Lake Dunstan was formed from the Clutha River after the completion of the Clyde Dam in 1992 - but it's slowly turning back into a river, leaving the community of Cromwell with a sludgy mess.
Silt that would have washed through from the Kawarau River has nowhere to go, and is mounting up on the town's lake banks.
The lake is helping to fuel the growth of invasive weed lagarosiphon, and the built-up sludge and weed is impacting on boaties and swimmers.
The job of mitigating some of the effects of the silt falls to Contact Energy as the Clyde Dam operator.
Could more be done? Kathryn talks to Boyd Brinsdon, head of hydro generation at Contact and Duncan Faulkner is project manager for the Lake Dunstan Charitable Trust Board.
