1911 Encyclopædia Britannica/Levée

LEVÉE (from Fr. lever, to raise), an embankment which keeps a river in its channel. A river such as the Mississippi (q.v.), draining a large area, carries a great amount of sediment from its swifter head-streams to the lower ground. As soon as a stream’s velocity is checked, it drops a portion of its load of sediment and spreads an alluvial fan in the lower part of its course. This deposition of material takes place particularly at the sides of the stream where the velocity is least, and the banks are in consequence raised above the main channel, so that the river becomes lifted bodily upwards in its bed, and flows above the level of the surrounding country. In flood-time the muddy water flows over the river’s banks, where its velocity is at once checked as it flows gently down the outer side, causing more material to be deposited there, and a long alluvial ridge, called a natural levée, to be built up on either side of the stream. These ridges may be wide or narrow, but they slope from the stream’s outer banks to the plain below, and in consequence require careful watching, for if the levée is broken by a “crevasse,” the whole body of the river may pour through and flood the country below. In 1890 the Mississippi near New Orleans broke through the Nita crevasse and flowed eastward with a current of 15 m. an hour, spreading destruction in its path. The Hwang-ho river in China is peculiarly liable to these inundations. The word levée is also sometimes used to denote a riverside quay or landing-place.