At least 1.3 lakh people in northern California have been told to evacuate their homes after engineers said the emergency spillway – the structure used for controlling the release of water from a dam – on the Oroville Dam was likely to collapse. At least eight cities including Oroville, Gridley and Marysville were issued evacuation orders by officials, who said that heavy rain and snowfall had weakened the dam, AP reported.

The potential collapse of the spillway was first reported after engineers on Thursday discovered a hole that was eroding material towards the top of the spillway. Butte County Sheriff Koney Honea said that while the erosion was not taking place as fast as it would, they “had to make the very difficult and critical decision to initiate the evacuation of the Orville area and all locations south of that”. “We needed to get people moving quickly to save lives.”

Meanwhile, California’s water resources department said it was releasing as much as one lakh cubic feet of water per second from the dam’s main spillway, BBC reported. Honea said officials were considering using helicopters to drop rocks into the hole to contain the spillover. Roads leading out Oroville and the surrounding areas were reportedly jammed as residents sought to leave the flooding zone. Evacuation centres were also set up around 30 km northwest of Oroville.

On Friday, water levels at the Oroville Dam – the tallest in the country – were less than seven feet from its topmost point, according to Reuters. California Governor Jerry Brown had asked the United States’ Federal Emergency Management Agency to declare a major disaster in the state because of flooding and mudslides caused by the heavy rains.