I have an 8 year old dam (3 acre pond). Dam is about 20 feet tall. Just had 6 inch rain in 3 days. Draining water fast as it could, emergency spillway never required. BUT I suspect water at least 18 inches higher than normal level. Right behind the down drain is suddenly a pocket of missing dam!! 3 feet by 4 feet and about 3 to 4 feet deep (to present level of the pond) at normal level. No obvious flow into the pocket but soil is fully saturated at least 2 feet below bottom of the pocket. No obvious leak on back side of dam but minor sink hole depression 10 feet below water level about 2x2 feet (I do not know if this is new but it is NOT wet). But no water seen on back side near this depression. What do I do? Venturi effect from drain and high water on the dam caused erosion where dam rip-rap was poor? Or is this how a dam failure starts?