and ask what's best for the long-term health of the land."

Horizons Regional Council monitors the river at Pukeokahu. Between 2007 and 2017 the amount of both sediment and periphyton (weed and algae growth) increased, science and innovation manager Abby Matthews said.

The council has rules for hill country cultivation. Land manager Grant Cooper said cultivation on slopes of over 20 degrees needs consent, and there are rules about how close it can come to waterways.

Cultivation has risks that farmers need to be aware of and mitigate, he said.

"For people following the rules, the risks should be fairly low."

There's a better way to improve farming practice than rules, Roger Dalrymple said. He's the chairman and initiator of the Rangitīkei River Community Catchment Group.

It formed last year, has had four meetings and "100 per cent buy-in", he said. It includes the Turakina River and groups such as iwi and Fish & Game.

"Our farming industry has to realise that it's got to start making some changes," Dalrymple said.

He envisages smaller catchments within the rivers banding together to set their own goals, guided by educators from organisations like the New Zealand Landcare Trust.

They could decide to keep all their animals out of waterways, for example, or to cultivate hillsides differently.

Dalrymple loathes the term "spray and pray", which has been blamed for poor water quality. It refers to helicopters spraying herbicide to kill pasture, then broadcasting seed and fertiliser to grow more nutritious plants.

There's a lot more skill going into the practice now, he said. He's involved with a three-year Sustainable Farming Fund initiative trialling it in the King Country.

Another contributor to silt in the Rangitīkei is energy company Genesis. It diverts Central Plateau water to generate electricity and releases flushing flows from the Moawhango River into the Rangitīkei four times a year.

The purpose of the flushing is to clear nuisance periphyton and sediment, a spokesperson said. A flushing flow was successfully completed on December 12, almost two weeks before the Christmas rainfall events.