The Department of the Interior sent a letter to the State Water Resources Control Board on Friday warning of possible legal action over a proposed plan to divert more water from the Stanislaus, Tuolumne and Merced rivers, particularly as it pertains to the potential impacts on water storage in New Melones Reservoir.

According to the letter, the state’s proposal to require 40 percent unimpaired flows from the three rivers between February and June could have a “devastating” effect on recreation in the area and undermine congressionally mandated objectives for the reservoir, a federal asset operated by the U.S. Bureau of Reclamation as part of the Central Valley Project.

If Interior Secretary Ryan Zinke determines the proposal is inconsistent with the federal responsibilities, the letter stated that he “will request the Attorney General of the United States to bring an action against the Board.”

The letter, sent one week after Zinke toured New Melones and Don Pedro reservoirs with Republican lawmakers, asks the board to reconsider the proposal and delay a scheduled public meeting on Aug. 21 and 22 in Sacramento, where the board could approve the plan that’s been in the works for nine years.

The state’s final draft of the Bay-Delta Plan Update for the Lower San Joaquin River and Southern Delta has caused an uproar in the region since it was released earlier this month because of the proposed requirements for increased flows from the three tributaries of the San Joaquin River, which flows into the Sacramento-San Joaquin Delta.

State water regulators say the increased flows are needed to prevent an ecological crisis in the Delta, the primary source of drinking water for more than half of all residents in California and irrigation for a third of its farmland.

According to the Department of the Interior’s letter, the proposed requirements would reduce the storage in New Melones by an average of 315,000 acre-feet of water per year.

One acre-foot is roughly equivalent to filling an area the size of a football field 1 foot deep in water and about the amount an average California household uses in a year.

The reservoir can store a maximum of 2.4 million acre-feet of water, but federal officials stated the average inflow is about 1.1 million acre-feet per year due to variabilities in precipitation. The average annual demand for all of the current uses and regulations is about 1.2 million acre-feet.

Oakdale Irrigation District and South San Joaquin Irrigation District hold senior rights to divert a combined total of 600,000 acre-feet per year from the Stanislaus River that pre-date Congress’s approval of the New Melones Project in 1944 primarily for the purpose of flood control.

In crafting the proposed flow standards, the state determined there would be minimal impacts on storage in the reservoir based on targets for how much water to hold onto from one year to the next.

Federal officials contended in Friday’s letter that the state’s conclusion was based on the “erroneous assumption that Reclamation would be able to prioritize” the carryover storage targets over the needs of senior water rights holders.

The letter stated that previous studies by the Bureau of Reclamation determined the reservoir loses storage from one water year to the next about 61 percent of the time, so an annual reduction of an additional 315,000 acre-feet of water per year would mean the reservoir would “rarely, if ever, see gains in storage year over year.”

“This is not a sustainable operation for New Melones Reservoir and does not provide a reliable water supply for Reclamation’s CVP (Central Valley Project) water service contractors,” the letter stated. “As a result, full use of the dam as Congress contemplated would be prevented, significantly undermining Congress’s design for the long-term operation of the project to satisfy multiple policy objectives.”

Although the reservoir isn’t a source of drinking water or irrigation for residents in Tuolumne County — that distinction belongs mostly to Lyons and Pinecrest reservoirs upstream of New Melones on the South Fork of the Stanislaus River — local officials have said it’s a boon to the economy as a draw for recreation and provides public agencies with low-cost power.

The letter stated that the reservoir attracted roughly 450,000 visitors in the 2016-17 fiscal year, up from 286,842 visitors in the 2014-15 fiscal year when the water level was at a near-historic low in the midst of a five-year drought.

If the state’s proposed flow requirements are implemented, the letter stated that the potential economic impacts on the local area could be significant due to “reduced visitation caused by consistently lower lake levels.”

Government agencies in both Tuolumne and Calaveras counties also receive low-cost electricity through hydropower generated by the reservoir, including the city of Sonora, all K-12 public schools, Tuolumne Utilities District, Columbia College and various other special districts.

The Bureau of Reclamation anticipates that the state’s plan would cut power generation to levels similar to those during the drought, when electricity rates on local agencies increased by more than 16 percent.

Contact Alex MacLean at or (209) 588-4530.