Construction costs to build the largest new reservoir in California in 50 years, a vast 13-mile-long off-stream lake that would provide water to 500,000 acres of Central Valley farmlands and 24 million people, including residents of Santa Clara County, parts of the East Bay and Los Angeles, have risen sharply. The price tag for Sites Reservoir, proposed to be located in the rural ranchlands of Colusa County 70 miles northwest of Sacramento, have increased from $4.5 billion to at least $6.2 billion, and potentially as much as $6.8 billion, the project’s planners confirmed Wednesday. The increase is due to inflation for concrete, steel and other construction materials since 2021, when the original estimate was generated, planners said. Factory shutdowns during the COVID pandemic caused many construction materials to increase in price, and tariffs imposed by President Trump have led to more cost increases in recent months.