A special tribunal, appointed by the Ministry for the Environment, began hearing evidence a year ago on an application to amend the Water Conservation Order on the river.
New Zealand and Otago Fish and Game Councils have applied to change the Water Conservation Order so damming or diversion is prohibited.
The hearing was adjourned in August, resuming yesterday to hear expert evidence on flora and fauna which might be affected by any dam on the river.
Pioneer Generation is one of the groups which has opposed any change to the Water Conservation Order.
Pioneer has been considering two options for a hydro-electricity scheme on the Nevis but has not finalised its plans.
It already owns the leases for the land it needs.
The tribunal heard conflicting views yesterday from experts on plants, moths, beetles and skinks about the impact a dam would have.
A dam and reservoir on the Nevis would destroy the only known population of some nationally-threatened plants and wipe out much of the habitat for other rare plants, ecologist and botanist Kelvin Lloyd said.
"The concentration of nationally threatened plant species in the Nevis Valley floor is an outstanding feature of the site.
It is unusual to have so many nationally threatened plant species occurring at close proximity in a single location," he said.
A dam would destroy the only known population of Myosurus minimus novae-zealandii, (mousetail) which was a threatened plant, classified as nationally critical.
It would also swamp all the known occurrences of another plant - a bidibid (Acaena buchananii), one of two important sites for a tiny forget-me-not (Myosotis pygmea glauca) and a considerable portion of the habitats for those plants, Mr Lloyd said.
Department of Conservation botanist John Barkla said of six nationally threatened plants in the valley floor, all but two were outside the footprint of any potential dam.
Another population of the mousetail plant existed outside the dam area, he said.
Consultant scientist Brian Patrick said a dam would possibly wipe out a rare Nevis skink but herpetolgist Tony Jewell said the skink was "abundant" over a wide area in the Nevis Valley.
Scientist Barbara Barratt said a large beetle, Mecodemia chiltoni, mentioned in some of the submissions, was not restricted to the Nevis valley.
Ecologist Mark Sanders said although three threatened species of birds were found in the Nevis, the river provided only a small area of suitable bird habitat.
The tribunal is made up of Richard Fowler (chairman), Carolyn Burns and Rauru Kirikiri.
It has considered 248 submissions on the matter and spent 17 days so far hearing evidence, in Cromwell and in Dunedin.
Today is the final day of the hearing and then the tribunal will make a recommendation to the Minister for the Environment
Day 17
• Tribunal: Richard Fowler (chairman), Carolyn Burns and Rauru Kirikiri.
• Application: To amend the existing Water Conservation Order to prevent damming or diversion of the Nevis River.
• Players: New Zealand and Otago Fish and Game councils want the changes, Pioneer Generation, Contact, TrustPower and local authorities are among those in opposition.
• Yesterday: Submissions on flora and fauna from Royal Forest and Bird Protection Society solicitor Sally Gepp, ecologist and botanist Kelvin Lloyd, consultant scientist Brian Patrick, wildflower expert and tour guide John Douglas, Department of Conservation lawyer Pene Williams, botanist John Barkla, Doc technical support officer Bruce McKinlay, ecologist Mark Sanders, herpetologist Tony Jewell, AgResearch scientist Barbara Barratt.
• Today: Closing submissions on flora and fauna by lawyers for the various parties.
• Quote of the Day: "I wouldn't normally find biology quite so riveting. I certainly didn't find it so at school." - Tribunal chairman Richard Fowler.
