“PMF” stands for “Probable Maximum Flood”. (In other words, a flood that is probably never going to happen.)
The PMF is an event which is calculated by FERC to occur once every 10,000 years. (Compare that with the 100-year floodplain that is being defined right now for Gladwin County.) The Edenville Dam is classified by the government as a “high hazard” dam, which means that if it were to fail, significant property damage and loss of life would likely occur downstream (at Sanford Lake, in this case). The Federal government mandates all “high hazard” dams to comply with the PMF dam safety standard.
After years of engineering analysis it has been determined that, to pass this massive flood event, the spillway capacity of the Edenville dam must be roughly doubled. The Edenville Dam has two sets of concrete spillways that can together pass 32,000 cubic feet per second of water (cfs) when fully open. The PMF at Wixom Lake would require those same spillways to pass 64,000 cfs. In addition, the spillway modification would require the installation of gates that are more than twice the size of the current gates. See this graphic comparison: