A riled scientist is calling out claims that dams benefit the environment, as the cost of building a single Hawke's Bay water storage project balloons to $900 million.
Freshwater ecologists say the suggestions of environmental positives smack of desperation from lobby groups.
Dams are back in the spotlight as a tussle over the massive Ruataniwha water storage project continues in Hawke's Bay, with the most recent costings ballooning 50 percent to more than $900 million.
There have been recent suggestions dams can provide some environmental positives.
Massey University freshwater ecologist Mike Joy was so annoyed at that he pulled together relevant research on the effect of dams on rivers.
The reality was, he said, the natural regime of rivers was what had driven the life and shape of rivers - and dams were impeding that.
"You come along and you start controlling that and it has just an incredible number of impacts on the life in the river.
"And to try and claim that putting some water back in after you've taken it out is somehow good for a river is just bizarre beyond belief."
Otago University freshwater ecologist Marc Schallenberg said it was not well supported that dams were good for the environment.
He said dams modified river flows and took out water from systems.
"So really if you're stepping back and saying, in general, are they good for the natural catchments and the natural flows of water? No, they alter them. I mean I think that's probably the only generality you can say is that they alter them and they create barriers for the migration of fish."
Seventy-two percent of New Zealand's indigenous freshwater fish species are classified as at-risk or threatened with extinction.
Dr Joy said the barriers to freshwater fish migration caused by dams was a big part of that statistic.
He believed there was a drive towards large-scale irrigation and to pay for the cost of irrigation farmers had to intensify the farming to match that extra requirement for water.
"So then you put more cows on the land, there's more diffuse pollution from that, mostly through urine patches, so just more cows on the land, more nitrogen coming out of the land ending up in the groundwater, or the rivers, or the lakes."
Irrigation for other land uses such as horticulture had have less of an impact than dairying, he said.
Irrigation New Zealand chief executive Andrew Curtis conceded that if a feature of an ecosystem was changed it would have an impact - but there were other benefits from dams, such as managing river flows.
He said New Zealand had periods of dry during the summer where quite often flows of rivers became very minimal.
"That's predicted to increase more with climate change, so if people want to protect the ecosystems in rivers then actually having stored water and being able to release that to ensure flow for aquatic life is actually quite beneficial."
Mr Curtis said an example was the South Canterbury Opuha Dam, which he said had helped manage water flows to the Opihi catchment in the last few years of bad drought.
He said the irrigation industry had learned a lot over the past few years and improvements were being made to new structures.
Dr Schallenberg said there was potential that further systems would have to be put in place to try to undo the damage done by dams on ecosystems.