What a difference a day makes!  As of Friday afternoon, it appeared unlikely that Oroville Reservoir would rise enough for water to flow over the emergency spillway, as inflow into the lake had been declining since early Friday, the rain had stopped, and the primary spillway continued to be used to manage outflow. But the gaping hole in the spillway, coupled with concerns about downstream flooding, resulted in the story traveling all the way back to New York.  And with the latest inflow figures and DWR’s modified release schedule from the Primary Spillway, the emergency spillway actually started spilling on Saturday morning.

Here is a really helpful summary for going into the weekend:

1. As of Friday afternoon DWR did not believe water would flow over the emergency/auxiliary spillway, and assumed it could continue to release 65,000 cfs through the dam’s primary spillway. 65,000 cfs releases were anticipated to keep the lake level below the emergency spillway.  Watch footage from Action News Now of Chico-Redding for what 65,000 cfs running through a damaged spillway looks like.  However, late Friday night DWR changed the releases to 55,000 to arrest erosion.
2. As inflow into the lake continued to drop (from a high of over 190,000 cfs late Thursday night to about 125,000 cfs Friday afternoon), DWR anticipated that the lake level would start to drop sometime on Sunday. In fact, though, late Friday night the inflow did not drop as quickly as anticipated, and as of Saturday morning it was still at 95,000 cfs.  Thus, some water has started running over the emergency spillway.
3. The National Weather Service predicts some precipitation starting next Wednesday, but the anticipated precipitation from this next storm may not be enough to result in a lake level that will again spill over the emergency spillway.

   

4.  Authorities have placed boom lines in the diversion pool upstream of the Thermalito diversion dam to capture any debris that is collected along the way from water running down the emergency spillway. Authorities cleared some trees in the area immediately below the emergency spillway to minimize any debris that could end up in the river.  Here is a picture from Google Earth taken by user Briantravelman that shows the trees that could become debris in the river (hopefully the cows are no longer there).
5. DWR continues to monitor erosion from the crater in the spillway, and now believes it will continue to be able to release 55,000 cfs without additional erosion (erosion to date has moved about 200 feet up the spillway, and if not controlled much beyond that could damage the spillway gates).  However, since DWR believed it could run the Primary Spillway at 65,000 cos, this is obviously subject to change.  Officials believe further erosion is unlikely to occur because of hard bedrock underneath the spillway at the current point where erosion stops.

For more detailed information about current storage, inflow and outflow from Oroville Dam, we recommend the California Data Exchange Center.

Additional sources of information on this incident can be found on the DWR website.

Look for further updates as we consider the consequences of future storm events until the primary spillway can be repaired.