SALT LAKE CITY, United States (AP) — Banks along parts of the Colorado River where water once streamed are now just caked mud and rock as climate change makes the western US hotter and drier.

More than two decades of drought have done little to deter the region from diverting more water than flows through it, depleting key reservoirs to levels that now jeopardise water delivery and hydropower production.

Cities and farms in seven US states are bracing for cuts this week as officials stare down a deadline to propose unprecedented reductions to their use of the water, setting up what’s expected to be the most consequential week for Colorado River policy in years.

The US Bureau of Reclamation in June told the states — Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming — to determine how to use at least 15 per cent less water next year, or have restrictions imposed on them. The bureau is also expected to publish hydrology projections that will trigger additional cuts already agreed to.

“The challenges we are seeing today are unlike anything we have seen in our history,” Camille Touton, the bureau’s commissioner, said at a US Senate hearing.

Tensions over the extent of the cuts and how to spread them equitably have flared, with states pointing fingers and stubbornly clinging to their water rights despite the looming crisis.

“It’s not fun sitting around a table figuring out who is going to sacrifice and how much,” said Bill Hasencamp, the Colorado River resources manager at Metropolitan Water District, which provides water to most of Southern California.

Representatives from the seven states convened in Denver last week for last-minute negotiations behind closed doors. Officials party to discussions said the most likely targets for cuts are Arizona and California farmers. Agricultural districts in those states are asking to be paid generously to bear that burden.

But the tentative agreements fall short of what the Bureau of Reclamation has demanded, and state officials say they hope for more time to negotiate details.

The Colorado River cascades from the Rocky Mountains into the arid deserts of the south-west. It’s the primary water supply for 40 million people. About 70 per cent of its water goes toward irrigation, sustaining a US$15-billion-a-year agricultural industry that supplies 90 per cent of the United States’s winter vegetables.

Water from the river is divided among Mexico and the seven US states under a series of agreements that date back a century, to a time when more flowed.

But climate change has transformed the river’s hydrology, providing less snowmelt and causing hotter temperatures and more evaporation. As the river yielded less water, the states agreed to cuts tied to the levels of reservoirs that store its water.

Last year federal officials for the first time declared a water shortage, triggering cuts to Nevada, Arizona and Mexico’s share of the river to help prevent the two largest reservoirs — Lake Powell and Lake Mead — from dropping low enough to threaten hydropower production and stop water from flowing through their dams.

The proposals for supplemental cuts due this week have inflamed disagreements between upper basin states — Colorado, New Mexico, Utah and Wyoming — and lower basin states — Arizona, California and Nevada — over how to spread the pain.

The lower basin states use most of the water and have thus far shouldered most of the cuts. The upper basin states have historically not used their full allocations but want to maintain water rights to plan for population growth.

Gene Shawcroft, the chairman of Utah’s Colorado River Authority, believes the lower basin states should take most of the cuts because they use most of the water and their full allocations.

He said it was his job to protect Utah’s allocation for growth projected for decades ahead: “The direction we’ve been given as water purveyors is to make sure we have water for the future.”

In a letter last month, representatives from the upper basin states proposed a five-point conservation plan they said would save water, but argued most cuts needed to come from the lower basin. The plan didn’t commit to any numbers.

“The focus is getting the tools in place and working with water users to get as much as we can rather than projecting a water number,” Chuck Cullom, executive director of the Upper Colorado River Commission, told The Associated Press.