Our third clue was that someone had recently been pushing some dirt around, as though the water and sediment had actually flowed across the road.
Mt. Airy Summit is a broad, almost unnoticeable summit about 15 miles west of Austin, and all the flooding activity had seemingly taken place on the east side of the summit. It didn't look like there was a whole lot of drainage area upstream from the flash-flooded dry washes - some washes on the north side of the highway and some small drainages right along the road.
Mt. Airy Mesa, the hill shown in the photo above, is formed of columnar-jointed ash-flow tuff sitting on some white to light greenish-yellow poorly welded tuff or lake deposits, the latter of which are strongly clayey.These rocks units are denoted as Tt2 and Ts2 on the Geologic Map of the North-Central Nevada, which are designated as being in the 34 to 17 Ma range. The large square in the map above is a township, about six miles on each side. [Note: this map shows Highway 50 going through Railroad Pass to the south on the road now known as Nevada S.R. 722, formerly known as Route 2, and before that known as Highway 50.]
The drainage area for the flooding seemed small to me, and possibly the clay in the Ts2 unit contributed to runoff. The thunderstorm must have dumped a lot of water really fast in the highlands at least on the north side of the highway, and possibly on the south side. [Note: "dumped", "a lot", and "really fast" are technical terms - well, I mean, I wasn't there when it was raining, and can't find a single thing written about this little flash flood that must have taken place on July 24th.]
If you're working in an area, you can examine flash-flood cuts and deposits like this one when they are fresh. In a year, come back and look again: the dry wash will look much the same, but the bushes will be in better condition. If you come back year after year, you can gradually get an idea of what a dry wash looks like after a certain time has passed, and you will be able to estimate the date of the last flash flood by looking at any dry wash anywhere.
The fact that a particular dry wash flooded within the last year, or hasn't flooded for ten years - using the above estimating method - doesn't really say anything about whether the dry wash is ripe to flood again anytime soon, so don't use that estimate as a guide to whether you are safe from being caught in a flash flood. Some drainages seem a little more prone to flash flooding, and some types of drainage areas are more dangerous to be in during a rainstorm.
It would have been fairly easy to get above the flash-flood at Mt. Airy Summit because very few of the dry washes overflowed their regular stream banks; when they did, the water wasn't very deep - judging from the muddy watermark on the sagebrush maybe a foot or two at most. However, there would have been no way of knowing at the time how long the rain would fall or what size the flash flood would be. So it's a good idea to get to high ground when rain is pouring all around you or when it's falling upstream. Flash floods can run for miles and can carry large boulders or even pickup trucks with them.We saw only minimal evidence for other flash flooding between Austin and Fallon, Nevada - above, a little flash flooding had been going on on the west side of Sand Springs Pass, about 25 miles east of Fallon. Location of the photo is near Bench Mark 4372 in the lower left (southwest) part of the linked-to MSRMaps map.
I've only been near one stream when it was in flash-flood mode. Water was roaring down the little stream on the west side of the Virginia Range south of Reno, Nevada. It never rose high enough to overflow the banks, but the sound of boulders smashing against one another was impressive.
Stewart, J. H. and Carlson, J. E., 1976, Geologic map of north-central Nevada: Nevada Bur. Mines and Geol. Map 50, 1:250,000.
UPDATE 11Jul2010: A good place to view the results of the flash flood, some of which can still be seen east of Mt. Airy Summit, is to park south of Highway 50 on the dirt road shown below, and to cross to the north side where the wash is about the go under the road.
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