HORIZONTAL 

SCALE delta in front of an outlet is proportionate to the size of the channel, and the length of the jetties required for lowering the bar by scour in front of any channel is proportionate to the discharge of the channel . Consequently, the conditions are more unfavourable for the improvement of the outlets of the larger delta channels than of the smaller ones; though, on the other hand, the larger channels crossing the delta are generally more suitable for navigation on account of their size, and the natural depth over their bars is greater owing to the larger discharge . The discharge of the main branch of the Rhone, which formerly flowed into the Mediterranean and the Gulf of Foz through six Rhone. mouths, was in 1852—57 concentrated in the direct eastern channel by embankments along sides, which closed all the lateral channels . The entire flow of the river, being thus discharged through the eastern outlets, increased for a time the depth over its bar from 41 ft. to 91 ft.; but as the great volume of alluvium brought down, including an unusually large proportion of sand rolled along the bed of the river, was also all discharged through the. one outlet, the bar soon formed again farther out, and naturally advanced with the delta in front of the outlet more rapidly than formerly when the deposit was distributed through six divergent mouths . Accordingly, the very moderate deepening produced by the embankments was not iong maintained, and the average depth over the bar has not exceeded 61 ft. for many years past; the St Louis Canal was constructed to provide a deeper outlet for the navigation.' This want oI success was due to the selection of an outlet opening on a sheltered, somewhat shallow bay, instead of a southern outlet discharging into deep water in the Mediterranean and having a deep littoral current flowing across it, and also resulted from the closing of all the-other outlets, whereby the whole of the deposit, as well as all the discharge, was concentrated in front of the badly situated eastern outlet . The southern Roustan branch was reopened in 1893 to prevent the silting-up of the outlet of the St Louis Canal . The Danube traverses its delta in three branches, the northern one of which, though conveying nearly two-thirds of the discharge Danube. 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 250 ft. annually across a shallow foreshore . The central Sulina branch was selected for improvement in 1858 in preference to the southern St George's branch, which had a more favourably situated outlet and a better channel through the delta, on account of the much smaller expenditure required for carrying out jetties to the bar in front of the Sulina outlet, which was only half the distance from the shore of the bar of the St George's outlet, owing to the much smaller discharge of the Sulina branch.2 The jetties, begun provisionally in 1858 and subsequently consolidated and somewhat extended, were finally completed in 1877 . They increased the depth over the bar from an average of about 9 ft. previously to 1858 up to 201 ft. in 1873, which was maintained for many years . In 1893, 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 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 necessary to cope with the shoaling . The somewhat small ratio of sediment to discharge in the Danube, the fineness of the greater portion of this sediment, its comparatively moderate amount owing to the small proportion of the discharge flowing through the Sulina branch, and its partial dispersion by the southerly littoral current and wave action, have prevented the rapid formation of a shoal in front of the Sulina outlet . Nevertheless, the lines of soundings are gradually advancing seawards in the line of the outlet channel, and there are signs of the formation of a new bar farther out, whilst the deposit to the south by the current and waves has deflected the deepest channel northwards .

Accordingly, a prolongation of the jetties will eventually be necessary, notwithstanding the removal of a portion of the deposit from the outlet channel by dredging . 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 Mlsals- south-west pass possessing a larger channel and a better slppl 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 . 13) . There fascine mattress jetties, weighted with limestone, and with large concrete blocks at their exposed ends (see JETTY), 21 and 12 m. long, and curved slightly southwards at their outer ends to direct the sediment-bearing current more directly at right angles to the westerly littoral current, increased the depth of 8 ft. over the bar in 1875 up to 31 ft. between the jetties and out to deep water (fig . 14) . The prolonged current 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 theforeshore in front of the south pass has occurred in the 7o-ft. line of soundings, though the shallower soundings have also advanced .