The 1953 Steamboat Slough Bridge is one of four operating swing spans remaining in Washington and it was the last steel swing span built in Washington. It incorporates several innovative design features including its central bearing support and the unusual live load bearing supports at the center and end piers.

The slough is one of several branches of the Skagit River which discharge into the shallow waters of Skagit Bay on the Skagit Delta. The name is for its use by stern wheel steamboats that once navigated the Skagit River. In the early 1880s, this was the only channel of the Skagit River not blocked by driftwood and snags.