Aspen Journalism is compiling a data dashboard highlighting metrics of local public interest, updated weekly on Tuesdays.

Twin Lakes diversions resume, dropping upper Roaring Fork streamflows

The state of the Roaring Fork River this late summer season is in flux and depends on where in the valley you’re measuring.

Below Maroon Creek, the USGS gauge measured streamflow at 173 cfs on Aug. 21, or 88.7% of average. That’s down from Aug. 14, when the river was flowing at 181 cfs; however, as a percentage of average, this week’s value rose from last week’s 79.7%.

At Emma, below the confluence with the dam-controlled Fryingpan, the Aug. 21 streamflow of 540 cfs represented about 104.9% of average. That’s up from 477 cfs, and 89.3% of average, on Aug. 14.

But at Stillwater, located upstream of Aspen, the Fork on Aug. 21 ran at a paltry 25.4 cfs or 44.6% of average, down from 56 cfs, or 86.2% of average, on Aug. 14. That put the river below the decreed minimum instream flow for the stretch of 35 cfs.

A major factor is that the Cameo call to protect senior water rights in the Grand Valley has come off, likely due to recent rains. When the Cameo call is on, as it was for a portion of mid August, it curtails junior water rights upstream, including the Twin Lakes diversion under Independence Pass. That means significantly more water in the upper Roaring Fork, which helped bolster streamflows in the previous week. Now, with no Cameo call, the Front Range cities that own the Twin Lakes water rights can resume diverting from the Roaring Fork basin’s headwaters via Grizzly Reservoir, at a rate of 40.3 cfs on Aug. 21.

With less water flowing down Lincoln Creek to the Fork, the state placed an instream flow call on the Fork at Difficult Creek on Aug. 23 to try and boost low flows for the sake of the environment.

Meanwhile, the Crystal River above Avalanche Creek, which is not impacted by dams or transbasin diversions, flowed at 171 cfs, or 108.2% of average, on Aug. 21. Last week, the river ran at 109 cfs, or 58.6% of average.

Lake Powell’s water level is going down

Lake Powell‘s storage remains at one of its lowest levels recorded since it began filling in the 1960s. On Aug. 21, the reservoir was 24.83% full (with a total capacity based on a 1986 sedimentation survey) or 25.9% full (based on 2017-18 sedimentation data), down from Aug. 14, when it was 24.97% full (1986 data) or 26.05% (2017-18 data).

On July 1, the Bureau of Reclamation revised its data on the amount of water stored in Lake Powell, with a new, lower tally taking into account a 4% drop in the reservoir’s total available capacity between 1986 and 2018 due to sedimentation.

“After inputting the new data on July 1, 2022, storage values at the current elevation were updated, resulting in a decrease of 443,000 acre-feet,“ bureau officials wrote in an email. 

Aspen Journalism published a story explaining the recent drop in storage due to sedimentation.

The reservoir’s capacity has fallen since last year, when on Aug. 21, 2021, it was 31.42% full (1986 data).

On Aug. 21, Lake Powell’s elevation reached 3,533.4 feet, or 166.6 feet from full pool, down from 3,533.9 feet on Aug. 14. That puts the water level just 8.4 feet above the target elevation of 3,525. Powell’s surface elevation this year peaked at 3,539.84 feet on July 3, after it dipped to its lowest level since filling of 3,522.24 on April 22. Last year, on Aug. 21, the reservoir reached 3,550.8 feet, or 149.2 feet from full pool.

The “minimum power pool” for turbines generating hydropower at the Glen Canyon Dam is 3,490 feet, and 3,525 feet has been set as a buffer to ensure that the reservoir and the turbines can continue to function properly.

The first two weeks of August saw above-average air temperatures

High air temperatures at the Aspen airport were above-average in the first half of August, from 79.5°F on Aug. 1 to 87°F on Aug. 9, or about 10 degrees above normal. Maximum temperatures dropped from 83°F on Aug. 14 to 78°F on Aug. 15, which is less than one degree above normal. Meanwhile, low temperatures remained above normal, reaching up to 58°F on Aug. 10, or about 12 degrees above normal. On Aug. 15, minimum temperature was hovering around 51°F, or about five degrees above normal.

Clean air reported in Aspen last week

The air quality in Aspen was “good” this past week. The AQI index for ozone ranged from 26 Aug. 16 and 44 on Aug. 18.

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Laurine Lassalle

Laurine Lassalle is Aspen Journalism’s data desk editor, where she works to catalogue and analyze local public data. She also heads our our “Tracking the Curve” project, documenting COVID-19 in Pitkin,...