Monday, October 1, 2012

OMN Willamette Valley Geology Presentation Part Two: Streams, Processes and Landforms

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.
I used the above image (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.
Corvallis lies on the west/left side of the above image, and near the top there's a nice example of an oxbow on the east side of the Willamette River. Also, if you enlarge the image to full size, you can see that the Benton/Linn County line was established along the historical Willamette River channel, but that the channel has moved since settlement. An area on the other side of the river, labeled "Fisher Island" in the image above, is technically part of Benton rather than Linn County. Rivers are not a good basis for boundaries; they move.
Another oxbow, near Keizer, Oregon
A somewhat randomly chosen area north and west of Eugene to illustrate how much of our farmland has been shaped by stream processes. You can see traces of meander scrolls almost everywhere on the valley floor if you think to look.
Nice examples of point bars at the Willamette/McKenzie confluence north of Eugene. Point bars "point" downstream.
Willamette Falls, at Oregon City. Oregon City became an important industrial center in the Oregon Territory early in its history, due to the availability of hydropower. Much of the 19th Century commerce in the region centered around the Willamette River as a transportation corridor, and while there is a canal and lock system to get by the falls (which has fallen into disrepair and was closed a couple years ago), Oregon City was also a transportation hub. The drop here is about 40 feet, and I hadn't realized it until I was doing some background reading in preparation for this talk, but in terms of volume, this is the 4th largest waterfall in the US. The bedrock is Columbia River Basalt, and there are some nice paleosols exposed in the roadcuts along I-205 a bit farther north from this image.
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 is a few miles east of Corvallis on route 34. It's subtle, but you can see the area where the tractor and power pole are, on the left, are a bit lower than the surrounding area. This is typical swell-and-swale topography. It's much more obvious in the winter when it's raining; water often accumulates in the swales.
This was a quick photo, taken along Parker Creek on the south side of Marys Peak, to point out that especially in smaller streams, but even in large streams and rivers in the absence of human intervention, large woody debris plays a complex but important role in shaping channels and their forms. In this case, a splintered slab of wood is moderating the rate at which the stream can erode lithic sediment.
Erosion can be much quicker than we perceive. The branching roots of this tree would have been established when it was a seedling. But during its lifetime the stream has removed about 3 or 4 feet of soil, leaving the top of the root system above ground. (Below North Falls, Silver Falls State Park, Oregon.)
The sediment in stream bottoms can tell you quite a bit too... the large, angular nature of the cobbles and gravel above tells me there's not too much upstream here. If the watershed was larger, the clasts would be more rounded. That they're as large as they are tells me that either it's a very steep watershed, or that it gets occasional extreme precipitation and corresponding flow rates. (In fact, it has both a steep gradient and occasional downpours- this is just a bit down the trail from the tree above.)
An old river bank on the east side of the OSU Campus. More specifically, this is the transition from a medium-level to a high-level terrace
Benton Hall, right (the oldest standing building on the OSU Campus), and Education Hall, left, standing on or near the high-medium terrace transition.
Looking east from about the same spot as the previous photo was shot, looking out over the mid-level terrace. These same terrace deposits underlie downtown Corvallis. Corvallis was originally established as Marysville, on the other side of the river. That side, though, is about 15-20 feet lower in elevation, and Marysville was destroyed in a flood during the late 1800's. It was rebuilt as Corvallis in subsequent years, between Oregon State College and the Willamette River. This helps to explain why paying attention to the niceties of low, middle and high terraces is important for would-be property owners. Low terraces are good for golf courses, parks and agriculture- around here, at least, where flooding is almost completely restricted to winter and early spring months.

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