Wednesday, August 13, 2014

Day Creek Mesa

This DEM (digital elevation model) of the Skagit River valley shows the river has been systematically eroding away an older river valley floor. In the image above dark green is lowest elevation white/gray is highest.

A closer look shows the active channel migration area in the green colors shows lots of active channel movement back and forth that on occasions bumps up against the sides of the old valley floor in this area near Day Creek. The older valley floor can be seen in the brown to light green shades. Streams flowing across this elevated remnant valley floor are incised down through land surface and the broad channel migration zone of Day Creek can be seen on the right also cut down below the old valley surface.The features suggest this reach of the Skagit is a net erosion area. An even higher elevation remnant valley floor can be seen in center of the image. This remnant isolated fragment forms a small mesa that some how survived previous erosion. The mesa correlates with the elevated terrace in gray on the east side of the image. As far as I know this is the only isolated mesa like feature in the valley. Most of the older valley floor areas are connected to the sidewalls of the valley as terraces. 

The shape of the mesa has been altered a fair bit in recent years by anthropogenic erosion. The mesa is mostly underlain by high quality sand and gravel and has been actively mined in the early 2000s.   

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