I’ve been pretty successfully checked out of Colorado River work while I put the finishing touches on the new book (pre-order now!) but my colleagues are on it with a new post looking at the over-winter storage at the big reservoirs behind Hoover and Glen Canyon dams:

Key Points

• The rules that control releases from Lake Powell and Lake Mead are very different. Lake Powell’s releases are determined by an Annual Operating Plan that has little flexibility during the year. Lake Mead’s releases change each month in response to changing delivery requirements to Lower Basin users. The impact of these different release rules on each reservoir’s storage was illustrated this autumn and early winter when Lake Powell steadily declined and Lake Mead steadily increased. The magnitude of Powell’s decline and Mead’s increase compensated for one another, and the total combined storage in Lake Powell and Lake Mead did not change.
• During the four months between October 1 and February 1, Lake Mead’s releases were reduced in response to decreasing Lower Basin demands, but Lake Powell’s releases were not similarly reduced. Lake Powell lost 615,000 af during the four-month study period, and Lake Mead gained the same amount.
• On February 1, Lake Mead had 2,714,000 af more water than Lake Powell, the largest difference between the two reservoirs since April 2022.
• Modest flood inflows in early October delayed drawdown of Lake Powell by six weeks. Releases during the four-month study period were the second smallest since at least 2010[1]. Releases from Lake Mead were the smallest since at least 2010. Despite the small inflows to Lake Mead, the increase in storage in Lake Mead during the study period was the largest since 2019.
• The four-month delay in depletion of the combined storage in Lake Powell and Lake Mead saved between 400,000 to 900,000 af.
• Forecasts for spring snowmelt inflow to Lake Powell are not encouraging and have been declining all winter, because Rocky Mountain snowpack remains meager.

[1]We compared the inflows, outflows, changes in storage, and Lower Basin consumptive uses between 2010 and 2026.

This feels like good news, or least not bad news, except for that last bullet. The Upper Basin median forecast is just 40 percent of the median.