A public inquiry into calls for water to be pumped from the Lake District to top up urban reservoirs, has heard of the possible effects on wildlife.
More water would be taken from Ullswater under the measures
Water supplier United Utilities says it needs to pump supplies from the Lake District to top-up two drought-threatened reservoirs.
An exceptionally dry summer and autumn has almost halved water levels compared to last year at the reservoirs, which serve Manchester and other parts of the north west.
A public inquiry is looking into United Utilities' application to the Environment Agency for "drought measures" to safeguard supplies.
It wants to pump water from Ullswater and Windermere, two of the most popular lakes in Cumbria's Lake District.
But opponents have told the inquiry the move would have a "devastating" effect on salmon.
Around 12m people visit the area every year
The deepest lake is Wastwater at 74 metres (243 feet)
Windermere is 62 metres (203 feet) deep and Ullswater 61 metres (200 feet) deep
The lakes provide homes for birds such as coots and mallards
The biggest reservoir in the Lake District is Haweswater, which holds 18,600m gallons
Some experts claim the Lake District's salmon rivers could suffer.
They say diverting water from the lakes would reduce the level of water in the rivers Eamont and Leven, which flow into the Ullswater and Windermere.
Dickon Knight, from the River Leven salmon fishery, said: "A reduced river could be a disaster for the salmon."
He said the last drought order, in 1995, almost wiped out a full year's salmon breeding.
He said the species was already close to extinction on the river and it was crucial water levels were not allowed to fall drastically.
United Utilities has pledged to maintain a "minimum" level, below which salmon rivers would not be allowed to fall.
The government is expected to receive a report from the inquiry before the end of the month.
United Utilities has stressed the contingency plan would not be needed if a "prolonged wet spell fills up the reservoirs".
Winter is normally the time when reservoirs are re-filled but low rainfall recently has forced United Utilities to consider alternative plans.
It argues that taking action now will have less of an impact on the river environment than if water is diverted during the spring or summer.
The problems in the north west mirror similar worries in Yorkshire and southern England, where low rainfall has been aggravated by high domestic water consumption.