Friday, May 13, 2011

Warm then Storm

Last weekend's overcast and rain caused the Merced volume (and the tributary waterfalls) to drop back down to near average flow. One might think that rain would raise the river level, but, in fact, the cloud cover and cooler temperatures reduce the snow melting rate much more than a bit of rain can increase the runoff. Warm sun brought the flows up again through the week. Again this weekend we expect a cool storm to bring a bit of rain (and snow -which, of course, reduces the near-term runoff much more than rain does), so the waterfalls will be smaller at the start of next week than they are now.
On most days, the river (and tributary) volume exhibits a nice 24-hour cycle of rise and fall. The hottest part of the day is when the most snow melts up at 7-13,000'; this surge of melt doesn't reach the two Yosemite Valley river gauges until many hours later. It's odd that our highest water flows in the Valley happen after midnight - about 12 hours after the hottest part of the day. That's the lag time for high country runoff to make it down to 4000'.
It was cold enough in the Valley this past Tuesday morning (9 May) for people to observe frazil ice in Yosemite Creek and in Ribbon Creek. A deposit outside the eastern channel at Ribbon Creek persisted into the next day, looking like an anomalous patch of snow.
NPS road crews have reached Glacier Point, and they've reached the White Wolf area on Tioga Road, through average snow depths of 8' and 10' respectively.
The bizarre red snowplant is emerging from the ground in Yosemite Valley. Bracken fern is tall and unfolding its fiddlehead fronds. Dogwoods are fully alight now. Western Wood Pewees arrived this week in the Valley. The concentrations of yellow-rumped warblers and some other birds have dispersed from the Valley to higher elevations a bit. I've been seeing 1-2 bears a week in the Valley, more than has been my usual experience.
If you're up early and have a good eastern view, check out the concentration of planets in the east before dawn.

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