As the lake levels continued to rise and with the main spillway weakened, the only way to reduce the water levels was to utilize the emergency spillway. Lake Oroville can be seen towards the top of the photo, with the emergency spillway highlighted red.
On February 11th, water from Lake Oroville began flowing over the emergency spillway as designed. Several hours later, near the top of the emergency spillway’s concrete weir and along the ramp erosion was present. Below is an example of the erosional damages from the overflow. The second image displays how close the erosion was to the weir. Officials feared the erosion and down cutting into the weak layer of bedrock would continue upstream, further damaging the weir.
On February 12th, officials were forced to reopen the main spillway to reduce strain on the weir, and with a flow less than 5% of the approved rate, the emergency spillway was determined to be unsafe and unpredictable. If the erosion occurring near the weir to fail, uncontrolled flow would cause the Feather River downstream to crest higher than levees were built to withstand, and towns from Oroville to Sacramento, along with Highway 70, would possibly flood.
February 15th, with the expectation of more storms moving in, the Department of Water Resources, along with the help from construction crews, Caltrans, and other state agencies, worked quickly to reinforce the emergency spillway. More than 1200 tons of rock was being deposited in the damaged areas every hour as seen in the picture below, and there were over 125 construction crews on site. As this was happening, the Department of Water Resources was releasing water in an attempt to lower Lake Oroville below 840 feet.