Many different types of bridges are categorized as movable bridges: draws, swing-spans, bascules, and vertical lifts, to name a few. Vermont's movable bridges were primarily located within the Champlain Islands.
View a 1911 map of Lake Champlain that marks the location of Vermont's movable bridges, only one of which survives today.
Swing-span and bascule bridges were the dominant types of movable bridges in Vermont. A number of swing-span bridges were built by the railroads during the late 19th century.
Swing-span bridges generally pivot horizontally at their midpoint, rotating to open two navigation channels. "Bob-tail" swing-spans pivot from one end and rotate to open one large channel. Swing-span bridges became disadvantageous, though. Their large center piers allowed only narrow channels; piers created deposits of silt; opening and closing was very time-consuming; construction costs were expensive; and they could not be widened without first demolishing the rotating spans. Bascule bridges were eventually preferred.
The bascule bridge, French for teeter or seesaw, gained popularity at the end of the 19th century. The most common type of bascule bridge, the trunnion, is counterweighted to rise up in the center, thus opening the channel, and drop at then end, much like a seesaw. A double-leaf bascule is seen below.