A dangerous condition continues at the nation’s tallest dam.
A light flow of water began washing into the emergency spillway Saturday and the volume of water began to increase. Around 3 p.m. (6 p.m. ET) Sunday, authorities learned that the dam's emergency spillway was also eroding, Honea said.
This is the first time in the dam’s history that water levels have reached the point of flowing into not just the main spillway, but the emergency spillway. The volume of water flowing into the emergency spillway ripped a hole in the concrete which rapidly expanded to 250’ width. Releasing additional water down the spillway risks erosion that could rapidly undercut the structure. If the spillway failed it could still release a 30’ wall of water down the valley, damaging a significant region and endangering tens of thousands.
To reduce the strain on the emergency spillway, authorities have increased the flow from the main spillway to 100,000 cubic feet per second—double the normal rate. This risks damage to the main spillway, but should bring the lake level down to the point where more normal operation is possible.
Just two years ago, the lake was at less than 25% of its normal volume due to severe, ongoing drought, but an exceptionally rainy year with heavy precipitation over the winter has not just filled the lake, but raised it to the highest levels ever recorded, with water still flowing into the reservoir at over 50,000 cubic feet per second.
Evacuation orders have been issued for parts of several counties, including the area of Yuba City and Marysville City.