Monday, May 8, 2017

Liveblogging the Deluge: The Other Shoe Drops!

Wait...it hasn't rained in weeks. Why am I still talking about "liveblogging the deluge"? This is the case of the other shoe dropping. For months we have had storm after storm, adding up to a record year of precipitation in Northern California, and nearly a record in the central state. The snowpack a week ago was nearly 200% of normal. So the storms have stopped, but now a tremendous amount of snow is about to melt, so we have another round of possible flooding.
This was all brought home to me this last weekend when I took my class on a field trip to Yosemite Valley. We had a short heat wave during the week prior, and the river shot above flood stage on Wednesday and stayed there until Saturday when we arrived. The valley floor was flooded in many places, and the Merced was raging downstream (below).
On Thursday, the river peaked at nearly 12 feet (9,000 cubic feet per second), and was still at 10 feet (6,500 cfs) on Saturday. With a weak storm system passing through over the weekend, the flows backed off a bit, but the next heat wave will started the cycle all over again. For perspective, the average flow at this time of year is about 2,000 cubic feet per second.

Needless to say, the waterfalls were booming. Yosemite Falls were exploding and the echoes were resounding across the valley. It sounded like boulders were falling off the cliffs.
There was a change back home, but for a different reason, I suspect. The Tuolumne River was down for the first time since early January! I took my customary walk along the Parkway River Trail in Waterford, and the water had receded some 2 or 3 feet! I checked the numbers, and indeed the flow had declined in the last week from flood stage at 11,000 cubic feet per second to about 8,500 cfs. My first thought is that the dam operators finally felt confident enough about the flood capacity at Don Pedro Reservoir, and this has to be partly true, as the lake is down to 798 feet after nearly spilling over in January at 830 feet.
A check of the flow data told a different story. For months the outflow has been at 11,000 cfs or so, and inflow at the reservoir has been around 8,000-9,000 cfs. But during the heat wave last week inflows climbed to 11,000-12,000 cfs. The releases have been fairly constant at about the same level, but now the irrigation system is starting to take a fair fraction of the river at LaGrange Dam, thus the drop in river level.
The river is still astounding. The normal channel still lies hidden beneath 10-15 feet of water, and the river is filling the flood plain in a way that hasn't been seen in decades. And it will continue, according to reports, well into the summer. Extraordinary...