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?

Sounds like you need a dam pro to examine the situation, Chip. Especially if more rain is in the forecast!

You'll, at least, get more opinions if you can supply some pictures. I am having a hard time imagining your situation with any confidence, but it seems like your drain pipe is washing out below the output and cutting back into the dam. I'm no expert and am having a hard time understanding what's washing out exactly.

You probably won't know until the water goes but it sounds like you have water that traveled next to the pipe and eventually washed out the dirt. This is just a guess.

I second what quarter acre says, a pic would be nice. sounds like you have a straight pipe thru the dam instead of a traditional L shaped stand pipe, right? when this event is over, i would recommend lowering the emergency spillway. I've always heard the e-spillway should be 1 foot higher than full pool and top of the dam should be 2ft higher than e spillway. 6 inches in 3 days isn't unheard of, it will happen again for sure.