The Anderson Dam, located in Santa Clara County, California, will be no more by the end of this year.
There is a risk of a catastrophic failure of the dam when a big earthquake strikes California.
The 72-metre high earthen dam measures 440 metres long by 270 metres wide.
The Federal Energy Regulatory Commission has said that there is a risk of catastrophic failure of the dam and ordered that it should be drained on starting October 1, 2020.
It holds over 110,000,000 m3 of water when full, more than the other nine reservoirs in the county combined.