Semiahmoo Spit is essentially an appendage of Birch Point, which is a large headland lying between Drayton Harbor to the north and Birch Bay to the south.
Easterbrook (1973) shows Birch Point to be composed primarily of Bellingham glaciomarine drift with a veneer of sand and gravel. This material, when wet, has low strength and is prone to slumping or flow, particularly along the sea cliffs.
The headland reaches maximum elevations of approximately 80 meters on the northeastern side near Drayton Harbor. From that point, the topography gently lowers westerly and southwesterly to the 25 and 30 meter high sea cliffs that rim the headland.
At the northern most reach of Whatcom County’s shoreline is a spit approximately 2000 meters long trending northeasterly from the north side of Birch Point (fig.2.). The spit separates Drayton Harbor from Semiahmoo Bay and provides some protection for small boats and shoreline developments within the bay.
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/places/capes/birch-point/birch-point.md
Northwesterly waves travel down Georgia Strait and are refracted around Birch Point into Birch Bay. Waves sweep counter-clockwise around Birch Bay transporting beach sediment in the same direction.
The northeasterly growth of Semiahmoo Spit indicates that sediment eroded from Birch Point has been transported by littoral drift northward, leading to the growth of the spit.
Winds up to 30 knots from the south and southwest could generate waves up to 2 meters (6 feet) high. During the summer, these waves are lower, corresponding to a weakening of the prevailing southerly wind velocities. Westerly winds up to 20 knots may build waves up to 11/2 meters (5 feet) high to attack Birch Point’s western shore.
