Dune Fields of the Upper Columbia River Region, WA
Palouse windstorms thicken Okanogan soils.
- Silviculture/Soils Consultant 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 are vegetated, so its unlikely you would even notice them when driving by. Bare earth lidar images cut through the vegetation to reveal the dunes. Remarkable just how many square miles of the region are covered by sand dunes. Images are from Washington Lidar Portal and are shown at the same scale. I've added dune migration direction arrows - best guesses based on their forms.
So does Palouse silt thicken Okangan soil? Maybe, but its not sand doing the thickening, its silt, which doesn't form dunes. Sand dunes move along the ground, driven by ground-level winds (SW to NE flow). Silt moves higher in the atomosphere. But if we assume the silt is subject to the prevailing SW flow, we can look upwind for a source area. Turns out we end up in places like Quincy Basin, Moses Lake, or even Yakima Valley - areas west of the Palouse.
Perhaps dust carried from the Palouse to the Okanogan moves at higher levels of the atmosphere where circulation patterns are different (SE to NW). Or late Pleistocene wind patterns were different from today. Alternatively, Okanogan silt may come from the Channeled Scablands.
Whence Okanogan silt? Prevailing wind direction, determined by migration of Holocene sand dunes, suggests dust in the Okanogan Highlands may not derive from the Palouse Hills, but from the Channeled Scablands. Base map shows EPA Ecoregions of Washington.
Dune fields. Sand dune sites located along the upper Columbia and Okanogan Rivers. Circles indicate 8 of 14 sites depicted below. Five others are north of the map area. One at Brewster Flat was a late arrival.
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.
Bissell Flat just west of the Nespelem River mouth.
Sand Hills just south of Redford Canyon along Ninemile-Hellgate Rd, north bank Lake Roosevelt.