Tuesday, July 29, 2008

Flash Flood on Highway 50

brown bushes As MOH and I were driving over Mt. Airy Summit west of Austin the other day, our first clue that thunderstorms had caused a little flash flooding was the brown, muddy, knocked over bushes on the side of the road.

mud Our second clue was - lo and behold, some actual mud and wet ground - in Nevada!

disturbed 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.

equipment Ah, and who's been moving the dirt? Yes, there was a little construction activity in progress as we drove slowly through!

summit1 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.

mapThese 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.]

summit2 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.]

deposits 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.

sand springsWe 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.

Reference:
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.


View Larger Map

4 comments:

Lockwood said...

I'm curious what kind of rocks we're looking at in the last picture (Sand Springs Pass). At the risk of looking like a fool, I'm going to guess somewhat altered volcaniclastics that have been tipped over pretty well. And given that word, I have to say those are pretty pretty rocks!

Silver Fox said...

Check out this map, which shows Sand Springs Pass at the south edge of the map. The tilted-looking, dark gray stuff is supposed to be post-lower-Jurrasic to pre-middle-Mioccene basalt (yikes on the age range! - I've always thought it was pre-Tertiary) - probably altered, possibly tilted, and intruded by sills or dikes (the light colored to buff rocks, I think). The basaltic rocks in the map are in contact with Triassic limestone. All that is overlain structurally along a nearly flat-lying contact by gray, blocky-weathering limestone of unknown age.

Would be a good place to look around, I think!

Lockwood said...

Thanks for pointing me to the map! Don't see a date on it, but I'm guessing (from latest citations) early-mid 60's. That spread of dates on the basalts and other volcanics is pretty crazy until you realize this is 45+/- years ago. I'm inferring from the terraserver topo and the geomap that you're looking basically east in the photo from near the benchmark. Behind you and up the hill is whatever's left of the Summit King Mine, which according to the notes was one the top 3 producers in Nevada '48-51. If you switch to the aerial photo in terra server, you can kind of see that the trend of the pits/addits is basically east-west, so the trend crosses the road right near where you took the picture. I'll bet with the current price of metals, prospectors think it's a good place to look around too!

My reaction to the picture was that if I was to see that out the window, I'd be off across the sagebrush to check it out. Don't know what I'd find, but as I've been telling people for years, every time you break open a rock, you get to see something no one else in the history of the planet has had a chance to see.

I'd love to see a flash flood out in the desert. There's an area up in the north of Nevada, near Denio, called Virgin Valley. A good-sized basin, the whole thing drains out from a deep, relatively narrow canyon incised through a gorgeous set of rhyolite flows (or possibly very welded tuffs). There's driftwood and piles of brush stranded 15-20 feet off the floor. Trying to imagine a flash flood of that magnitude in the middle of the desert is a little jaw-dropping.

Thanks Again!

Silver Fox said...

The map was published in 1965, which was quite a while ago, indeed! Even before my time. :)

A more recent pub has the black rocks in the photo north of the highway being black shales of Mesozoic age (which fits more with what I generally recall - they look rather slaty, overall). And the Geologic Map of Nevada - 34 MB - has all the pre-Tertiary mapped as Jurassic-Triassic seds, metaseds, and metavolcanics, and local carbonates. The light colored to buff rocks in my photo could be some ash-flow tuffs. I guess someone will have to get out and look someday!

From the highway near the benchmark, or maybe just a bit to the east, the photo is looking north to northwest. Tertiary basalt flows and underlying Tertiary sediments cap the high hills just behind/above the photo.

In the TerraServer aerial view, more or less in the center south of the highway is a kind of largish, white man-made configuration which has currently or recently been a gold heap leach pad, I think. A smaller man-made disturbance north of the highway was a heap leach pad put in during the 1980's. I'd recommend the area for prospecting, although I suspect there are already a lot of existing claims.

The largest flash flood I'm really familiar with was the Northumberland Canyon flood of 1979, in Big Smoky Valley (also with flooding on the east side of the Toquima Range), which took out one pickup truck (no on in it), and several 100's of tons of barite ore from the small barite mine no longer in existence up the canyon. It stripped bushes, dirt, and talus from the canyon walls to a height of more than 6 feet, unless time has exaggerated my memory (I think it was actually higher).

I left my hammer in a picture I took not too far from the Northumberland Canyon Road, right next to the dry wash, and now its buried somewhere under the flash flood deposits!