Earth Notes - Dams and Weather

Flagstaff, AZ – Earth Notes: Dams and the Weather

Dams like those on western rivers may be generating more than just hydropower. It turns out that large dams can do more than just store water and spin electric turbines. New research suggests they can, in some cases, increase local rain, perhaps even triggering extreme precipitation events.

Weather patterns change near dams because of reservoirs and because new, water-intensive land uses, like irrigated farms, tend to spring up nearby. The added moisture makes it into the air, and falls back as rain. China's huge Three Gorges Dam, for example, has increased rainfall by a millimeter a month hundreds of miles from the reservoir, and reduced temperatures in the region by just over a degree Fahrenheit.

The Folsom Dam on California's American River may be helping to cause the very flooding that's been forcing its spillways to stay open more than usual in recent years.

According to new research, dams in dry regions like the desert Southwest may increase the frequency of extreme storms, those in the 99th percentile of storm severity. The most severe storms have increased up to five percent in the American West in the past century, and that increase correlates with the construction of large dams.

Most southwestern residents don't worry too much about too much rain, but study co-author Faisal Hossain of the University of Tennessee says that dams that cause heavy precipitation may add extra trouble to a water-supply system that's already facing the dual stresses of drought and climate change.

-Anne Minard