The water level of Lake Burley Griffin will be half a metre lower for the next 18 months and will expose mud flats and rubbish while causing headaches for rowers, yachties and other lake users.
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But the National Capital Authority said it lowered the water level to do essential work on the Scrivener Dam and would do all it could to ease the pain, including extending ramps, bringing in temporary pontoons and excavating soil to allow access to the lake.
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And the authority maintains it does not believe the lowering of the lake sparked the mysterious death of scores of fish in the lake in recent weeks.
The authority's Gary Rake said water was released via the dam sluice gates being opened to lower the lake initially by 20cm on November 25 and then by the full 50cm last Friday, taking it to the same lower levels reached during drought conditions in February, 2003.
The water level had to be reduced to replace 120 corroded anchor bolts securing the dam's gates, a project likely to cost up to $20million and take 18 months to complete.
The bolts were on the dry side of the dam but the water had been lowered to reduce the force on the dam gates and to avoid a ''sudden rise over the gates'' when work was being done.
He said the work environment had to be kept dry and if it became wet it could cause delays which would blow the duration of the maintenance project out to 2 to 2 years.
A computerised system would monitor the lake level and open and close the sluice gates to keep the water level constant. ''It's a lot better than having to drain the lake entirely, which was a worse-case scenario,'' he said.
Mr Rake acknowledged most lake users would have received little to no warning of the lake being lowered, with the authority sending out an email just hours before it was done.
''There were really no other viable options,'' he said.
The authority was working to improve access to the lake, including towing a temporary pontoon to Sullivans Lake for the ANU Rowing Club, excavating some soil from the lake bed to allow boats to launch at the Canberra Yacht Club and putting in extra ramps to get paddle boats into the water.
Lake Burley Griffin Boat Hire owner Jim Seears said he now had to physically drag the paddleboats some distance into the water which was a ''massive headache''.
''I'm 60 years old and it's bloody hard to push them down,'' he said. ''But the NCA's been very good about it and we just have to put up with it.''
Rowing ACT president David Bagnall said the lower water had exposed mud flats and increased the risk of walking on broken glass and fishing hooks.
Lake users were discovering ''lots of hidden rubbish that pose a severe health risk from infection of any cuts''.
''Two of our clubs have been unable to get their boats on to the water because their landing stages are not floating, and it is not possible to safely launch from these two venues.
''These clubs are located on Sullivan's and Jerrabomberra creeks which are both muddy and dangerous for launching'' he said.
Dr Bagnall was concerned some clubs might not ''survive this crisis''. ''There will be difficulty for some clubs in maintaining membership and financial viability, the likelihood of increased damage to equipment,'' he said.
Canberra Yacht Club chief executive Matt Owen said he was concerned about obstacles such as trees presenting a problem. Some boats had run aground but most sailed without incident.