DELTAIC OUTLETS OF TIDELESS RIVERS Large rivers heavily charged with sand and silt, when their current is gradually arrested on entering a tideless, or nearly tideless, sea, deposit these materials as a constantly advancing fan-shaped shoal through which comparatively shallow diverging channels, almost devoid of fall, have to force their way in order to convey the fresh-water discharge into the sea. These deltaic channels deposit their burden of sediment in front of their out lets, forming bars which advance with the delta and whose rate of progress seawards and distance in front of each outlet are proportionate to the discharge of the several channels. A channel dredged on the bar in front of one of the outlets of a deltaic river retains its depth only for a moderate period on account of the deposit continually accumulating at the outlet.
The construction of parallel jetties prolonging seaward the banks of the channel, concentrates the scour of the issuing current on the bar at the outlet and under favourable conditions will procure and maintain for some time an adequate depth for navigation. The requisite conditions for the success of this system of improvement are (I) a sufficient depth in the sea beyond the bar, to allow of a con siderable deposit of alluvium before the increased depth is inter fered with, and (2) a littoral current carrying a portion of the alluvium away from the outlet. Both of these conditions retard the progression of the delta in front of the outlet and the in evitable eventual formation of a new bar farther out. The rate of advance of a delta depends also on the proportion of solid matter carried in suspension by the river and on the specific gravity and size of the particles of alluvium discharged into the sea ; for the heavier and coarser materials, and especially those which are rolled along the bed of the channels, come first to rest. Moreover, the larger channels of a delta bring down a larger volume of alluvium on account of their larger discharge, and their bars form farther seawards from their outlets owing to the issuing current being less rapidly arrested in proportion to the volume discharged. Thus the rate of advance of the delta in front of an outlet is proportionate to the size of the channel, and the length of the jetties required is proportionate to the discharge of the channel. Consequently, the conditions are more unfavour
able for the improvement of the outlets of the larger delta chan nels 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 at the gulf of Foz through six mouths, was in 5852-57 concentrated in the direct eastern channel by embankments along its sides, which closed all the lateral channels. The entire flow of the river, being thus discharged through the eastern outlet, increased for a time the depth over its bar from 41 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 long maintained, and the average depth over the bar has not exceeded 5 ft. for many years past. The St. Louis lateral canal, an artificial waterway between the gulf of Foz and the river 4 m. above its bar, was constructed in 1864-73 to afford a deeper navigable outlet, the entrance being 23- m. north of the river outlet. The want of success of the RhOne outlet jetties was due to the selection for training of a channel opening on a sheltered, somewhat shallow bay, instead of a south ern outlet discharging into deep water in the open Mediterranean and having a deep littoral current flowing across it. 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 situ ated eastern outlet likewise contributed to the failure. The south ern Roustan branch was reopened in 1893 to prevent the silting up of the outlet of the St. Louis canal.