The Sno-King Upland Plain

The cities of southwest Snohomish county are on a wishbone-shaped plain or upland that extends into King county. It is mostly above 400 ft in elevation and at places above 600 ft. The plain is neatly defined by Puget Sound on the NW side and the Snohomish river valley and the Snoqualmie valley on its NE side. The Everett peninsula is the northernmost tip of the plain. Shoreline and North Seattle, including Ballard and the University District, are the western prong of the wishbone. Maltby, Cottage Lake, and Union Hill-Novelty Hill are the less urban eastern prong of the wishbone.

The southern edge of the plain is defined by the Lake Washington Ship Canal, Lake Washington, the Sammmamish River, Lake Sammamish, and a valley containing SR 202 which is drained by Evans Creek to the west and Patterson Creek to the east. The surface of Lake Sammamish is 30 ft in elevation. It drains via the Sammamish River into Lake Washington, which is 16 ft in elevation.

The Lake Washington Ship Canal, with its locks at Ballard and its cuts at Fremont and Montlake, is a man-made feature. The creation of the Lake Washington Ship Canal lowered Lake Washington by 9 ft. The canal became the new outlet for Lake Washington, replacing the Black River. If one wanted a natural boundary for the plain, one could use the now partially dry Black River and the Duwamish instead of the canal.

The plain does not have a name that I am aware of. It is one of the "upland till plains" mentioned in the paper by Goetz etal:

Upland till plains in many areas are cut by recessional meltwater channels and modern river channels. Till plains display north-south drumlins with long axes oriented in the ice-flow direction.

The paper by Fevold etal refers to a "Snohomish bench":

The three watersheds have their headwaters at about 500 ft elevation on the Snohomish “bench” area of glacial till and outwash soils.

Fevold etal don't appear to be referring to the entire plain—just a high portion of it that includes the Paine Field area.

High points on the plain are at Hwy 99 and 148th (643 ft), Clearview (653 ft), and Union Hill (679 ft). The col or highest point on the border of the bench appears to be in the Evans Creek/Patterson Creek valley and is about 85 ft in elevation. The topographical prominence of the plain is thus almost 600 feet. The elevation of the Panama Canal is also 85 ft—if the elevation of the col is higher than that, the encirclement peak of the plain is Denali, otherwise it is Aconcagua.

The western edge of the plain consists of a slender watershed about a mile across that drains into Puget Sound and features ravines or gulches with year-round streams in them. The interior of the plain drains south to the Sammamish river and Lake Washington. From west to east the major south-draining creeks are: Swamp Creek, North Creek, and Little Bear Creek.

The plain is covered by glacial till which was deposited during the Vashon glaciation. During this glaciation, ice advanced south from Canada into the Puget Sound, reaching as far as the midpoint between Olympia and Chehalis about 16950 years ago. The glacier was at times and in places over 1000m thick. The till it deposited is a mixture of sand, clay, and rounded rocks and can be up to 125 feet thick in places, at least further south in Seattle.

CLARK GRUBB