The Danube traverses its delta in three branches, the northern one of which, though conveying nearly two-thirds of the discharge of the river, is unsuitable for improvement owing to its splitting up along portions of its course into several channels, and eventually flowing into the sea through twelve mouths of a small independent delta advancing about 25o ft. annually across a shallow foreshore. The central Sulina branch was selected for improvement in 1858 in preference to the south ern St. George's branch, which had a more favourable outlet and a better channel through the delta. The distance of the Sulina bar from the shore was only half that of the St. George's bar owing to the much smaller discharge of the Sulina branch. The jetties, begun provisionally in 1858 and subsequently consolidated and somewhat extended, were completed in 1877. They increased the depth over the bar from an average of about 9 ft. previously to 1858 up to 202 ft. in 1873, which was maintained for many years. In 1894, however, the increasing draught of vessels rendered a greater depth necessary; the wide inshore portion of the jetty channel was therefore narrowed by inner parallel jetties, and a powerful suction dredger was set to work in the jetty channel and outside, whereby the depth was increased to 24 ft. in 1897, and was fairly maintained up to 1907, when a second dredger became neces sary to cope with the shoaling. The depth contours gradually ad vanced seawards whilst the deepest channel was deflected north wards by the action of current and waves. During the World War the decrease in depth, accelerated by the interference with dredg ing which occurred, was so rapid that the navigable depth was reduced at one time to 18 feet. In 1922 the seaward extension of the jetties was commenced leaving temporary gaps, between the old heads and the new works, which were closed after the new channel had been opened for traffic (fig. 5). This extension of the jetties for about 6,000 ft. and dredging operations restored for the time being the ruling depth of 24 ft. in the direct channel. A further extension of 2,000 ft. is proposed. The new jetties, like the original ones, are formed of fascine-mattresses covered with stone-rubble.
The selection of the outlet of the south pass of the Mississippi delta for improvement by parallel jetties in 1876-79, in spite of the south-west pass possessing a larger channel and a better depth over its bar, was due, as at the Danube, to motives of economy, as the bar of the south-west pass was twice as far off from the shore as that of the south pass (fig. 4). Fascine-mattress jetties, 2* and i2 m. long, weighted with limestone, and with large concrete blocks at their exposed ends, were constructed. The jetties, which were curved slightly south wards at their outer ends to direct the sediment-bearing current more directly at right angles to the westerly littoral current, in creased the depth of 8 ft. over the bar in 1875 up to 35 ft. between the jetties and out to deep water. The prolonged flow of the river produced by the jetties has, as at the Sulina outlet, carried the main portion of the heavier sediment into fairly deep water, so that the greatest advance of the foreshore in front of the south pass has occurred in the 7o-ft. line of soundings, though the shal lower soundings have also advanced. The shoaling, however, in
the jetty channel necessitated its reduction in width by mattresses and spurs from i,000 ft. to 600 ft., and eventually the jetties were rebuilt on lines reducing the channel width to about 65o ft. Dredging was also required to maintain the stipulated central depth of 3o ft., and 26 ft. depth for a width of 200 ft. out to deep water; whilst the outer channel was deflected to the east and narrowed by the alluvium carried westwards by the littoral current and also deposited in front of the jetty outlet. Since 1901 suction dredging, the construction of additional sills, and the increased discharge at the south pass (due to the works car ried out in the south-west pass) have widened the channel across the bar to about 600 ft., and given it a minimum depth of 3o feet.
In order to provide for the increasing requirements of sea-going vessels, the formation of a channel 35 ft. deep and i,000 ft. wide through the larger south-west pass and its 9 ft. bar to deep water in the gulf was begun at the end of 1903. The discharge through this pass is rather more than three times that through the south pass and the bar was double the distance seaward of the outlet. Converging jetties, about 5,600 ft. apart at their land ends, and about 3,000 ft. apart at the seaward outlet, were substituted for the parallel jetties constructed at the south pass, and suction dredging was relied upon to maintain the channel between the jet ties. The channel dredging was soon found to be excessive in quantity and in 1916 the project was modified by limiting the channel to a width of 2,400 ft. between two parallel interior jetties, and the extension of the latter to the 3o ft. contour beyond the bar. In 1923 the width was still further restricted to 1,750 ft. by building spur-dikes or groynes, and by 1924 a depth of 35 ft. was secured by dredging aided by the scour of the current. The jetties are formed of
weighted with stone and capped with rubble and concrete (see JETTY). They have been extended seawards from time to time and in 1927 the east jetty was about 4/ m. in length from the shore. The west jetty has a length of about 3i m. The amount of dredging required to maintain the 35 ft. deep channel through the pass and over the bar has been much reduced since the contraction of the width has been effected. The artificial improvement and simultaneous mainte nance of two mouths of a river is an unique experiment.
The United States Congress through the River and Harbour act of Jantiary 21, 1927, assigned to the Secretary of War and to the Chief of Engineers, U.S. Army, the duty of making surveys in accordance with House Document 3o8, 69th Congress, ist Ses sion, with a view to the formulation of general plans for the most effective improvement of the navigable streams of the United States and their tributaries. The Flood Control act of May 15, 1928 amplified this duty with respect to the tributaries of the Mississippi river.
Under this authorization the Army Engineers have investigated and reported on practically every river of any importance in the United States. Some two hundred of these studies, called "308 Reports," have been published. These reports present plans for river improvement with an aggregate cost of more than eight billion dollars.