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A 'wing dam' is a manmade barrier that, unlike a conventional dam, only extends partway into a river. These structures force water into a fast-moving center channel which reduces the rate of sediment accumulation, while slowing water flow near the riverbanks.
The Mississippi River in North America has thousands of wing dams which were originally constructed to reduce the amount of dredging required when the main navigation channel was maintained to at least 4½ feet (1.37 m). Since that time, additional conventional dams have been built to increase the water level in the river, doubling the depth of the navigation channel to 9 feet (2.75 m). The wing dams still serve their purpose, but to a lesser extent than before.
While wing dams assist in assuring that rivers are navigable, they can also pose a threat to boaters. Many wing dams in the Mississippi are usually underwater and may be difficult to see, but can be easily hit by propellers or other parts of a vessel.
Wing dams are typically constructed so that they point slightly into the current (meaning that the riverbank end is slightly downstream of the riverward end).