The city of Seattle has multiple bridges that are significant due to their function, historical status, or engineering. Bridges are needed to cross the city's waterways and hilly topography. Twelve bridges have been granted historical status by the city, federal government, or both. Seattle also has some of the only permanent floating pontoon bridges in the world. Lake Washington and Puget Sound are to the east and west of the city, respectively. They connect via a series of canals and Lake Union that are collectively known as the Lake Washington Ship Canal. The four double-leaf bascule bridges crossing the Ship Canal are the oldest in the city, having opened between 1917 and 1930. The floating bridges carry Interstate 90 and State Route 520 across Lake Washington to the Eastside suburbs. The neighborhoods that make up West Seattle are on a peninsula separated from downtown by the Duwamish River. The West Seattle Bridge is the primary roadway crossing the river. (Full list...)