In the second part of my presentation on Willamette Valley Geology, I talked about streams and stream processes. I have thus far neglected to mention that my presentation was preceded by a talk by Bob Lillie (Emeritus Prof from OSU Geology), on plate tectonics and the overall tectonic setting of this region. I bring that up at this point, because I generally mention that technically speaking, the Willamette Valley isn't really a valley. A valley, by definition, is a water created/carved feature, and the Willamette Valley exists, first and foremost, because of its plate tectonic setting. With a forearc ridge to the west- the Coast Range- and a volcanic arc to the east- the Cascades- the Willamette Valley is a structural depression. Even that can be a bit misleading, as it could lead people to think it's actively descended compared to its surroundings. The Coast Range has risen due to compression, folding, and underplating. The Cascades have been elevated due to warming (hence more buoyancy) from below and addition of lava from active volcanism. The Willamette Valley, in contrast, has basically just sat where it is, near sea level.
That is not to say this is a geologically static environment, by any means. While the Willamette River exists where it does due to tectonics, rather than the valley existing because of the river, the river and its tributaries are constantly moving and reworking the sediment that sits on the valley floor. There have been marked changes in the landscape even in the brief period since Europeans moved in, starting a bit more than 150 years ago.
from here) to talk briefly about stream meandering and the landforms created during that process: point bars, cutoffs, oxbow lakes, meander scrolls, swell-and-swale topography and so on.
A cross section of the alluvial fill in the mid-valley area. During the ice ages (Pleistocene), glaciers did not reach the valley floor. There is no firm evidence of glaciation in the Coast Range, though there is some suspicion that there may have been an ice field on the north flank of Marys Peak. However, alpine glaciers were extensive in the Cascades, and major streams coming out of those mountains show clear evidence (U-shaped valleys) of glacial erosion. While the exact elevation of maximum extent varies, the general rule of thumb is to look for terminal moraines at around 1000 feet elevation- note that this is the maximum extent of how far down the valleys glaciers were able to push. Actual accumulation of ice would have been at much higher elevations. Ice is much more erosive and competent (that is, much better in its ability to carry sediment) than is water, so it's thought that major streams coming out of the Cascades, and indeed much of the Willamette Valley, was sediment-choked, and would have been dominated by braided streams. A relevant feature of the Willamette River is that it follows the west side of the valley for much of its length. It has been suggested that this may be a result of it being "pushed" to the west as a great pile of sediment accumulated from the east. Towards the end of the Pleistocene, as I discussed in Part 1, variable amounts of sediment accumulated during repeated inundations from the Missoula floods. In the Corvallis area, if memory serves, the amount of silt is about 10 meters/30 feet.
This Week's Geo-Quiz: Geologic Time
2 days ago