State engineers on Thursday discovered new damage to the Oroville Dam spillway in Northern California, the tallest in the United States, though they said there is no harm to the nearby dam and no danger to the public.
California Department of Water Resources personnel monitor water flowing through a damaged spillway on February 10 at the Oroville Dam in Oroville, California.
Once the reservoir reaches its elevation capacity of 901 feet, the emergency spillway will direct the overflowing water to a diversion pool at the base of the hillside, Carlson said.
A gaping hole - some 250 feet long and 45 feet deep - appeared Tuesday in the lower part of the main spillway, which is made of concrete and rests on dirt.
Video on the sheriff department's Facebook page showed about 35,000 cubic feet of water per second being released down the enormous slide into the river, but officials said the additional flow would not necessarily cause flooding.
This is the first time the uncontrolled emergency spillway has been used in the dam's 48-year history. Yet flows had to continue as the reservoir continued to rise.
They expect that water flowing out of the dam will catch up with the torrent flowing in from recent storms by sometime Saturday.
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Water topping the Oroville Dam may cause severe property damage, and it would be catastrophic for downstream salmon and steelhead populations. Doug Carlson, a spokesman for the Department of Water Resources, called it a "last resort".
Department of Water Resources officials said the flow over the emergency spillway will range between 5,000 cubic feet per second (cfs) and 10,000 cfs.
The water further eroded the gated spillway channel, but "not to a point that would compromise the integrity of the dam upstream", state officials said in a statement Friday.
The hatchery, on the Feather River, is vital to the survival of the state's native chinook salmon, whose numbers have dwindled during the drought. But they also said erosion is an ongoing concern.
Located about 150 miles northeast of San Francisco, Oroville Lake is one of the largest man-made lakes in California and the 770-foot-tall Oroville Dam is the nation's tallest.
But faced with little choice, the state Department of Water Resources resumed ramping up the outflow from Lake Oroville over the damaged spillway to keep up with all the runoff from torrential rainfall in the Sierra Nevada foothills.