Work began in Rotterdam in 1908 on Erie Barge Canal Lock 8 and a three-span movable dam about 510 feet wide. When activated, the dam raises boats heading upstream as much as 15 feet. Often mistaken for a bridge, it is instead a walkway for necessary maintenance. Beneath the dam's trusses are steel frames, each containing a giant steel plate, and below the dam's base a concrete sill on the river bed spans the river's width. When the frames are swung open and lowered by chains onto the sill, they form a dam. Dam gates can be adjusted up or down almost like window shades to control the volume of water and its rate of flow. Raising or lowering the dam must be carefully coordinated with lock operators downstream to prevent the release of too much water that might flood the river banks, so lock gates downstream must be raised or lowered accordingly. During the winter, when the canal is not operational, the dam's steel structures are lifted completely clear of the water, allowing the river to resume its natural flow.