During the storm of 10 July 1968 , the lake gained an extra 471 million imperial gallons ( 2,140,000 m ³ ) and rose 19 inches ( 480 mm ) in under 12 hours . At one point worried Bristol police issued a warning that the dam might not hold , prompting localised evacuation of populated valley areas downstream including Pensford and Keynsham
storm of 10 July 1968
Chew Stoke flood of 1968
Chew Valley Lake