By Jack Schmidt | January 9, 2024
There was not much loss in reservoir storage in the Colorado River basin in December 2023. Total storage in the basin’s reservoirs only declined by 17,000 acre feet during the month, and the combined contents of Lake Mead and Lake Powell increased by 68,000 acre feet. At year’s end, the basin’s water users have only consumed 21% of the gain in storage caused by the large snowmelt of 2023.
Here are a few graphs depicting where we stand at the start of the new year.
1. The amount of water stored in the basin’s reservoirs remains at an unprecedented low condition. On 31 December 2023, total basin storage was 28.0 million acre feet (af), of which 17.5 million af was in Lake Mead and Lake Powell (Fig. 1). The total amount of water stored in the basin is the same as it was in early May 2021. At that time, storage was less than at any other time in the 21st century, but we drained the reservoirs much more in the summer and fall of 2021 and 2022. The recovery of storage caused by the large runoff in 2023 provided some relief to the ongoing water-supply crisis, but water storage remains critically low.
This winter’s snowfall has been meager so far, and Reclamation’s January 1 prediction of unregulated inflow to Lake Powell is only 66% of average. Water managers are beginning to anticipate another summer of water-use restrictions. No one should think that last year’s runoff eliminated the ongoing crisis.
Figure 1. Graph showing active water storage in 42 reservoirs in different parts of the Colorado River basin. Conditions at the end of December 2023 are comparable to conditions in early May 2021, indicated by the black arrows. Data downloaded at https://www.usbr.gov/uc/water/hydrodata/reservoir_data/site_map.html.
2. Most of the basin’s water storage is in Lake Mead and Lake Powell (Fig. 2). Releases from Lake Powell and reductions in Lower Basin water use were sufficiently large that there was significant recovery of storage in Lake Mead. At the end of December, storage in Lake Mead (9.05 million acre feet) exceeded storage in Lake Powell (8.44 million acre feet) by approximately 600,000 acre feet. The difference in storage between the two reservoirs is much less than during the previous two years when more water was stored in Lake Mead.
Figure 2. Graph showing water storage since January 2021. Note that storage in Lake Mead was significantly greater than in Lake Powell in 2021 and 2022. Large spring runoff in 2023 was captured in Lake Powell, and some of that accumulated inflow was subsequently released to Lake Mead. The rate of reduction in storage in reservoirs upstream from Lake Powell significantly slowed after mid-fall 2023. The category “other Upper Basin reservoirs” includes Strawberry, Granby, McPhee, Dillon, Starvation, Nighthorse, and smaller reservoirs. Water storage in Lake Mohave and Lake Havasu remains nearly constant. Note that the vertical axis is an arithmetic scale that has a break. Data downloaded at https://www.usbr.gov/uc/water/hydrodata/reservoir_data/site_map.html.
3. The rate of loss in reservoir storage this year remains low relative to the rate of loss in previous years (Fig. 3), especially the rate of decline of the combined storage in Lake Mead and Lake Powell. (Fig. 4) The basin’s water managers are doing a good job of reducing use and conserving water in reservoirs. Reclamation’s estimate of probable consumptive water use in the Lower Basin in 2023, issued 31 December 2023, is 5.78 million acre feet, nearly 900,000 acre feet less than Lower Basin consumptive use in 2022. Will that degree of water conservation be enough? That depends on how much snowmelt occurs this spring.
Figure 3. Graph showing the rate of reduction in basin-wide reservoir storage in each of the past ten years. The reduction in storage has been at a much slower rate than in other years. Each year that plots lower than 2023 on this graph reflects a higher rate of loss in storage than this year.
Figure 4. Graph showing the rate of reduction in the combined storage in Lake Mead and Lake Powell in each of the past ten years. The reduction in storage has been slower than in any other recent year. Each year plotting lower than 2023 on this graph reflects a higher rate of loss in storage than in this year.
Acknowledgement: Eric Kuhn and John Fleck provided helpful suggestions that improved this posting.