Anglers claim a beauty spot in west Cumbria will be destroyed if the reservoir at its centre is drained.
Cogra Moss, which is no longer part of the water network, is under threat because a dam there needs refurbishment.
United Utilities, which owns the reservoir, says it's costing it money, but that the closure is just one option being considered.
United Utilities are reinforcing calls for people not to swim in the region's reservoirs, rivers and lakes, as the school holidays approach.
The warnings come after a man died in Windermere and another in the River Esk in the past fortnight.
Two years ago a man drowned in Thirlmere, which is managed by the company.
Plans are being drawn up to increase the size of two reservoirs. Scottish Water want to raise the height of the dam by 1.5m at Penwhirn Reservoir, which supplies most of Wigtownshire, and Black Elk reservoir, which supplies Dumfreisshire. It would increase the capacity of both by 35%
A public consultation event on the proposals is planned later this month.**
Nigel Graham-Smith, asset planner for the Wigtownshire area, said: "This proposed work will enable Scottish Water to meet water supply demands in times of low rainfall by enhancing the amount of water the reservoir can hold."