by drift of shingle or sand carried along the coast by the waves in the direction of the prevailing winds. When the flow falls very low in dry weather, the outlet of a river is sometimes completely closed by a continuous line of beach, any inland or tidal waters merely trickling through the obstruction; and it is only on the descent of floods that the outlet is opened out. In rivers which always have a fair fresh-water discharge, or a small fresh-water flow combined with a tidal flow and ebb, the channel sometimes has its direct outlet closed, and is deflected parallel to the shore till it reaches a weak place in the line of beach, through which a new outlet is formed; or, where the current is strong enough to keep the outlet open, a bar is formed across the entrance by the littoral drift, reducing the navigable depth.
Jetties at River Outlets.—The bar formed by littoral drift across the outlet of a river not charged with sediment and flowing into a tideless sea can be lowered by carrying out solid jetties on each side so as to scour the bar by concentrating the issuing current over it. Thus by means of jetties, aided by dredging, the depth at the entrance to the Swine mouth of the Oder has been increased from 7 ft. to 2212 ft.; the approach channels to the river Pernau (fig. 9) and other Russian rivers flowing of the outlet across the foreshore, into the Baltic have been deepened by jetties, and the outlet channels of some of the rivers flowing into the Great Lakes of North America have been improved by crib-work jetties and dredging.
Where the littoral drift is powerful enough to divert the outlet of a river, as in the case of the river Yare, which at one time was driven to an outlet 4 m. south of its direct course into the sea at Yarmouth, and the river Adour in France, whose outlet, owing to the violent storms of the Bay of Biscay, was liable to be shifted 18 m. from its proper position, it has proved practicable to fix as well as to deepen the outlet by means of jetties (fig. 10).
Fig. 9.—Jetty Outlet into Baltic: River Pernau.
Fig. 10.—Shifting Outlet, fixed by Jetties: River Yare.
In such cases, however, where the rivers flow into tidal seas, it is important to place the jetties sufficiently apart to avoid any loss of tidal influx, since the tidal flow assists the fresh-water discharge in keeping the outlet open; whereas, with rivers flowing into tideless seas, a moderate restriction of the width between the jetties increases the scour. The tortuous and somewhat shifting outlet channel of the Scheur branch of the river Maas, emerging on to a sandy coast where the rise of tide is small, and obstructed at its mouth by a bar, has been replaced by a straight cut across the Hook of Holland, and by an outlet guided across the foreshore and fixed in position by fascine mattress jetties (see Jetty), the maintenance of the depth at the mouth by the tidal and fresh waters being aided by frequent dredging (figs. 11 and 12).
Deltaic Outlets of Tideless Rivers.
Large rivers heavily charged with sand and silt, when their current is gradually arrested on entering a tideless sea, deposit these materials as a constantly advancing fan-shaped shoal in front of their mouths, 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 (fig. 13).
Figs. 11 and 12.—Jetty Outlet into North Sea: River Maas.
Fig. 13.—Mississippi Delta.
These deltaic channels deposit their burden of sediment in front of their outlets, 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 simply dredged on the bar in front of one of the outlets of a deltaic river is only maintained for a moderate period on account of the large volume of deposit continually, accumulating at the outlet. Thus the channel in front of the outlet of the south-west pass of the Mississippi delta, when deepened from 13 ft. to 18 ft. over its bar by dredging many years ago, was soon silted up again on the discontinuance of the dredging; Whilst the depth of the outlet channel of one of the branches of the Volga delta, which was increased from 4 ft. to 8 ft., could only be maintained by regular yearly dredging.
