Thursday, August 19, 2010
The Merced River has dropped below 100 cubic feet per second (cfs) at the Pohono Bridge gauging station now. This is about 15% of the annual mean daily flow, but is still a bit higher than usual for this date. Locals who commute into Yosemite Valley for work refer to a different measuring gauge: Table Rock, which is located just upstream of the junction of Hwy. 140 and the Big Oak Flat Road. Table Rock is now 3-4 feet above the surface, but it was underwater without a ripple just a short time ago. It's hard to believe that the river was at nearly 7000 cfs just 2.5 months ago. That's our Mediterranean climate of strongly disparate seasonal precipitation. Rivers in most of our country do not regularly vary their volumes by two orders of magnitude within a year.
Some unexpected cloud cover yesterday moderated temperatures in El Portal, Yosemite Valley and Tuolumne Meadows. More monsoon moisture is probably on the way over the next few weeks, and we are likely to see some small spikes in river flow as rain runoff from the uppermost watershed is added to the low river volume. Any surges will probably not be that visible on Table Rock, but we should see them on the gauging station display.