Fishermen and other lake users are up in arms as many say they can't get their boats onto the lake as a result.
It's been put down to works by the ESB at Parteen Weir who says this has now been completed with levels back to normal.
However Sinn Fein Councillor Seamie Morris says the lack of communication from the company is not acceptable as the work was carried out at the height of the fishing season in May.
There are also concerns that the work is part of tests being carried out on Lough Derg to determine the impact of water extraction on the lake.
Proposals are currently being considered to take water from the Shannon to supply Dublin and the east of the country.
Councillor Morris says many people feel the drop in the level may have been part of the plans to take water from the lake.
