Tuesday, December 30, 2014

What a Geologist Sees - Part 22 (Originally published 10/10/08)

Most folks know a river or creek meander when they see one (or more).

Usually we see them in Coastal Plain settings or other places where the stream gradient (feet/mile drop in elevation) is low, especially if the materials underlying the stream are soft Coastal Plain sediments and/or floodplain deposits.

When a stream's gradient is steep, as in a mountain stream, there is a tendency for gravity to control erosion, i.e., the erosion is vertical - down-cutting as we call it. This vertical down-dutting (being slightly redundant) results in sharp "V"-shaped valleys, with no flood plains.

So when you see deeply-incised meanders, such as in the above photo, that suggests that the meanders were established under low-gradient conditions at a higher Base Level (see this post for an explanation of Base Level). Then a rapid drop in Base Level and/or a rapid uplift of the land "preserved" the meanders as the river cut downward through the Colorado Plateau sedimentary rock layers.
The Grand Canyon also illustrates this same rapid uplift/drop in Base Level, as does the area containing the Black Canyon of the Gunnison River, in Colorado, though without the meanders.

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