GIS Analysis of Okanogan Valley Terraces

A DEM-based analysis of a flight of glaciofluvial and kame terraces along the Okanogan Valley near Omak, WA. Data are for river right.

Each data point is a pixel value created from a simple calculation for 'relative elevation' made on the 30m DEM. Thus, the values are not actual elevation (but scale linearly with it) and many of the pixels in the original DEM were filtered out (pixels with slopes greater than about 10%, which is very flat). Each point shown in the chart satisfies the filter criteria (nearly level slope, within 2km from river, river right only). The color ramp represents distance away from the river (red=close, yellow = distant). My annotations/interpretation are in pen.

The blurriness in some terraces in the above example is the result of four factors:

1.) The size of the symbol we used to plot the data could only be made so small (we wanted to make them smaller). We used open circles here, but horizontal dashes might be better. We could vertically stretch the display to change how things look a bit, too.

2.) The terraces slope gently down-valley (to the left) as well as toward the river (in a plane out of the screen).

3.) The surfaces of glacial terraces like these often undulate a bit (kettles, drainages, etc).

4.) Terraces formed primarily by deposition of sediment by flowing water in the presence of ice. Such deposition is complex with local cuts and fills, which express as variations in the y-axis (elevation).

Since we know each charted point represents a flat-lying pixel, we can interpret individual terrace surfaces in a coarse way (lump all the blue pixels in the example) or split them into individual terraces (black lines drawn on blue pixels).

This analysis is probably best done for long river valleys using a combination of ArcGIS (DEM clipping, calculations) and R (looping, charting). There are other technologies that would work, too. Terrace interpretation is a human task no matter how you do it.

I am currently using this method to complete a regional analysis of the terrace system (the so-called "Great Terrace") along the Okanogan-Columbia River Valleys between Osoyoos, BC and Vantage, WA.

The Great Terrace near Chelan, WA from I.C. Russell's USGS reconnaissance survey report of 1893.

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