Where Denver Would Get Water If The Colorado Failed
Denver is the one major American city in this position where the answer is genuinely reassuring, and the reason is that it is upstream of the problem. Denver Water draws roughly half its supply from tributaries of the Colorado River on the western side of the Continental Divide and the other half from the South Platte on the eastern side, which means a failure of the Colorado costs it about 50 percent of its water rather than all of it. More importantly, Denver takes its Colorado water at the headwaters, before the river reaches Lake Powell or Lake Mead, so the reservoir elevations that dominate every other story about this river are not Denver's problem at all. Denver's nightmare is a different one entirely, and it is legal rather than hydrological.
Here is how the city actually gets its water, what it would fall back on, and the scenario that genuinely worries its planners. That last one has nothing to do with how much snow falls.
A Utility That Spans The Divide
Denver Water serves around a quarter of Colorado's population using roughly 2 percent of the water consumed in the state, and it does that by collecting from both sides of the Continental Divide. Around 50 percent comes from Colorado River tributaries on the West Slope, chiefly the Fraser, Williams Fork and Blue rivers, and the rest comes from the South Platte basin on the East Slope.
That split is unusual and it was entirely deliberate. A utility drawing from two separate watersheds has redundancy a single-basin system cannot buy at any price, because the two do not always have bad years at the same time.
The Moffat Tunnel
The first of the two transmountain systems began operating in 1936, using the pilot bore of a railway tunnel driven under the Continental Divide. It increased Denver's water supply by roughly 30 percent on completion, which is what made the postwar growth of the city possible.
The Moffat system collects from 36 streams in Grand County, sends the water under the divide into South Boulder Creek, stores it in Gross Reservoir, and then moves it by canal to Ralston Reservoir and the Moffat treatment plant.
Water from the Williams Fork takes a route that is difficult to believe on first reading. It is diverted into the Gumlick Tunnel under Jones Pass, brought back over the divide through the Vasquez Tunnel, and then passes under the divide again through the Moffat. By the time it reaches a tap in Denver it has crossed the Continental Divide three times.
The Roberts Tunnel
The second system is the larger piece of engineering. The Harold D. Roberts Tunnel runs 23.3 miles from Dillon Reservoir beneath the divide to the North Fork of the South Platte, and it is among the longest water tunnels in the world.
Excavation began in 1946 and it was not finished until 1962, sixteen years later, through fault zones and highly variable rock. The finished tunnel is a concrete-lined pressure conduit just over ten feet in diameter.
Dillon sits at the confluence of the Blue River, the Snake River and Ten Mile Creek and holds around 257,000 acre-feet, making it the largest reservoir Denver Water owns. In the 2002 drought the Roberts Tunnel is generally credited with saving the city.
The East Slope Half
The other 50 percent is native South Platte water, captured on Denver's own side of the mountains and stored in a chain of reservoirs running through Park, Douglas and Jefferson counties.
Cheesman, Strontia Springs, Antero, Eleven Mile Canyon and Marston are the main storage on this side. Across both slopes, Denver Water's nine major reservoirs hold approximately 750,000 acre-feet when full.
This is the half that would carry the city if the West Slope diversions stopped, and it is the reason the answer to the headline question is a reduction rather than a catastrophe.
What Losing The Colorado Would Actually Mean
A total loss of West Slope water would cut Denver Water's supply roughly in half. That is severe, and it is survivable in a way the same event would not be for Phoenix or Las Vegas.
The response would be sequenced and familiar. Mandatory restrictions first, with outdoor watering the initial target because landscape irrigation is the largest discretionary use in any Front Range utility. Then drawdown of East Slope storage, then emergency agreements with neighboring providers, then rationing if the shortage persisted across years.
What Denver would not face is the scenario Las Vegas spent half a billion dollars preparing for. There is no single reservoir whose surface elevation determines whether the city gets water, because Denver takes its share before the river reaches any of them.
The Compact Call Is The Real Risk
Here is the scenario that actually occupies Denver Water's planners, and it has nothing to do with a reservoir running dry.
The 1922 Colorado River Compact obliges the Upper Basin states to deliver a set volume downstream. If they cannot, the Lower Basin can make what is known as a compact call, which would force Colorado to curtail water rights established after 1922 in order to meet the obligation.
Denver's transmountain diversions are post-1922 and therefore junior. Analysts expect that under a curtailment most transmountain diversions would be limited and possibly stopped altogether, which means the legal mechanism would take Denver's West Slope supply regardless of how much snow actually fell that year.
Water officials generally expect a compact call to trigger a lengthy legal fight rather than an immediate shutoff. That is its own kind of problem, because a utility cannot plan around litigation the way it can plan around a reservoir level.
The Reservoir Denver Never Got
Denver's entire supply picture would look different today if one 1990 decision had gone the other way.
Two Forks Reservoir was proposed on the South Platte southwest of the city, at around a million acre-feet, and it would have been filled largely with Colorado River water delivered through the Roberts Tunnel. It was intended as the anchor for a much larger expansion of Denver's Blue River collection system.
The federal permit was vetoed, and the project died. What followed was a shift in strategy across the entire Front Range toward conservation, reuse and efficiency rather than new large storage, which is a substantial part of why Denver uses so little water for so many people today.
Gross Reservoir Is The Current Fight
The modern version of that argument is over enlarging Gross Reservoir, the storage that receives Moffat Tunnel water, a project that would increase average annual diversions from the Fraser and Williams Fork.
Denver Water's case is that the Moffat side of its system has far less storage than the Roberts side, which leaves the city vulnerable if the South Platte has a bad year at the same time as the Fraser. Opponents argue it takes more water out of an already depleted Colorado headwaters.
Both positions follow directly from the dual-basin design. The redundancy that protects Denver is the same thing that puts pressure on the West Slope.
Where It Stands This Year
The current position is not comfortable. Three consecutive below-average snowpack years have drawn the system down to its lowest May 1 storage in more than a decade, and Denver Water has been operating under restrictions with escalation on the table if summer rainfall underperforms.
That is the dual-basin system's weakness showing. The redundancy works when one basin has a bad year, and it works far less well when the same warm dry pattern sits over both sides of the divide at once, which is increasingly what a bad year in Colorado looks like.
The Answer
If the Colorado failed, Denver would fall back on the South Platte, on roughly 750,000 acre-feet of system storage, and on restrictions severe enough to change how the city looks in summer. It would lose about half its supply and it would not run out of water.
The more realistic threat is that Denver never gets to find out whether the river fails, because a compact call takes its West Slope rights first on legal grounds. Being upstream protects a city from empty reservoirs. It does not protect it from being junior.
