Heavy rains pounded Arizona and the Four Corners region in recent weeks, and the effects were nearly impossible to miss. Storms drenched places like Flagstaff, Winslow and Durango, Colorado, surging through rivers, across streets and into homes.
Steady rain made for a soggy Halloween in the Phoenix area. Floods made it hard to reach the polls in some parts of Navajo Nation.
For all of the ways those rains reshaped life in the Southwest recently, they barely made a dent in the region's water supply crisis.
"Don't want to dim the news too much," said Nolie Templeton, who monitors Colorado River reservoirs for the Central Arizona Project. "Very happy to have more water in the system, but it is quite literally a drop in the bucket, particularly when we're looking at a 27 year — and counting — drought."
The Central Arizona Project, which brings water from the Colorado River to the Phoenix and Tucson areas, keeps a close eye on upstream conditions. Even after days of soaking rain, those conditions hardly changed.
Lake Powell, the nation's second-largest reservoir, went up 14 feet after the storms. However, its surface still sits about 15 feet lower than it did this time last year. Since Oct. 20, the reservoir went from about 23% full to about 26% full.
The rains were certainly unusual for this time of year. In the month of October, 232,000 acre-feet of water flowed into Lake Powell. That's more than triple the average October inflow across the past five years.
Ultimately, it's hard for any amount of rain to significantly increase the amount of water in Lake Powell. The vast majority of its contents come from Rocky Mountain snowmelt. This October, the rain-charged inflows added about 850,000 acre-feet of water to the reservoir. By comparison, average spring inflows to the reservoir are around 5 million acre-feet.
An acre-foot is roughly the amount of water it would take to cover a football field with one foot of water. The Arizona Department of Water resources says one acre-foot is roughly a year's supply of water for three homes in the Phoenix area.
The heavy rains boosted smaller reservoirs upstream. Navajo Reservoir in New Mexico, for example, holds back the San Juan River on its way to the Colorado River. It went from about 50% full at the beginning of October to about 80% full in early November. Its total capacity is relatively modest, though. Navajo Reservoir can hold about 7% as much water as Lake Powell.
Lake Powell and the broader Colorado River system are stretched thin by more than two decades of drought, supercharged by climate change, and steady demand from states that have failed to agree on a plan to reduce demand. As a result, water levels in Lake Powell reached a new record low.
Because of that, Templeton said, it would take about five back-to-back winters of above-average snowfall in the Rockies to get Lake Powell halfway full. That appears to be unlikely given that even average snowfall has been hard to come by. Drought and climate change have made winters warmer and drier across the region. Last winter was the driest in decades.
There are some small silver linings to the recent rains, though.
Water levels in Lake Powell had been tiptoeing towards dangerously low levels, threatening the ability to generate hydropower inside Glen Canyon Dam and, potentially, the ability to safely send water from the reservoir into the Colorado River on the other side. A new federal water management strategy is careful to keep water above that threshold. So, while the reservoir isn't completely empty, there isn't much left to send downstream.
That has serious consequences on the other side of the dam.
Limits on outflows from Lake Powell meant there wasn't enough to send a burst of cold water into the Grand Canyon this summer, making it harder for threatened native fish to compete against invasive predators. The limits have also accelerated dropping water levels in Lake Mead, the nation's largest reservoir.
States that depend on downstream deliveries from Lake Powell — Arizona, California and Nevada — are taking issue with their upstream counterparts. Arizona has openly threatened to sue those upstream states, arguing that downstream releases have fallen low enough to violate the foundational, century-old agreement that governs the Colorado River.
Kathryn Sorensen, a water policy researcher at Arizona State University, said given the slim margins behind Glen Canyon Dam, this modest boost from autumn rains makes a difference for the amount of water that is "accessible" without risking damage to the dam.
"Given how critically low elevations were relative to the need to protect that critical infrastructure," Sorensen said, "I would argue, yes, these flood flows were really important and helpful. But in the overall picture, it does not boost overall storage, and it doesn't buy us a lot of time. I would say it just gives us some breathing room."
That breathing room likely won't change the planned cutbacks to Arizona's water supply. Federal water management rules are unlikely to change, and big reductions to the state's allocation from the Colorado River are still on track to begin in the new year.
"It's not like a tap you can turn off and on," Templeton said. "These are things that require a little bit more long-term planning on where that water is going."
Templeton and Sorensen highlighted a few other small benefits from the big rains.
For one, they may have helped soak soil throughout the mountain ranges that supply the Colorado River with snowmelt. Dry soils absorb fresh snowmelt like a sponge, so pre-soaked dirt from these autumn storms may help more spring runoff actually reach the streams, rivers and reservoirs where people divert and collect water.
While the rains did not move the needle much on the Colorado River, they certainly boosted a smaller river system that is also important to the Phoenix area. Reservoirs along the Salt and Verde Rivers, which store water in the foothills east of the metro, are now 69% full. On October 1, they were only 48% full.
The Salt River reservoir system is much smaller than the Colorado River system, and more prone to quick refills. Many cities in the Phoenix area draw water from both the Colorado River and the Salt and Verde Rivers, and can rely on more water from the latter during acutely dry periods on the Colorado.
So much water entered the Verde River that one of its reservoirs got too full. The Salt River Project released massive volumes from Bartlett Lake downstream, sending it surging through the Salt River's channel in downtown Phoenix. Sorensen said those flows will help recharge the groundwater aquifer beneath the metro, which is another important resource for nearby cities.
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