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 damage, 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.