Looking up the highway from our impromptu stop on the way up to Mount Hood, you can see the lava flow on the right, with talus raveling down from the overlying lahar deposits. The road crews have added some sturdy fencing as a further barrier to prevent rocks from rolling out into traffic. But just as we saw with the block knocked out of the Jersey barrier recently, that fencing has taken a lot beating from falling debris.
I was not able to get a good, clear photo of the lahar deposits around the river at this stop, but this is the best of them. In the little slice of whitewater right near the center of the shot, you can see some of the rounded boulders that characterize much of the upper stretch of this river. And though it's over exposed, you can see more of the poorly sorted and essentially un-bedded deposits on the far bank. Near the top, it looks as if we're seeing a talus pile from a still higher (though not necessarily younger) lava flow, based on the angular, un-transported nature of the rock, and the consistency of color and apparent composition.
Well, okay, yes, this photo is very similar to the previous one in the series, and maybe doesn't add much in the way of information. On the other hand, look at that talus pile of loose boulders behind the barrier; it has taken a lot of beating. And the fact that one of those boulders was able to knock that large chip right off the reinforced concrete is really pretty impressive. At least it was to me.
Looking over later photos of this outcrop, I found I'd misremembered it somewhat in yesterday's description- it appears to be a lava flow- likely andesite or similar- overlain by lahar deposits, not simply piles of those debris flows. However, there's still plenty of bouldery, loosely consolidated and poorly sorted debris up above that andesite (?) cliff, and pieces obviously fall out and off frequently. This poor, beaten up Jersey barrier testifies clearly to that.
There's a tendency to think of alluvial fans as an arid landscape landform, and they often are. But they can occur anywhere debris flows are a major component of sediment transport. (I almost called the phenomenon "erosion," which technically, debris flows aren't.) And volcanoes generally are highly vulnerable to debris flows; in that setting, they're called "lahars." Volcanoes are steep, catch a lot of precipitation as a result of orographic lift, and often have large amounts of unconsolidated to poorly consolidated rock. It's clearly a setting where saturated ground can fail, leading to a torrent of mud to boulder-sized sediment along with trees, houses, cars, and any furry creatures unfortunate enough to be in the way. I suspect most of what we're seeing in this mini-fan is the result of single rocks and boulders falling down the stream bed- not one or more lahars. As we'll see soon though, this whole area is composed of lahar deposits, young and old.
Followup, 3:33 PM- It just dawned on me, if this pile formed the way I described, it should more accurately be described as a talus cone.
Getting closer to Mount Hood, some details, such as the upper snowfields, started to emerge. However, the smoke still veils much of the scenery. I imagine with better clarity, this is a beautiful drive. In these conditions, though, it was kind of sad. Good stuff lay ahead of us though...
After our stop at Cascade Locks, we drove through the gorge, and got off I-84 at Hood River. The smoke and low water flows at the falls made spending any further time in the gorge pointless. I believe this photo was taken just a bit out of Hood River, on Route 35, AKA the Mount Hood Highway. Right smack in the middle of the photo, the eponymous mountain is well camouflaged in the smoke.