Lake Oroville will go over the top sometime in the next 24 hours, and begin pouring down the emergency spillway officials conceded last night.
Oroville Dam itself is sound and there is no imminent threat to the public, said DWR Acting Director William Croyle.
The concrete spillway, which helps drain the reservoir, began breaking up Wednesday. Officials were forced to reduce the outflow from the structure to limit further damage. But the rate with which water is entering the lake is far outstripping the rate at which it can be drained.
Total discharges are not expected to exceed the capacity of any channel downstream as the water flows through the Feather River, into the Sacramento River and on to the San Francisco Bay. Inflow to Lake Oroville was expected to peak late Thursday and begin to decline through the weekend.
Thursday evening operators increased the releases down the damaged, gated spillway from 35,000 cubic feet per second to 40,000 cubic feet per second, with another 7,000 cfs passing through the dam’s hydroelectric power plant outlets. But those discharges are not enough to match an inflow of approximately 192,000 cfs. The reservoir, at 884 feet elevation above sea level Thursday, will naturally begin to spill into the emergency spillway channel at elevation 901 feet.
The emergency spillway has never been used since Oroville Dam was completed in 1968.
Using the emergency spillway would wash large amounts of soil, trees, brush and debris into the river. Officials have been clearing brush. Officials are also evacuating fish from a downstream hatchery.
The lake stood at 96 percent of capacity earlier this morning. As of 9 a.m. the lake level was just under 6 feet from spilling over. The reservoir level is currently rising at a rate of about a third of a foot per hour.
The emergency spillway, separate from the damaged spillway, isn’t gated. Water will start pouring over the emergency spillway automatically if the reservoir reaches capacity at 901 feet.