How Dams Can Kick Up a Storm and Change Our Climate

Only 140 kilometers from our Berkeley office, the Folsom Dam towers 100 meter high over the American River. When it was built in the 1950s, the project was supposed to withstand the most severe flood in 250 years. Yet after it was completed, strong floods suddenly became more frequent and overtopped the dam at several instances. Until a safety upgrade goes forward, 440,000 people in the downstream area are exposed to the highest level of flood risk in the US. Scientists have now found evidence that the project's problem may be partly of its own making, and that dams can in fact kick up a storm.

We have known for a long time that dams can influence local rainfalls. Humidity evaporates from reservoirs and irrigated fields and gets recycled as rainfall. Dams and levees can reduce evaporation and rainfalls when they drain wetlands and open up woodlands for deforestation. The Niger Delta in West Africa is an example. In September, the delta's wetlands extend to 30,000 square kilometers -- roughly the size of Belgium -- and feed rainfalls over a much larger region. Yet existing and proposed dams on the Niger would reduce the river's flow by almost half. Christopher Taylor of the Centre for Ecology and Hydrology warns that this could "significantly reduce" the seasonal window when the delta induces rainfalls.

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