Rivers in Lancashire could dry up unless emergency measures are adopted, according to a report by the Environment Agency (EA).
The level of rivers including the Lune near Lancaster could fall by 80% in summer by 2050.
The report blames the fall on climate change and the growing population.
Trevor Bishop, head of water resources at the EA, said it was "critical" to change the way river water is used as pressure increases on water resources.
He said: "We need to plan carefully to secure sustainable water supplies for people, businesses and the environment.
"The current system of allowing water to be taken from rivers by farmers, businesses and water companies has been in place since the 1960s, and will not be able to ensure secure supplies of water to people and businesses in the future.
"Reform of abstraction management is critical if people and businesses using water are to adapt to the impacts of climate change and economic growth is to continue."