Use of Oroville Dam's Half-Finished Spillway Now 'Unlikely'

We'll have to wait awhile to see how the rebuilt Oroville Dam spillway stands up to water pounding down its 3,000-foot-long concrete chute.

After giving the public a heads-up that the half-repaired structure might be pressed into service because of last week's expected heavy rains, the agency announced Sunday that use of the spillway is now "unlikely."

The extra attention to lake levels grows out of last year's spillway breach -- a near-calamity that triggered mass evacuations and exposed the flood-control outlet's history of poor design, construction and maintenance.

DWR issued last week's advisories after a 50-foot rise in Lake Oroville in the second half of March -- due to a series of storms and dam operators' decision to limit releases from the reservoir to almost nothing.

By early last week, the lake's surface elevation had risen to 793 feet. That was just 20 feet below the sill of the spillway gates and 37 feet below the level that would trigger specially adopted flood-control releases down the spillway.

Last week's widely advertised atmospheric river storm was forecast to dump 7 inches of rain in the wettest parts of the Feather River watershed above the reservoir, setting up the possibility of a huge runoff event.

In response, DWR began what it called "aggressive" releases from Lake Oroville through the dam's only currently operating outlets, the hydroelectric plant at the base. While the half-completed spillway itself is said to be able to handle a release of about 100,000 cubic feet of water per second -- the daytime volume of Niagara Falls -- the power plant can release up to 15,000 cfs. Over the weekend, releases peaked at about 13,500 cfs.

The rain arrived on schedule, and the wettest recording station in the Feather basin, a site called Four Trees, got 7.12 inches of rain. Other totals ranged from 4.06 inches at Oroville Dam to about 6 inches.

But for whatever reason -- perhaps partly a function of the relatively short duration of the storm and the fact it was preceded by 10 days of dry weather -- the big runoff did not materialize. The lake has risen 8 feet in the past week.

Flows into the lake peaked at 54,000 cubic feet per second on Saturday. That compares to 80,000-plus cfs last month and 190,000 cfs in February 2017 immediately after the failure of the main spillway -- an event that led to a breathtakingly fast rise in the reservoir, the rapid erosion of the facility's unpaved emergency spillway and the evacuation of 188,000 people from downstream communities.

More rain is expected this week -- but not enough to make a significant change in the outlook for Lake Oroville or the dam's spillway. The National Weather Services's California-Nevada River Forecast Center forecasts that the wettest parts of the Feather River watershed will get between an inch and an inch and a half of rain in a pair of small storms later this week.