BEVERLY, Wash. — Divers for the Grant County PUD have discovered a crack in one of Wanapum Dam’s concrete spillgate supports. The dam continues to generate electricity as a probe into what caused the crack continues. Dam officials have lowered the reservoir level behind the dam by 6 feet to relieve pressure on the dam, PUD spokesman Tom Stredwick said this morning. Federal inspectors were at the dam Thursday, he said. Wanapum Dam is capable of generating 1,092 megawatts of electricity, enough to supply nearly 900,000 Northwest homes. It is near Vantage where Interstate 90 crosses the Columbia River. The crack was discovered Thursday in one of the concrete “piers” that support the fourth spillgate, Stredwick said. Engineers were tipped to a possible problem Tuesday when they noticed a slight bowing of a concrete curb on the dam deck. They also noticed that the affected spillgate was misaligned. Dams have “expansion joints” that let them flex with changes in water pressure and temperature, Stredwick said. But measurements taken at the dam showed that the movement at Wanapum was “outside historical norms.” Officials don’t yet know what could have caused the crack. They’re studying a video that divers took of the affected area, he said.