<Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (May 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (May 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as beavers . Man - made dams are typically classified according to their size (height), intended purpose or structure . </P> <P> Based on structure and material used, dams are classified as easily created without materials, arch - gravity dams, embankment dams or masonry dams, with several subtypes . </P>

When was the first dam built in the us and why