Several people making submissions to a review of the Waikato River wanted hydro dams on the waterway removed entirely.
The review was done by the National Institute of Water and Atmospheric Research (NIWA) on behalf of the newly formed Waikato River Authority.
It found cleaning up the river could cost between $1 billion and nearly $5 billion.
The report says it was suggested at several community consultation sessions that the river's eight hydro dams should be decommissioned and removed.
But it says the dams generate 13% of New Zealand's electricity and there is no alternative power source.
The report listed several other reasons for keeping the dams, including the recreational value of the lakes behind them and their role in controlling river flow and reducing flood risks downstream.