Environment Canterbury (ECan) yesterday said at least 20% of vital vegetation cover along the 65km stretch of river from Kurow to the coast had been illegally cleared.
Timaru-based senior engineering officer Bruce Scarlett said the vegetation removal had accelerated in recent years due to the dairying boom.
"Basically, it has been a land grab. Farmers have cleared the land to increase the pasture for their herds. But some are finding out almost too late that the impact of this during flooding, as happened in May this year, can be severe," he said.
The trees and vegetation are growing in the buffer zone between the established river channels and existing farmland.
Averaging about 250m wide, it constitutes about 3000ha in all.
Mr Scarlett estimated about 600ha of the protection had been lost in the past five to 10 years, cleared to create more pasture.
"This land is either privately owned into the river channel or the property of either the Department of Conservation or Land Information New Zealand. But there is legal protection for trees in the buffer zone, even on private land," he said.
By clearing the buffer zone, farmers were not only increasing the risk of severe flood damage on their own properties, but were also endangering neighbours' land.
Valuable riparian vegetation, habitat for wildlife and important in the overall ecology of one of New Zealand's most important salmon rivers, had also been destroyed.
Since the beginning of this year, ECan had been approaching farmers offering to replant the buffer zones.
In some cases, the offer had been positively received.
"Some landowners are new to the district and had no idea of what damage had been done by previous owners. They've been only too pleased to see us. However, not all farmers are receptive.
"We are still concerned that other farmers, who are either unaware of the potential increased flood risk or who are gambling on increased production outweighing any possible future damage, are still clearing the river buffer zone," Mr Scarlett said.
One measure ECan was now promoting to enable possible successful prosecutions in the future was defining more accurately the buffer zones and the actual river boundaries.
This was being carried out in consultation with the Lower Waitaki River Ratepayers' Liaison Committee.
In the past 18 months, ECan's southern enforcement team had four formal complaints involving vegetation removal.
Willow-pole planting was occurring at some places where ECan had been able to gain access.
