It’s the big Denver Water reservoirs in Colorado’s mountainous backyard that get most of the attention — Dillon Reservoir, visible from Interstate 70 in Summit County, and Gross Reservoir, popular with anglers and campers west of Boulder.

But in many ways the most important reservoir to Denver Water is some 600 miles and an entire state away.

Lake Powell straddles Utah and Arizona and, with a capacity of 27 million acre-feet is the second-largest reservoir in the United States, storing water from the mighty Colorado River. And right now, it’s at the center of Denver Water’s efforts to protect its own water supplies — as well as Colorado’s as a whole.

How can this faraway reservoir best known as a red-rock ringed recreational mecca for house boaters play such a major role for Denver Water?

Here’s how: Under a law created in 1922 called the Colorado River Compact, every year Colorado and three neighboring “upper basin” states, Wyoming, New Mexico and Utah, have to send large volumes of Colorado River water downstream to the “lower basin” states of Nevada, California and Arizona.

That’s where Lake Powell comes in.

“Lake Powell is our bank account for the upper basin where we store all of the upper basin’s water from the Colorado River to meet obligations to the lower basin,” said Jim Lochhead, Denver Water’s CEO/Manager, who visited the reservoir this summer, on the eve of a trip down the Grand Canyon.

If there’s enough water to release from Lake Powell, upper basin states can meet their legal obligation to the lower basin states.

But drought and climate change are taking a toll on the big reservoir. And concerns are mounting that levels in Lake Powell could fall below the levels needed to ensure enough water is delivered for lower basin states.

“You can see the bathtub ring”

In Lochhead’s video journey of the trip, he pointed out the band of faded color above the water line, where nature has marked with perfect clarity where far higher water levels used to reach.

“You can see the bathtub ring,” he said. “We’ve got a long way to go to fill this thing up. We’ve got a lot of work to do in the upper basin to try and make this reservoir (work) for our benefit.”

Lake Powell is less than half full and during this past water year, which started Oct. 1, 2017, and ended Sept. 30, 2018, the reservoir had its third-lowest inflow ever at 4.6 million acre-feet. That compares to an average of 10.8 million acre-feet. In a drought that has plagued the southwestern United States over nearly 20 years, 2018 proved one of the driest years on record.

“With a repeat of historic hydrology since 2000, Lake Powell would be dry — empty — within about three years,” Lochhead urgently told a meeting of the Colorado Water Conservation Board in September 2018. “If you doubt this possibility, consider that the August inflow into Lake Powell was 2 percent of average.”

The Upper Colorado River Basin has another key reason to push Lake Powell levels up: If the reservoir sinks too much more, it would jeopardize its ability to produce hydropower. Revenues from the dam’s hydropower sales fund a suite of projects crucial to water users in the basin, including operations at other federal reservoirs, desalination efforts and work to protect endangered fish — all programs with a big presence in Colorado.

The nearly century-old Colorado River Compact was an agreement between the upper and lower basin states to provide an average of 7.5 million acre-feet a year at Lees Ferry, a measuring point just below Lake Powell. The figure is calculated using a 10-year rolling average of 75 million acre-feet.

In the decades since, the upper basin has met its obligations. And it continues to do so — with the current rolling average at about 91 million acre-feet, well above what the law requires.

So, what’s the worry?

Simple: As Lake Powell loses elevation, the bank account shrinks. Just like for a family, the smaller the bank account, the less flexibility you have in times of stress. And years of drought are creating just that stress. If the bank account runs too low, Colorado and its fellow upper basin states would have to pull from their own supplies — water these states use for farms, cities, recreation and industry.

To extend the family comparison further, when a family runs out of money it may have to take extreme steps, such as selling possessions to pay the bills.

This kind of situation — when applied to the Colorado River — translates to the lower basin states seeking obligated water payments from the upper basin. It’s called a “compact call,” and the prospect makes everyone in both basins uncomfortable.

“There’s no precedent for it, and no one can quite say how it would unfold, only that it would create confusion and controversy as states scramble to comply and leave water users — including Denver Water and its customers — at risk of surrendering half of Denver’s supplies," Lochhead said.

“Reservoir within a reservoir”

That brings us to today, and evolving efforts to develop a plan to address this possibility; one that leaves Denver Water and water users across Colorado better prepared for such a “call” on the river.

Lochhead is leading the charge for the creation of a “storage pool” in Lake Powell. This would be a pool of water, about 500,000 acre-feet, that could be dispatched to the lower basin in times of major shortages. Think of it as a “reservoir within a reservoir” to be filled with contributions across the upper basin states. Just how it would be filled, and who would contribute how much, are complicated matters still to be worked out.

How would this pool differ from water the upper basin already sends downstream?

It would stay in Lake Powell, and not be subject to typical releases into the Grand Canyon and into an even larger reservoir roughly 300 river miles away called Lake Mead.

The water in the storage pool would amount to its own kind of bank account, there to be used specifically for a call on the upper basin. Think of it as a kind of insurance policy that would spare Colorado, Wyoming, New Mexico and Utah from a full-out scramble if they fell short of their delivery obligations.

“The mere prospect of these events argues that it would be foolish to do nothing and wait until we are in a crisis,” Lochhead told the CWCB. “I certainly hope we don’t have to implement any countermeasures to an extended drought. But if we need to, we should have them in place before the system collapses.”

All of this comes against the backdrop of so-called “drought contingency planning” in both the upper and lower basins. Both basins are working on plans to address water levels in lakes Mead and Powell in the face of ongoing drought. Creation of a storage pool in Powell is a key element of the upper basin’s drought planning.

Another key component involves managing other federal reservoirs in the upper basin — Blue Mesa in Colorado, Navajo in New Mexico and Flaming Gorge in Utah — in helping to ensure levels in Lake Powell remain high enough for the reservoir to remain viable.

In the lower basin, water users in California, Nevada and Arizona are working on plans that would add more than a million acre-feet in Lake Mead to shore up that reservoir’s also-sagging water levels.

Colorado has long set the tone among the upper basin states for working through issues affecting the Colorado River Basin and the relationship between upper and lower basin. And Lochhead believes the state can, once again, play an important role.

“In the negotiation of the 1922 compact, and since, Colorado has led the basin in developing farsighted solutions that protect our state and also benefit the greater good of the American West,” he said. “It is time, once again, for Colorado to lead.”