RIVERDALE, N.D. — The U.S. Army Corps of Engineers will repair and improve Garrison Dam’s spillway for $1.8 billion, the largest dam safety modification project in the federal agency’s history.

Old Dominion Electric Cooperative (ODEC) has announced its decision to pull out of the project to build a third nuclear reactor at the North Anna nuclear power plant in Virginia. ODEC owns an 11.6% ...