Washington's 1st congressional district encompasses the majority of Whatcom, Skagit, and Snohomish counties, as well as nearly one-third of King County. The eastern edge of the district follows county lines from the Canada–US border down to the I-90 corridor. Then it follows I-90 west to West Lake Sammamish, and from there north to I-5. The western border follows the I-5 corridor north to Bellingham, then along the coast to Canada.