Thursday, February 23, 2017

Liveblogging the Deluge: Checking out the Spillway at Don Pedro Reservoir

It's probably not too hard to figure out what happened. I get home from a five day trip only to find that the spillway at Don Pedro Dam has been opened for the first time since the floods of 1997. It's a big event in these parts, an acknowledgement that the reservoir was full and in danger of spilling over in an uncontrollable manner. So I had to go and have a look. Mrs. Geotripper and I had a few hours this afternoon and headed on up the river. We had no idea what to expect, or whether we would be allowed anywhere near the spillway.
We were pleasantly surprised to find that there is some limited access for the public. If you want to have a look, you need to drive Bond's Flat Road to the Blue Oak Campground at Don Pedro (trying to approach from the south side - the Fleming Meadows side -  doesn't work as Bonds Flat Road is closed two miles away from the dam). The campground and day use area is open, and provides a limited view of the spillway and the spray from the water releases.
The spillway opens up into a former meadow that was carved down to bedrock during the floods of 1997. A portion of the flow can just be glimpsed from the viewing area. The spillway is similar to the one that failed at Oroville Dam, but at Don Pedro the slope is more gentle, so uncontrolled erosion of the rock is unlikely (especially since the flow rates are not nearly as high as they were in 1997).
The long concrete structure in the top two photographs is the weir that serves as kind of a last ditch spillway. The crest lies at an elevation of 830 feet, and the water this afternoon had reached 829.5 feet. As a consequence, the outflow through the spillway has been increased to 14,600 cubic feet per second (well above flood level) to start bringing the water level down.
There is a high point in the campground that provides a better look at the spillway from the lake side. On this sparklingly clear day, the view of the valley was also spectacular.
For the first time in twenty years the lake is absolutely full. It is quite a sight!

Just a note, in case you want to see the sights for yourself...stay behind the fences and follow the directions of the authorities. I didn't find out for myself ("But sir, I'm a GEOLOGIST!"), but the ranger told me stories of the people that they had caught and cited. They were rather vigilant.