U.S. water reservoirs are shrinking and becoming less reliable, new study finds

Major water reservoirs in the continental United States are experiencing longer, more severe and more variable periods of low storage than they did decades ago, according to a new study. The problems are most severe in the western and central United States, but reservoirs in the eastern and southeastern United States are not immune, the study says. Overall, reservoirs are less reliable and more vulnerable to climate change than they used to be.

The findings, which update critical information on water storage, are expected to improve water forecasting, helping water managers at national, regional and local levels make more informed decisions about when and how much water to release. The study is published in Geophysical Research Letters.

Water storage reservoirs are becoming increasingly important as more ephemeral natural storage becomes less reliable: in many areas, snowpack is diminishing, rivers are depleting, and humans are pumping groundwater supplies.

Reservoirs can help limit the spread of drought downstream, but interruptions to their normal operation can lead to widespread water availability problems. Consider, for example, the low storage levels of Lakes Mead and Powell from 2000 to 2021, the driest 22-year period in the region in 1,200 years. This drought triggered widespread water use restrictions across the southwestern United States.

Drought, water withdrawals and sediment buildup behind dams determine how much water can be stored in a reservoir. Each of these factors has evolved, in many cases causing reservoirs to operate in conditions that are not what they were designed for.

“Reservoirs are a key part of the modern water cycle, and water managers can influence them,” said Caelan Simeone, a hydrologist at the U.S. Geological Survey’s Oregon Water Science Center who led the study. “We know that reservoirs are changing and were designed for historical hydrologic conditions, so there’s now uncertainty about how reservoirs will adapt.”

National Water Overview

Much of the information and research on reservoirs is local or regional, limiting scientists’ understanding of how climate and anthropogenic changes impact water storage at a national scale.

“Water reservoir managers could benefit from this knowledge,” Simeone said. “It would allow them to take into account broader national water trends as well as more local trends.”

To get a national perspective on reservoir changes, Simeone and his colleagues analyzed water levels in 250 large reservoirs from 1981 to 2020, looking for changes in base, maximum, and minimum water levels. They compared water levels to management practices and climate, looking for patterns that could explain water level changes. Because data for reservoirs in the northeastern United States are not available, that region is excluded from the study.

Reservoirs in more arid regions of the western and central United States tend to experience longer, more severe, and more variable periods of low storage. This is partly to be expected, because reservoirs in drier regions are designed and engineered to handle variable annual runoff and drought conditions. But current drought conditions are pushing low-flow conditions to the extreme.

The problem is not unique to the arid West. Reservoirs in the wetter Southeast and Pacific Northwest, as well as arid regions, have seen their annual maximum storage capacity decline. Of the 250 reservoirs studied, 169 saw their maximum storage capacity decline, and 89 of them had significant declines. Across all reservoirs, the median decline in maximum storage capacity relative to the average was 2.2 percent; for reservoirs with significant declines, the median decline was 8.1 percent.

Simeone did not expect these falls.

“The reduction in annual peak storage levels has been widespread, which really surprised us,” Simeone said. “Many reservoirs are no longer filling to their original levels. Overall, we are seeing a decline in peak water levels across the United States. This was the case even in places where periods of low storage were not more frequent.”

According to Simeone, a combination of increased sediment and changes in hydroclimatic conditions is likely behind the observed increase in water storage variability and the overall decrease in water levels.

Reservoir managers are trying to adapt to these changing conditions, which can be difficult when reservoirs were designed decades ago, assuming that the climate and society would be relatively similar. (Most of the reservoirs and dams studied were built between about 1930 and 1970.)

“We thought conditions would be more or less stationary,” Simeone said. “Climate change has put an end to that assumption. Managers now have to try to mitigate the hydrological changes we’re seeing.”