In 1844 Roebling won a bid to replace the wooden canal aqueduct across the Allegheny River with the Allegheny Aqueduct. His design encompassed seven spans of 163 feet (50 m), each consisting of a wooden trunk to hold the water, supported by a continuous cable made of many parallel wires, wrapped tightly together, on each side of the trunk. He followed this innovation in 1845 by building a suspension bridge over the Monongahela River at Pittsburgh. It is known as the Smithfield Street Bridge. The confluence of the Allegheny and Monongahela at Pittsburgh forms the Ohio River.