Complex sentence: A river crossing has been at the bridge's site long before it was constructed; a nineteenth-century pontoon bridge on the site was replaced in 1889 by a through truss bridge, which permitted river access because it was a swing bridge.
Split into simpler sentences: A river crossing had been at the bridge's site long before it was constructed. In 1889, a 19th century pontoon bridge on the site was replaced by a through truss bridge, which permitted river access because it was a swing bridge.