This was the scene just as we started across the bridge at White River. Its headwaters are up in that mostly snow-covered valley on Hood's flank. I initially interpreted that line of dead trees at the top of the bank in the lower center as more of those damaged by the 2006 lahar that roared down the valley here. Comparing it to other pictures from this crossing though, I think a better take is that the bank was developed as one or more older debris flows were eroded by the river- perhaps farther by the 2006 event. Subsequent collapse of the loose, unconsolidated rubble exposed and damaged roots, leading to the trees' deaths.
...and there was Mount Hood in all its glory. Mount Hood is mostly composed of andesitic to dacitic dome lavas, and is apparently not in the habit of temperamental explosive eruption, like its nearby neighbor to the north, Mount St Helens. While I'd imagine that during ongoing dome-building eruptions, pyroclastic flows could be a serious risk, the biggest risk in historical times is that of lahars, when saturated rubble rushes down the mountain and inundates nearby landscapes, just as we've been seeing in numerous previous posts.
This was a spot where the lahar on the White River inundated the forest floor more heavily, and most of them beyond the few survivors in the mid-ground appear quite dead. This may look like an ecological disaster, but keep in mind, these Douglas firs have evolved along with this volcanic environment for millions of years. They'll rebound, in time, quite well, thank you. There was a terrific blog post a few days ago from Hollis Marriott at In The Company of Plants and Rocks discussing how debris flows (non-volcanic, in that case) create a highly desirable setting for new Doug fir forests to become established.
There was a lot of road work going on this this area, and while we weren't stopped until we got to the bridge, traffic was slow, so it's difficult to estimate just how far we traveled through this damaged forest. It felt like miles, though it might have just been a couple. We still hadn't had a decent view of Mount Hood through the haze and smoke, but we were now up close to pass level, above a lot of it. And then we turned a corner...
Continuing to drive up toward the pass north of Mount Hood, I suddenly realized we were traveling through a very unhealthy-looking forest. Unlike forests killed off by pine bark beetles (where pretty much all the trees are dead), this one ran the gamut of dead, dying, and damaged trees, with a few looking fairly well off. That, in addition to the long stretch of road work, had me puzzled for a bit. Then something clicked, and I remembered there had been a very damaging lahar in the area some years earlier. (I just checked- it was 2006. See the bottom of this page for a short description and a very impressive photo of the aftermath at the Route 35/Mount Hood Highway Bridge over the White River.) What I hadn't realized until this trip was that the debris flow inundated a large area of forest in the area as well. Burying a tree's roots, for most trees, causes them to slowly suffocate, explaining the problem here.
In the FlashEarth view, you can see why it's called White River, but that's all loose volcanic rubble, not snow. You can also see some post-lahar incision, and how the debris fanned out into the forest when it reached the vicinity of the highway.
I'll leave this spot the same way I arrived: quite impressed with the power of falling rock to smash human constructs. Here, the concrete has been shattered, and the rebar visibly bent by rock falling off the cliff face. Judging by the freshly chipped face, it may have been the medium-sized rock to the front, not the big boulder behind it, that caused this damage.
A closer view of the same outcrop as yesterday's shot, showing lahar deposits over grey volcanic rock- likely something in the andesite/dacite range. Given the clean contact- as opposed to a brecciated lava surface, I'm guessing the top of the lava was an erosional surface for a period of time before the lahar deposits were emplaced. Again, the line of alders is a result of water following that contact until it reaches the cut.
Turning around to look at the outcrop across the road, we see the jointed lava flow overlain by rubbly lahar deposits. I (and others) have mentioned before that alders- the trees growing in a horizontal row across this cut- like to have their feet wet, and a tight cluster like that suggests a concentration of water. See comments 12 and 13 here, and my write up on the "Cheshire Cat outcrop," here, particularly clue #2. So that line of trees tells you that there's a bit more water in that portion of the cut than elsewhere, and that makes perfect sense. The lava, while jointed, is less permeable than the loose lahar deposits, so water tends to flow laterally along that contact until it reaches the surface, and signals its presence by supporting a stand of alders.