Sand Dunes? Really?


Big Dune, Bruneau Sand Dunes, Idaho

You probably think of sand dunes as a desert phenomenon. But you can get sand dunes wherever you have (1) a decent supply of coarse-grained sand, (2) an area where there is enough topography to create a trap for the sand, and (3) relatively equal distribution of the winds, or at least no prevailing single wind direction.

Most of the time in the northern half of North America the sand comes, directly or indirectly, from the Pleistocene glaciation. Few things are better than glaciers at generating ground up rock. But sometimes it just takes a huge flood. And sometimes it’s a combination of the two. WC will offer notes on an example of each.

As you are floating – well, paddling – down the Kobuk River in Kobuk River Valley National Park in northwestern Alaska, you get glimpses of some distinctly odd-looking hills on the northerly side of the river. If you pull ashore and walk just under two miles, you[ll find the Great Kobuk Sand Dunes, 28 square miles of sand dunes, some about 100 feet high. The sand is from the continental glaciers of the Pleistocene, which covered much of the area as recently as 10,000-11,000 years ago. The grains are quire coarse, showing little indication of water erosion or smoothing. The dunes, in summer, are probably the hottest place north of the Arctic Circle, sometimes reaching more than 100° F. It’s certainly not something you expect to find that far north.

The sand dunes are where they are because that stretch of the Kobuk River Valley lies between the Baird Mountains to the north and the Waring Mountains to the south, and the winds there blow almost equally from the east and the west. And so the sand has piled up, sometimes shifting enough to bury some of the taiga forest, but basically held in place.

You can find sand dunes in Central Washington, too, near the community of Moses Lake, in the channeled scablands created by the Missoula Floods. The Moses Lake Dunes are a product of the combination of glaciation and the Missoula Floods. Much of the sands were generated by the Wisconsian Glaciation, a bit earlier than the sands at Kobuk Valley, and then were transported a few hundred miles by the Missoula Floods. The dunes are in the lee of the Moses Lake Coulee, and are composed of basalt carved out by the Missoula Floods and quartz and quartzite generated by the continental glaciation. They are rather grandly called volcaniclastic aeolian dunes. These dunes have been largely sacrificed to the ORV crowd.

The Moses Lake Dunes are much smaller than the Great Kobuk Sand Dunes, only about 3,000 acres, less than five square miles, some of it seasonally flooded. But they are still impressive. The dunes are presently being slowly moved up the coulee, the flood channel created by the Missoula Floods, and at least in geologic terms are ephemeral.

Bruneau Dunes in the background; the foreground is a pond created by the higher water table consequential to the reservoir behind C. J. Strike Dam on the Snake River ar the confluence with the Bruneau River.

The final set of dunes are the ones in the photos, the Bruneau Sand Dunes, in the Idaho State Park of the same name, in south-central Idaho. The dunes are in an immense abandoned meander (or maybe a giant eddy; geologists disagree) of the Snake River generated in the Bonneville Flood. It’s called Eagle Cove, but there aren’t many eagles and it isn’t a cove. It is, however, a nearly circular valley and serves as a perfect trap for blowing sand. The result is the tallest dunes in Idaho, at 470 feet. The sand was likely generated by the Bonneville Flood, when the flood waters tore through the lake bottom sediments of fossil Lake Idaho.

Flood sediments, likely some of the eroded sandstone from the native rock of Eagle Cove and windblown loess all contribute to the dunes. The seasonally varying winds blow from nearly opposite directions. Unusually for a sand dune created by geomorphology, Bureau Dunes are in the middle of Eagle Cove, not in the side as most dunes are.

This is by no means an exhaustive list of the mechanisms that create sand dunes. But it does illustrate that different geologic processes can create the same geologic features.

One thought on “Sand Dunes? Really?

  1. Thank you for the explanation of, particularly, the Moses lake sand dunes. I appreciate it. Now I need to travel to see your other two examples.
    Take care. Tom Richards

    Like

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