Sunday, January 8, 2017

Liveblogging the Deluge: Perspectives on the Biggest Flood in a Decade, Part Two

Check back for updates at the end of this blog...
First off, let me emphasize that I am writing almost exclusively about the central Sierra Nevada and adjacent Great Valley, particularly the drainage of the Merced, Tuolumne, and Stanislaus Rivers. I say this because some very serious things are happening farther to the north, and flooding is occurring in a number of places. If you live in those areas, you need very much to be listening to emergency response officials, and not reading random blogs!
Screen capture of Yosemite Falls from video on NPS Facebook Page

That being said, it looks like we have good news emerging from the storm. Although the rain has been heavy at times, it did not reach the apocalyptic levels that seemed possible earlier in the week. The Merced River in Yosemite Valley is certainly rising towards, and will exceed, flood stage (below), but not nearly to the extent that had been predicted in the previous week.

Early predictions had suggested discharges in the range of 20,000 cubic feet per second, but the latest projection is pointing more towards 9,000 cfs (below). This will raise the river level to nearly 12 feet, and that will inundate some roadways, but nothing like the floods of 1997 when wide parts of the valley floor were under 8-10 feet of floodwaters. That's good news.

There is also good news indicated by the actions of the watermasters at Don Pedro Reservoir on the Tuolumne River. Two days ago, a lot of water was being released from the reservoir, trying to make room for possibly catastrophic river flows. Flows were up to 10,000 cubic feet per second for a while (see some pictures here). This morning, I checked the flow (below), and it has dropped back to the usual level of several hundred cubic feet per second. This means that the dam operators are confident that they can capture and save the runoff and are not worried about the dam being overtopped the way it was in 1997 (the damage from that event is still visible 20 years later). (*see new update below)
Discharge of the Tuolumne River at LaGrange Jan 8, 2017

The river surge will have some benefits: River Hyacinth, an invasive aquatic plant has been choking the channel of the Tuolumne River during the last few years. The surge of the artificial flood has no doubt cleansed the channel of the river in many places. A lot of silt that has been smothering the best spots for salmon eggs has probably been swept way, and new sand bars have perhaps been formed. Although some minor flood damage may have taken place, there is a lot of good too.
River hyacinth in the lower Tuolumne River in 2015. Those mats of green aren't supposed to be there.

The other area of concern is Dry Creek. It is an unconstrained drainage, and for a time it seemed possible to have a record flow of 8,000 cubic feet per second. Those projections have backed off a bit too, but the expected 5,600 cfs will still be the highest runoff that I've seen, and some minor flooding damage will be possible in Modesto. Notice in the diagram below that there are three peaks predicted.

I've been thinking about something else today. Will this spell the end of the drought that has afflicted California since 2011? The short answer is no, it will not. The storm is building reservoir levels to more healthy levels, but filled reservoirs are but a single metric in determining droughts. This warm storm has melted the snowpack, and the snowpack is what is critical to sufficient supplies during the dry summer season. The good news is that the storms later in the week will be much colder, and the snowpack may be building up. The storm does nothing about global warming, which will continue to put a "finger on the scale" each year in California towards continued droughts.

Even more concerning is the fact that the storm will only help a bit to recharge the groundwater of the Great Valley. We have developed a huge deficit in groundwater storage during the drought, as agricultural interests have pumped increasing amounts of groundwater to replace the missing irrigation supply. Even if we have the foresight to put water back into the ground, some of the storage space has been lost due to compaction. This becomes obvious when one observes the amount of subsidence of the ground surface throughout much of the San Joaquin Valley (the southern portion of the Great Valley).
UPDATE (1/8/17 4:50PM): The story on the drop in the discharge on the Tuolumne River is a bit more nuanced than I suggested above. I've been notified that they are trying to modulate the flow with the high discharge in Dry Creek to prevent flooding downstream of the confluence of the two watercourses in Modesto. In other words, flooding occurs on the Tuolumne at about 9,000 cfs. Dry Creek may contribute as much as 6,000 cfs tomorrow, so they've cut back on the flow of the Tuolumne to compensate.

UPDATE  (1/8/17 8:50PM): The Merced River in Yosemite Valley has just reached flood stage at 10 feet (6,600 cfs). The river is expected to rise another 1-2 feet before subsiding.

UPDATE (1/8/17 11:20 PM): Among all the other stories of the day, word comes from Calaveras Big Trees State Park that the beloved "Tunnel Tree" has fallen in the windstorm. This is sad in one way, but it's clear that these kind of "touristy" developments like carving big holes in the base of the tree is damaging to them. We are changing nature for our own amusement instead of appreciating them for what they are. It's an unfortunate parallel that the Orca in the "Blackfish" story died this week as well.

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