This is one of the reasons why NYC flooded during Hurricane Sandy, the rivers were reclaiming their natural boundaries. The only thing that keeps the Hudson out of Lower Manhattan are the concrete sea walls.
quote: The engineering was so precise that when the last section of the main span was lifted into place, the final adjustment needed to join everything together was 1/2 inch (12.7 mm). Construction of the Hell Gate Bridge began on March 1, 1912 and ended on September 30, 1916. It was the world's longest steel arch bridge until the Bayonne Bridge was opened in 1931, and was surpassed again by the Sydney Harbour Bridge in 1932. In 1996 the bridge received a facelift, including its first comprehensive paint job in 80 years. It was painted "Hell Gate Red," a dark, natural red. The bridge would be the last New York City bridge to collapse if humans disappeared, taking at least a millennium to do so, according to the February 2005 issue of Discover magazine. Most other bridges would fall in about 300 years