Dune Fields of the Upper Columbia River Region, WA

Palouse dust storms thicken Okanogan soils.

- A silviculturist I once knew

In the Okanogan Highlands of northeastern Washington, forest soils are very thick on north- and east-facing slopes. Dust (windblown silt) delivered from the south is trapped by thicker vegetation on those relatively wetter, colder aspects. The result is grass-tree mosaic (i.e., Sanpoil Valley). Lower down, atop the broad, sagebrush-covered glacial terraces that line the Columbia River, dunefields have grown large in the past several thousand years. In contrast to silt particles that are easily swept up by strong winds, carried high and far, sand-sized particles are locally-sourced, hug the ground, and steadily march downwind. Dune migration direction is a product of sediment source, prevailing flow, and influence by the local topography.

Here are a few examples of dune fields along the upper Columbia River (Lake Roosevelt) and Okanogan River. Most of them are vegetated, so its unlikely you would even notice them when driving by. Bare earth lidar images cut through the vegetation to reveal their conspicuous forms. Remarkable just how many square miles of the Upper Columbia region are covered by partially stabilized sand dunes. Images are from Washington Lidar Portal and are all shown at the same scale. I've added migration direction arrows - best guesses based on morphology.

So do Palouse dust storms thicken Okanogan soils? Maybe, but its not sand doing the thickening. Silt, which doesn't form true dunes, does the thickening. Sand dunes move along the ground, driven by ground-level winds (SW to NE flow). Silt moves higher in the atmosphere. But if we assume the silt is subject to the prevailing southwesterly flow, we must look upwind for a source area. We end up in source basins like Quincy Basin, Moses Lake, Waterville Plateau, bars along the Columbia River between Wenatchee and Bridgeport, or even Kittitas Valley - areas west of the Palouse.

Is a Palouse source area correct? Again, maybe. Perhaps dust carried from the Palouse to the Okanogan moves at higher levels of the atmosphere in a complicated, less intuitive way. Or maybe late Pleistocene wind patterns were different from today. Maybe Summer winds were different 12,000 years ago and the dust from Canada came south. Alternatively, Okanogan silt may actually have come from upwind basins to the southwest, kindof where it seems like it should have come from.

Whence Okanogan silt? Prevailing wind direction, determined by migration directions of Holocene sand dunes, suggests dust in the Okanogan Highlands may not derive from the Palouse Hills to the south, but from the Columbia River corridor downstream of Brewster and from the western half of the Channeled Scablands.

Dune fields. Sand dune sites are numerous along the upper Columbia and Okanogan Rivers. Circles indicate 8 of 14 sites depicted in images below. Five others are north of the map area. Also missing a circle at Brewster Flat.

Intersection of Hwy 25 N and Miles-Creston Rd just east of the Spokane River mouth (above Fort Spokane).

Inchelium Hwy at Roper Creek inlet, west bank of Lake Roosevelt.

Near landfill on Carson Flats, north of the Colville River mouth south of Kettle Falls.

Northport-Flat Creek Rd east of Kelly Hill near Kettle River-Columbia River confluence.

Flat west of China Creek between Bossburg and Evans, east bank Lake Roosevelt.

Marble Flat, Crown Creek, and Hwy 25 North, upper Columbia River near US/Canada border. Location not shown on regional map.

Northport-Waneta Rd at Deep Creek, near US/Canada border. Location not shown on regional map.

South of Mitchell Mountain and east of Goodeve Creek. Location not shown on regional map.

Morris Creek just south of the Waneta US/Canada border crossing. Interesting how the margins of some dune fields can appear very straight, similar to trimlines formed by large floods. Location not shown on regional map.

Swede Flats along Miles-Creston Rd.

Oddball. Is this correct? Steep faces of dunes east of Hwy 97 are south-facing. Southern Okanogan Valley at Wakefield RR Bridge between Monse and Mallott.

Brewster Flat at the satellite dish array NW of Okanogan River mouth. Yellow-highlighted landforms have characteristics of both giant current ripples and sand dunes, but those noses look duney to me. Location found by Don Hruska. Not shown on reference map. Scale differs from others.