A drawbridge is a bridge that can be moved to allow tall vessels or vehicles to pass through or to prevent crossing. The bascule bridge, shown below, operates through the use of a counterweight that continuously balances the span throughout its upwards swing.
South Park Bridge, 14th/16th Avenues S: Duwamish River, June 6, 1932. (Series 474, Bridge Number 3179. Section 34, Township 24N, Range 4E.)