Jet′ty, an embankment or pier extending into the water, built of earth, stone and wood. Jetties are built in rivers and harbors to increase the depth of the water by narrowing the channel and thus increasing the scouring action of the current. Jetties are particularly valuable where a river empties into a sea which is more or less tideless, as the Danube into the Black Sea and the Mississippi into the Gulf of Mexico. In these cases the river is continually bringing down sediment, and this is deposited at the mouth, forming a delta. There is little or no scouring action due to the flow and ebb of the tide. The jetties at the mouth of the Danube were commenced in 1858, completed in 1861, and improved in 1868. The depth of water was increased from 9½ feet to 22 feet. The Mississippi jetties put in by James B. Eads in 1875–9 are notable among modern engineering achievements. They are embankments made by sinking mattresses of interwoven osiers and covering these with stones and concrete. These embankments prolonged a channel 1,000 feet in width for 2¼ miles seaward. The depth was increased from 8 feet to 30 feet by the scour of the river. As the deposit accumulates, it will be necessary to prolong the jetties seaward in order to scour away the fresh deposits. There are many smaller harbors and rivers in the United States which have been deepened and made safer by building jetties.