Drought to overflow

Almost 200 000 people were evacuated recently after channels draining the Oroville Dam in northern California were damaged because of unusually heavy rain.

Images captured by the Copernicus Sentinel-2A satellite on 21 December 2016 and 30 January 2017 show the remarkable difference in the height of the dam water. Far more of the shoreline is exposed in the earlier image, for example. There are a couple of islands visible to the north of the dam in the first image but they are almost entirely submerged after the rains. The animation also shows that the water is much lighter in colour in December but is much darker in January owing to sediment washed into the dam. The ‘spillway’ appears dry in the first image but water can be seen clearly gushing down in the later image. It is the first time that water had ever run down the spillway. In turn, damage to this spillway caused by excessive water led to fears about safety and the area being evacuated.

At a height of 235 m, Oroville is the highest dam and second largest reservoir in the USA. It provides an important water supply and is used to generate hydroelectricity. Until the recent rains it was a defining image of the state’s drought.