A wing dam is a manmade barrier that, unlike a conventional dam

Dam

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

, only extends partway into a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

. These structures force water into a fast-moving center channel which reduces the rate of sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 accumulation, while slowing water flow near the riverbanks.

The Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

 in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 has thousands of wing dams which were originally constructed to reduce the amount of dredging

Dredge

Dredging is an excavation activity or operation usually carried out at least partly underwater, in shallow seas or fresh water areas with the purpose of gathering up bottom sediments and disposing of them at a different location...

 required when the main navigation channel

Channel (geography)

In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...

 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 propeller

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

s 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).