New Report: US Water Reservoirs on the Decline

A recent study from the University of Virginia has found that major water reservoirs across the continental United States are experiencing longer, more severe, and more unpredictable periods of low storage than in past decades. This issue is particularly pronounced in the western and central United States, though reservoirs in the eastern and southeastern United States are also affected. Overall, the study reveals that reservoirs are becoming less reliable and more vulnerable to climate change.

The study, published in Geophysical Research Letters, aims to enhance water forecasting. By improving predictions, water managers at various levels—national, regional, and local—can make better decisions regarding water release timing and volume.

Reservoirs are becoming increasingly crucial as natural sources of water storage, like snowpack, rivers, and groundwater, are diminishing. Reservoirs play a key role in mitigating the impact of droughts, but issues like low storage can lead to widespread water shortages. For example, Lakes Mead and Powell faced their driest period in 1,200 years between 2000 and 2021, prompting severe water-use restrictions across the southwestern U.S.

Several factors affect how much water can be stored in reservoirs, including drought, water withdrawals, and sediment buildup. These factors are changing, often pushing reservoirs beyond their original design conditions. Caelan Simeone, a hydrologist at the U.S. Geological Survey’s Oregon Water Science Center, led the study. Simeone noted that the increasing variability and declining storage levels in reservoirs indicate that they may not adapt well to modern climate conditions.

National analysis on reservoir changes has been limited, often focusing on local or regional data. Simeone and his team studied water levels in 250 large reservoirs from 1981 to 2020 to understand national trends. They compared these levels to management practices and climate patterns. Notably, reservoirs in the arid West and Central U.S. experienced longer and more severe low storage periods. However, even in the Southeast and Pacific Northwest, reservoirs saw declines in maximum storage levels.

The study revealed that out of the 250 reservoirs analyzed, 169 showed a reduction in maximum storage, with 89 experiencing significant drops. On average, the maximum water storage declined by 2.2%, and in cases of significant declines, by 8.1%. This widespread reduction in maximum annual storage was unexpected.

The study attributes these trends to a combination of increased sediment and shifting hydroclimatic conditions. As reservoirs were typically designed based on past conditions, adapting them to current climate changes poses a significant challenge for managers.

Simeone emphasized that the changing conditions reveal the limitations of older reservoirs designed under the assumption of stable climate conditions. Now, water managers must address the hydrological shifts induced by climate change.