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.

Liveblogging the Deluge: Big Changes on the Tuolumne River

Source: http://www.capradio.org/articles/2017/02/20/don-pedro-reservoir-spillway-opens-for-first-time-since-97/

I've been away for the last five days, experiencing California's storms from an entirely different perspective, that of being exposed and out in the open country of Death Valley National Park. There will be plenty of information and pictures about our adventures soon enough, but there were plenty of events back home that have been rather significant as well. A great many people have been affected as the waters of the San Joaquin River and its tributaries have risen past flood level.

First and foremost, at Don Pedro Reservoir on the Tuolumne River the emergency spillway has been opened for the first time since the floods of 1997. This became necessary when the lake filled nearly to capacity, and a new storm battered the north state over the weekend. The channel downstream floods at around 9,000-10,000 cubic feet per second, but storm runoff threatened to exceed that level. The dam operators had to make a decision to cause some flooding downstream to avoid even greater flooding if runoff was too high.

Flows were ramped up to around 16,000 cubic per second over the weekend, and have since settled back to 13,500 cfs at the latest reading (below). Rain and snow are still falling in the region, so the concern about flooding remains for the time being. The lake fills at 830 feet, and the current level is 828.82 feet. There isn't much room for error.

Source: USGS (https://waterdata.usgs.gov/ca/nwis/uv?11289650)

People are understandably concerned about Don Pedro Reservoir as they compare notes with the events at Oroville Dam in the last few weeks. Both dams have an emergency spillway that flows over unlined and unreinforced rock rather than a concrete channel.

There are some important differences, however.

First, the emergency channel below Don Pedro is much less steep than Oroville's, and is therefore less subject to the headward erosion that threatened to undermine the spillway at Oroville. Second, the spillway was tested previously, during the floods of 1997. The runoff amounts in that event were almost astronomically higher than any flows expected in this year's event. A forty-foot channel was carved through the meadow, and the flowing waters are unlikely to do anything worse this time around.

Still, these are nervous times. I had my first chance to see the higher flows this morning as I headed to work. I stopped at the head of the Tuolumne River Parkway Trail to have a look. The picture below was taken on February 5th when flows were around 10,000 cfs. Note that the entire trail was above water level.

The picture below shows the same scene this morning with flows closer to 14,000 cfs. As you can see, the entire trail is under water now, which means I'll have to find another place for my birdwatching for a few weeks or so.

One senses that authorities are just barely keeping ahead of events, which is not surprising given the nearly nonstop barrage of atmospheric river storms battering California. The situation has not quite spiraled out of control, and the storms are letting up for a few days, which will allow for more floodwaters to flow into San Francisco Bay. I hope we'll have a breather.