The territory of the Semiahmoo included the eastern shore of Point Roberts, the shores of Boundary Bay, the drainage basins of Dakota, California, and Terrell Creeks, the shores of Semiahmoo Bay and Drayton Harbour, and the shores of Birch Bay. To the north of the Semiahmoo was a small Halkomelem speaking group call the Snokomish. Their territory included the shores of Boundary Bay, and the drainage basins of the Serpentine, Nicomekl and Campbell Rivers. They intermarried with the Semiahmoo, shared a weir site near the mouth of the Campbell River, and shared a common hunting territory. This was perhaps stimulated by trade with Fort Langley as the trade route went through Snokomish territory. Shortly before 1850 the Snokomish were almost entirely wiped out by a smallpox epidemic.
"... it seems likely that the Derby people, who we might call "Sokomish", occupied both a segment on the Fraser and a bit of saltwater shore-line at Mud Bay, together with the two streams that make canoe navigation from one place to the other possible with only one short portage. After the Snokomish were wiped out, the Semiahmoo took over the salt-water section of their territory..."
Katzie Ethnographic notes by Wayne Suttles; edited by Wilson Duff. 1918 Victoria, BC p 12.
The few Snokomish survivors joined the Semiahmoo and the Semiahmoo became heirs to the Snokomish territory at the mouths of the Nicomekl and Serpentine Rivers.