A crisis at California’s Oroville Dam in early 2017 dramatically demonstrated the incredible power of flowing water. The 235-m (770-ft) dam (figure 1) is a 250 km (160 mile) drive north-east of San Francisco in the foothills of the Sierra Nevada. It’s the tallest dam in the US.

In February 2017, heavy rainfall in the Feather River catchment raised the water level of Lake Oroville, and operators began a routine release of water down the main spillway. As they increased the flow, even though it was well below its maximum capacity, an unusual flow pattern developed.1 On shutting off the water, they discovered the water had ripped large slabs of concrete from the spillway and eroded the underlying rock. The crater extended almost the full width of the spillway (figure 2).

Rains persisted and the dam continued to fill. Because of concerns that the auxiliary spillway could fail, operators had no choice but to use the damaged spillway again. The situation was so serious that more than 180,000 people were evacuated at short notice from the valley below the dam.2

As the flow resumed it eroded more concrete and started to gush alongside the spillway down the embankment (figure 3). However, operators continued discharging water for more than two weeks until the lake had reduced to a safe level.3

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