Some farmers in one of the country’s most important agricultural regions will have to stop taking water out of major rivers and streams because of a severe drought that is threatening the drinking water supply for 25 million people, state regulators said Tuesday.

The Water Resources Control Board approved an emergency resolution empowering regulators to halt diversions from the state’s two largest river systems. The order could apply to roughly 86% of landowners who have legal rights to divert water from the San Joaquin and Sacramento river watersheds. The remaining 14% could be impacted if things get worse.