All of Utah’s partner states sharing the Colorado River’s water are raising legal objections to the Lake Powell pipeline, possibly opening a schism in the interstate compact that has divvied up the mighty river’s flow for the past century.

In a joint letter Tuesday to Interior Secretary David Bernhardt, those states asked him to block the Bureau of Reclamation from completing its ongoing environmental impact statement until the seven states achieve a “consensus regarding outstanding legal and operational concerns” having to do with the pipeline’s moving water from the Colorado River’s Upper Basin to a corner of Utah draining into the Lower Basin.

The bureau concluded the review’s public comment period Tuesday, with a final decision expected later this year. The pipeline is among several major projects whose environmental reviews are to be expedited under an executive order signed by President Donald Trump in June.

Utah shares the river and its tributaries with Nevada, Wyoming, Colorado, Arizona, New Mexico and California, along with Mexico. If the Interior Department grants the states' request, the project, designed to move 82,000 acre-feet of water from Lake Powell 143 miles to St. George, could wind up in limbo indefinitely.

The six states fear approval of the Utah project without first resolving the legal questions would upset the delicate web of agreements that has held the interstate compact together through a period when the river has experienced extraordinary pressure from drought and explosive population growth.

“As we have in our past efforts, we commit through this letter to act in good faith to identify consensus solutions to the interstate questions that the Lake Powell Pipeline raises for the entire basin,” the letter says. “But that work is undeniably best undertaken as part of a seven-state process rather than as an incident to the NEPA process or ensuing litigation with third parties conducted by courts.”

In its own comments, Nevada went further, insisting Utah do more to conserve water before embarking on a project that could further deplete Lake Mead, which has been less than half full for several years.

The states' shares are spelled out in an interstate compact, known as the Law of the River, first signed in 1922, then revised in 1948. But declining precipitation since 2000 has complicated this picture, resulting in an endless cycle of negotiations that has kept the peace and the river functioning.

The compact may not allow for water to be transferred between basins, according to Anne Castle, a senior fellow at the University of Colorado law school who served as assistant Interior secretary for water and science in the Obama administration.

“In the past, as the letter suggests, when that kind of transfer was occurring, specifically with the Navajo-Gallup Water Supply Project, congressional authorization was obtained,” Castle said. “They’re suggesting that it may be necessary for the Lake Powell pipeline to have congressional authorization as well.”

Under the 1948 revision, Utah and the three other Upper Basin states are obligated to deliver on average 7.5 million acre-feet of water to their three Lower Basin neighbors and Mexico. Glen Canyon Dam was built, in part, to ensure the Upper Basin could meet that obligation by storing water in Lake Powell, currently less than half full.

“It’s an unusual step for six states to combine and send a letter to the secretary of the interior expressing concern about a project that the seventh state is promoting,” Castle continued. “This letter indicates a pretty high level of concern about a number of different aspects of the proposed Lake Powell pipeline. One of those is the compliance with the 1922 compact and the Law of the River and whether additional approvals are necessary in order to make that happen. Another concern is the impact of this additional diversion from the Upper Basin on the risk to all the Upper Basin states.”

The seven basin states have resolved complex challenges in the past and will do so again, according to Utah water officials.

“We remain committed to working with the other basin states to mitigate their legal and operational concerns raised by Utah’s intent to use a portion of its Colorado River allotment to provide water to Washington County,” said Todd Adams, director of the Utah Division of Water Resources. “We will use the next several months to address their concerns.”

Since the beginning of the accord, Utah has not fully used its allotted share and, in 2006, the state Legislature authorized funding the pipeline, now projected to cost $1.1 billion to $1.8 billion, to some to “beneficial” use.

In the meantime, the river’s flow has become seriously depleted and levels on its two largest reservoirs have dropped to the point where their continued operation is in question. Lower Basin states have agreed to cutbacks needed to stabilize levels at Lake Mead and Lake Powell.

But that hasn’t deterred Utah from pursuing its plan to siphon off some to feed booming residential growth in the scenery-rich desert around St. George.

“More than 20 years of planning have gone into the Lake Powell pipeline to meet the needs of Washington County’s growing population and to diversify the area’s water supply,” Adams said. “Without the project, the county’s economic viability and water security will be harmed.”

Utah has spent more than $30 million in environmental and engineering studies on the shape-shifting project, whose reviews have bounced among federal agencies before landing with Reclamation last year. The project initially included Iron and Kane counties and major multiple hydropower components, but Washington County is the sole remaining participant and much of the power generation has been stripped from the project.

The six states' diplomatically worded missive acknowledged Utah’s fruitful participation in the accord but goes on to predict that Interior’s approval of the project would unleash a deluge of lawsuits that would invite the courts to resolve controversies among the compact states, rather than allowing them to work out their differences among themselves.

“That is not a recipe for creating the kind of meaningful and positive change needed to sustain the Colorado River in the coming decades,” the letter states.

Environmental opponents of the project contend Utah’s fast-growing Washington County can meet its water needs from local sources and conservation and that its construction costs will lead to crushing tax hikes. Despite some progress with conservation, per-capita water use in St. George remains some of the highest in the region.

“These letters of opposition from outside Utah demonstrate how wildly out of touch the Utah Division of Water Resources has been on this dinosaur water project,” said Zach Frankel of the Utah Rivers Council. “Both climate change and water conservation are real and it’s time Utah accept those realities and stop fighting them.”

The council is part of a consortium of several groups that filed joint comments Tuesday to the Bureau of Reclamation, spelling out an exhaustive case against the project.

In its submitted comments, Nevada also highlighted potential problems of moving water into a Lower Basin drainage. The pipeline would increase return flows into the Virgin River, but the bureau did not explore how that might degrade the water quality of that river flowing into Nevada.

“Impacts which failed to be analyzed in the DEIS include increased mobilization of contaminants, increased algal growth and the potential for harmful algal blooms, transport of aquatic invasive species, and impacts on sensitive and listed fish species,” state the comments, submitted by the Southern Nevada Water Authority and the Colorado River Commission of Nevada.

The Nevada comments insist Utah do more to curb water use, chiding its neighbor’s reluctance to embrace measures that have sharply lowered per-capita use in Las Vegas; Albuquerque, N.M.; Phoenix, Tucson, Ariz.; and other cities in the desert Southwest.