Sunday, December 7, 2014

Old Skagit Channels and a Tributary Alluvial Fan

Before the weather shifted back to wet, I had a venture up the Skagit Valley in an area of old river channels, old river terraces and a dissected alluvial fan. One of the old river channels on what is now a terrace above the river flood plain was ice covered.

The lower end of the channel showed that water had been flowing out of the channel prior to full freeze up. The valley in the background is a tributary stream valley to the Skagit.


There was lots of water flowing in this old Skagit River channel on the flood plain proper. This water was flowing through an abandoned meander loop that had been abandoned by the river but now was occupied by a tributary stream coming off the valley side. Note that rapid water flow is flowing through trees and with the cold weather this water flowing through tree stands was not the result of high water. This feature suggests the creek had relatively recently changed course.

Stumbling about a forested flood plain with lots of water has its pleasures; the above observations were secondary to my field excursion. To get a better handle on the flood plain, old channels, terraces, and the creek a view of a DEM of the area clarifies understanding of the landscape.

DEM of Skagit Valley
Red line roughly marks the outline of the alluvial fan of the creek and blue marks the creek route.
Skagit River is on the north side of the valley on this river reach

The wide meanders and old channels of the Skagit are readily visible. What is also apparent is that the Skagit River took a few bites out of the northern, distal part of the creek's alluvial fan. A small secondary fan has formed where the creek encounters the old river channel. A stream course change on this secondary fan has directed the creek into a slightly different route now through the trees shown in the image above.

I did not mark the Skagit flood plain the above image. Most of the meander bends and old channel traces on the south half of the valley are now well above the flood plain level and are level terrace areas bound by flood plain abandoned channels or in places the river itself. This indicates that though the river has a meander component in this reach, it has still been down cutting. This reach of the Skagit is a broad section of the valley that continues downstream to the Sauk River. Below the Sauk the river enters a much narrower reach where it is incised down through a thick section of glacial sediments including a high terrace with kettles.

DEM of Skagit - Sauk confluence
The narrow reach to the west cuts through glacial sediments
Upon my return from exploration of the flood plain my new friends on the terrace above the flood plain came over to greet me.


Anonymous said...

Is that possibly a young fault trending NNE just left of center cutting both valley sediments and upland rocks?

Dan McShane said...

Good eye. I had noticed that too. The apparent offset does not continue to the north or south. Not aware if anyone has looked into it, but I have not researched it yet. The South Skagit Highway crosses it right at where the Sauk makes a sharp bend against the base of the slope and then follows that odd straight line in an otherwise meander area.

doug clark said...

Hey Dan,

just stumbled across this in searching for other items on the web (alluvial fans); I think that the apparent lineation is unusual happenstance of unrelated linear features rather than a fault. The straight scarp along the modern channel would have to be REALLY young (i.e., historic) for it to not be destroyed by the river (it defines the modern channel edge). The apparent projection southwestward onto the higher glacial deposit is much fainter and more muted than the scarp next to the river (which is opposite what I would expect for a tectonic scarp, given the more stable nature of that surface). It also switches apparent throw where it heads towards the river (changing from south side up to south side down).

Our eyes are trained well to see patterns!