During a severe storm in December 1964 , the inflow into Folsom Lake reached a record high of per second , with a river release of per second . In February 1986 , nearly 500,000 people faced the possibility of flooding when engineers at Folsom Dam were forced to open the spillway gates after heavy rains . The flooding was made worse by the failure of the Auburn Dam cofferdam upstream which released an extra into the American River . A peak flow of entered Folsom Lake , forcing operators at Folsom Dam to open all the spillway gates , releasing into the American River . This was above the safe capacity of downstream levees . Although the dam and the Sacramento levee system held without major damages , the requisite winter flood control space was increased 50 % , from 400,000 to 600,000 acre feet , to protect against future floods . In addition , about of sediment carried down from the mountains was deposited in Folsom Lake , considerably reducing its capacity . The consequence was a reduced capacity to store winter rainfall for summer use . Folsom Dam may have prevented as much as $ 4.7 billion in damages in 1986 alone
a severe storm
Christmas flood of 1964
Folsom Dam