Some cannabis growers may soon find themselves with a lot less irrigation water if the U.S. government decides to block the use of federal water for state-legal marijuana cultivation.

The U.S. Bureau of Reclamation, which oversees management of federal water resources, "is evaluating how the Controlled Substances Act applies in the context of Reclamation project water being used to facilitate marijuana-related activities,” said Peter Soeth, a spokesman for the bureau. He said the evaluation was begun "at the request of various water districts in the West."

Local water districts in Washington state and Colorado, where recreational marijuana is now legal, contract with federal water projects for supplies. Officials from some of those water districts said they assume the feds are going to turn off the spigots for marijuana growers.

“Certainly every indication we are hearing is that their policy will be that federal water supplies cannot be used to grow marijuana,” said Brian Werner at Northern Colorado Water Conservancy District, which handles approximately one-third of all water for northeastern Colorado and is the Bureau of Reclamation's second-largest user in the number of irrigated acres.

Washington state’s Roza Irrigation District, which supplies federal water to approximately 72,000 acres in Yakima and Benton counties, has already issued a “precautionary message” to water customers that may be involved in state-legal cannabis growing.

“Local irrigation districts operating federal irrigation projects have recently been advised that under Federal Reclamation Law, it is likely project water cannot be delivered and utilized for purposes that are illegal under federal law,” wrote Roza district manager Scott Revell in letters to the Yakima and Benton county commissioners. “Presumably growing marijuana would fall into this category.”

Both Washington and Colorado legalized marijuana for medical use more than a decade ago. Pot remains illegal under federal law. Reclamation’s Soeth said that the issue of cutting off water supplies for marijuana has never come up before.

A Department of Justice official told HuffPost it has no comment on the water issue. The Bureau of Reclamation is likely to announce a decision this month. “We’re going to work with our water districts once that decision is made,” Soeth said.