All riverbeds north of the Waiau River should be treated as no-go zones in the wake of the Kaikoura earthquake, Environment Canterbury says.
River engineering manager Leigh Griffiths admits the advice isn't great timing for outdoor summer adventures.
But people had to be aware of the potential for sudden releases of water and keep themselves as safe as possible.
Ms Griffiths says more than 150 landslides have created dams in rivers and streams since the 7.8-magnitude quake on November 14 and subsequent aftershocks.
She says a number of rivers have changed significantly and have an increased sediment load coming down them, making them less predictable, particularly after rainfall.
People going into a riverbed would be doing so at their own risk and need to be alert and have an escape plan.
"If there is heavy rainfall, earthquakes, changes in stream flow or colour, or a loud noise, then you need to be prepared to get out of there quickly," she said.
"That's why our advice is stay out altogether."